ifl 

sassst 


)GY  LIBRARY 


TEXT-BOOK 


OF 


COMPARATIVE  ANATOMY 


BY 

DR.    ARNOLD   LANG 

PROFESSOR   OF   ZOOLOGY   IX   THE    UNIVERSITY   OF   ZURICH 
FORMERLY   RITTER   PROFESSOR    OF    PHYLOGEXY   IX   THE   UNIVERSITY    OF   JEXA 


TRANSLATED  INTO  ENGLISH  BY 

HENRY   M.   BERNARD,  M.A.  CANTAB. 

AND 

MATILDA    BERNARD 


PAET  IL 


ILontron 

MACMILLAN    AND    CO,   LTD. 

XEW  YORK  :   MACMILLAX  &  CO. 
1896 

All  rights  reserved 


QL9 


V. 


TRANSLATORS'  PREFACE 

THE  fact  that  this  second  volume  of  the  translation  appears  four  years 
after  the  first  is  due  partly  to  the  delay  in  the  issue  of  the  third  and 
fourth  German  parts  of  which  it  is  composed,  and  partly  to  the 
increased  difficulty  in  the  work  of  translation.  A  comparison  of  the 
two  volumes  will  show  at  a  glance  that  the  work  has  developed 
under  the  hands  of  the  author:  the  treatment  has  become  more 
elaborate.  The  two  "  chapters  "  which  practically  fill  this  volume  are 
in  reality  more  like  comprehensive  treatises  on  the  groups  with 
which  they  deal,  and  as  such  could  only  be  adequately  translated  from 
the  German  by  some  one  with  a  very  special  knowledge  of  both 
groups.  There  are  probably  few  zoologists  who  have  attempted  to 
make  a  special  study  of  two  such  heterogeneous  phyla  as  the  Mollusca 
and  the  Echinodermata.  In  addition,  therefore,  to  frequent  references 
to  the  original  literature  and  to  constant  applications  to  kind  friends, 
the  whole  of  the  text  relating  to  the  two  chief  groups  was  submitted 
to  specialists  for  revision.  The  translators  beg  to  tender  their 
warmest  thanks  to  their  friends  who  kindly  undertook  this  laborious 
task.  Mr.  B.  B.  AVoodward  read  the  text  of  the  chapter  dealing 
with  the  Mollusca,  revising  the  terminology,  and  suggesting  slight 
alterations,  which  have  been  either  adopted  without  comment  in  the 
text  or  else  placed  in  short  footnotes.  Mr.  W.  Percy  Sladen  and  Mr. 
F.  A.  Bather  revised  the  text  dealing  with  the  Echinodermata,  each 
with  special  reference  to  the  group  with  which  his  name  is  most  asso- 
ciated. Thanks  are  also  due  to  Professor  Jeffery  Bell  for  his  kind 
assistance  in  the  solution  of  difficulties.  We  have  no  hesitation  in 
saying  that  it  is  to  the  generous  help  of  these  gentlemen  that 


vi  COMPARATIVE  ANATOMY 

our  translation  owes  much  of  the  value  it  may  possess  for  the  English 
student. 

In  the  use  of  certain  technical  terms  we  have  given  the  English 
or  the  Latin  form  indifferently,  e.g.  pinnule  or  pinnula,  auricle  or 
auricula,  with  deliberate  inconsistency.  On  the  other  hand,  we  have 
throughout  used  the  terms  madreporite,  madreporitic,  and  Echinoder- 
mata,  although  some  authorities  are  more  in  favour  of  madrepore, 
madreporic,  and  Echinoderma.  We  feel  it  our  duty  to  call  the  atten- 
tion of  students  to  these  points. 

The  following  author's  preface  is  a  free  translation  of  the 
Nachwort  which  appeared  at  the  end  of  the  fourth  German  part. 
In  it  the  author  answers  the  only  serious  charge  against  the  work 
as  a  text-book  which  has  been  brought  to  our  notice.  It  finds  its 
most  appropriate  place  as  a  preface  to  the  second  volume  of  the 

translation. 

H.  &  M.  BEENAED. 


AUTHOB'S  PEEFACE  TO  THE  SECOND  VOLUME 

\YiTH  the  publication  of  the  last  two  chapters,  dealing  with  the 
Echinodermata  and  the  Enteropneusta — that  is  of  the  fourth  German 
portion — I  bring  this  text-book  to  a  close  for  the  time  being,  as  a 
comparative  anatomy  of  the  Invertebrata. 

I  feel  that  some  excuse  is  necessary  for  the  tardy  appearance  of 
the  separate  parts,  especially  of  the  third  (Mollusca).  This  was 
mainly  due  to  my  call  to  the  University  of  Zurich,  where  official 
duties  left  only  the  holidays  and  vacations  for  my  own  work  When 
I  add  that  the  greater  number  of  the  illustrations  were  drawn  by 
my  own  hand,  the  reader  will,  I  trust,  pardon  the  lapse  of  time. 
Indeed,  if  he  be  a  trained  zoologist,  he  will  be  specially  sympathetic 
and  indulgent,  and  will  be  able  to  realise  my  feelings  as  I  watched 
the  fresh  relays  of  books  piling  up  before  me  at  the  commencement 
of  each  new  chapter.  Original  sources  alone  have  been  relied  upon 
for  the  subject  matter  of  the  work. 

In  spite  of  the  imperfections  and  deficiencies  of  which  I  am 
only  too  conscious,  the  book  appears  to  have  been  found  useful, 
judging  from  the  favourable  reception  almost  universally  given  to 
it,  and  from  the  circumstances  that,  even  during  its  appearance, 
it  was  translated  into  foreign  languages. 

I  am  fully  aware  that  the  matter  is  unequally  worked  up.  The 
divisions  treated  in  the  first  volume  are  too  briefly  dealt  with,  a 
defect  which  must  be  remedied  in  a  new  edition.  Any  criticisms 
or  advice  with  which  my  colleagues  may  favour  me  will  be  gladly 
accepted  in  the  spirit  in  which  they  are  intended. 

I  have  been  blamed  by  many  for  not  mentioning  the  names  of 


viii  COMPARATIVE  ANATOMY 

authors  in  the  text.  From  the  very  first  this  question  caused  me 
much  perplexity,  and  I  made  repeated  attempts  to  indite  single 
chapters  so  as  to  bring  in  the  historical  development  of  the  branch 
dealt  with,  together  with  the  names  of  the  most  important  authors. 
I  then  found  that  if  this  course  were  pursued  the  book  would 
attain  twice  its  present  dimensions,  that  is,  if  strict  impartiality 
were  to  be  invariably  observed.  This  latter  I  was  resolved  on  no 
account  to  renounce,  and  I  therefore  determined  to  exclude  from 
the  text  the  names  of  all  authors  without  distinction.  Any  one  who 
is  interested  in  knowing  how  a  special  question  stands,  can  easily 
find  his  bearings  by  careful  comparison  of  the  text  with  the  illustra- 
tions (the  origin  of  which  is  everywhere  given),  and  by  consulting 
the  literature.  I  have  convinced  myself  of  this  among  my  own 
students. 

I  must  here  express  my  thanks  to  my  honoured  and  dear  friend, 
Mr.  Gustav  Fischer,  for  the   care  and  patience  he  has  exercised  in 

connection  with  this  work. 

AKNOLD  LANG. 

ZUKICH,  July  1894. 


CONTENTS 

CHAPTER   VII 

MOLLUSCA 

PAGE 

Systematic  Review         ........  2 

Class  I.  AMPHINEURA          .            .            .            .            .            .            .  2 

II.  GASTROPODA  (CEPHALOPHORA)      .....  3 

III.  SCAPHOPODA          .......  13 

IV.  LAMELLIBRANCHIA      (PELECYPODA,       BIVALYA,       ACEPHALA, 

AGLOSSA)           .            .            .           .           .  14 
V.  CEPHALOPODA       .            .            .            .            .            .            .21 

I.  ORGANISATION  OF  THE  PRIMITIVE  MOLLUSC           ...  26 

II.  REVIEW  OF   THE  OUTER  ORGANISATION  CHARACTERISING   THE 

CHIEF  GROUPS  OF  THE  MOLLUSCA           ....  28 

A.  PLACOPHORA  OR  POLYPLACOPHORA  (CHITONID^E)  .            .            .  .29 

B.  APLACOPHORA,  SOLENOGASTRES     .....  29 

C.  GASTROPODA  (CEPHALOPHORA)      .....  30 

D.  SCAPHOPODA  .  .  .  .  .  .  .34 

E.  LAMELLIBRANCHIA  .......  34 

F.  CEPHALOPODA          .......  36 

III.  THE  INTEGUMENT,  THE  MANTLE,  AND  THE  VISCERAL  DOME        .  39 

A.  PLACOPHORA            .......  39 

B.  SOLENOGASTRES       .  ..*          .  .  .  .  .41 

C.  GASTROPODA            .                                     ....  42 

D.  SCAPHOPODA            .                                     ....  49 

E.  LAMELLIBRANCHIA  .......  49 

F.  CEPHALOPODA          .......  53 

IV.  THE  SHELL     ...                                     ...  55 

A.  AMPHINEURA           .....'..  58 

B.  GASTROPODA  58 


:  COMPARATIVE  ANATOMY 

PAGE 

C.  LAMELLIBRANCHIA  .......          61 

D.  CEPHALOPODA         .......          67 

V.  ARRANGEMENT  OF  THE  ORGANS  IN  THE  MANTLE  CAVITY,  AND 

OF  THE  OUTLETS  OF  INNER  ORGANS  IN  THAT  CAVITY          .          71 

A.  GASTROPODA  .  .  .  ,  .  .  .71 

B.  SCAPHOPODA  .......          80 

C.  LAMELLIBRANCHIA  .......          81 

D.  CEPHALOPODA         .  .  .  .  .  .  .81 

VI.  THE  RESPIRATORY  ORGANS  ......          84 

THE  TRUE  GILLS  OR  CTENIDIA     .....          84 

A.  AMPHINEURA          .  .  .  .  .  .  .86 

B.  GASTROPODA  .......          88 

C.  LAMELLIBRANCHIA  .  .  .  .  .  .  .91 

D.  CEPHALOPODA         .  .  .  .  .  .  .96 

ADAPTIVE  GILLS     .  .  .  .  97 

LUNGS          ........  99 

VII.  THE  HYPOBRANCHIAL  GLAND  .....        101 

VIII.  THE  HEAD     ...  .  .  .101 

A.  GASTROPODA  .  .  .  .  .  .  .102 

B.  SCAPHOPODA  .  .  .  .  .  .  .104 

C.  CEPHALOPODA          .  .  .  .  .  .  .105 

IX.  THE  ORAL  LOBES  OF  THE  LAMELLIBRANCHIA        .  .  .105 

X.  THE  FOOT  AND  THE  PEDAL  GLANDS  ....        106 

A.  AMPHINEURA           .            .            .  .            .            .            .        106 

B.  GASTROPODA           .           .           .  .           .           .            .107 

C.  SCAPHOPODA            .            .            .  .            .                        .112 

D.  LAMELLIBRANCHIA  .            .            .  .            .                        .112 

E.  CEPHALOPODA          .            .            .  .            .            .            .115 

XL  SWELLING  OF  THE  FOOT  (Turgescence)         .  .  .  .118 

XII.  MUSCULATURE  AND  ENDOSKELETON  ....        119 

A.  AMPHINEURA  .  .  .  .  .  .  .120 

B.  GASTROPODA  .  .  .  .  .  .  .120 

C.  SCAPHOPODA  .  .  .  .  .  .  .123 

D.  LAMELLIBRANCHIA  .  .  .  .  .  .124 

E.  CEPHALOPODA         .  .  .  .  .  .  .126 

XIII.  THE  NERVOUS  SYSTEM          .  .  .128 

A.  AMPHINEURA          .......        128 

B.  GASTROPODA  132 


CONTENTS  xi 

PAGE 

1.  THE  AREAS  OF  INNERVATION  OF  THE  VARIOUS  GANGLIA    .        133 

2.  ORIGIN  OF  THE  CROSSING  OF  THE  PLEUROVISCERAL  CON- 

NECTIVE (CHIASTOXEURY)  .  .  .  .    V  135 

3.  SPECIAL  REMARKS  ON  THE  NERVOUS  SYSTEM  OF  THE  GAS- 

TROPODA     .......         137 

C.  SCAPHOPODA  .  .  .  .  .  .  .142 

D.  LAMELLIBRANCHIA  .......        143 

E.  CEPHALOPODA         .......        145 

XIV.  AN  ATTEMPT  TO  EXPLAIN  THE  ASYMMETRY  OF  THE  GASTROPODA        149 

XV.  THE  SENSORY  ORGANS          .  .  .  .  .  .162 

A.  INTEGUMENTAL  SENSORY  ORGANS  ....        162 

1.  TACTILE  ORGANS          ......        162 

2.  OLFACTORY  ORGANS     ......        162 

3.  THE  "LATERAL  ORGANS"  OF  THE  DIOTOCARDIA      .  .        165 

4.  GUSTATORY  ORGANS     .  .  .  .  .  .166 

5.  SUBRADULAR  SENSORY  ORGAN  OF  CHITON     .  .  .166 

6.  THE  SENSORY  ORGANS  ON  THE  SHELL  OF  CHITON   .  .        166 

B.  AUDITORY  ORGANS  .  .  .  .  .  .167 

C.  VISUAL  ORGANS      .  .  .  .  .  .-  .169 

1.  OPTIC  PITS       .            .            .            .            .  -    .        169 

2.  OPTIC  VESICLES  OR  VESICULAR  EYES            .            .  .170 

3.  THE  EYE  OF  THE  DIBRANCHIATE  CEPHALOPODA       .  .        170 

4.  THE  DORSAL  EYES  OF  ONCIDIUM  AND  THE  EYES  AT  THE 

EDGE  OF  THE  MANTLE  IN  PECTEN  .  .  .173 

5.  THE  EYES  ON  THE  SHELL  OF  CHITON  .  .  .175 

6.  THE  COMPOUND  EYES  OF  ARCA  AND  PECTUNCULUS  .        175 

7.  DEGENERATION  OF  THE  CEPHALIC  EYES        .  .  .        176 

XVI.  THE  ALIMENTARY  CANAL     .  .  .  .  .  .176 

A.  BUCCAL  CAVITY,  SNOUT,  PROBOSCIS         .  .  .  .178 

B.  THE    PHARYNX    AND    JAWS,    THE    TONGUE    AND    SALIVARY 

GLANDS              .......  180 

FORMATION  OF  THE  RADULA         .....  183 

C.  THE  (ESOPHAGUS    .            .            .            .            .            .            .  187 

D.  THE   MID-GUT   WITH  THE   STOMACH    AND    DIGESTIVE    GLAND 

(LlVER)    ........  190 

1.  AMPHINEURA    .......  191 

2.  GASTROPODA      .......  192 

3.  SCAPHOPODA      .......  193 

4.  LAMELLIBRANCHIA        ...  .  194 

5.  CEPHALOPODA  .......  194 

E.  HIND-GUT  (RECTUM  ......  195 

XA~II.  THE  CIRCULATORY  SYSTEM  ......        198 

A.  GENERAL     .  198 


xii  COMPARATIVE  ANATOMY 

PAGE 

B.  SPECIAL       ........        201 

1.  AMPHINETJRA    .  .  ...  .  .  .201 

2.  GASTROPODA      .......        201 

3.  SCAPHOPODA      .......        206 

4.  LAMELLIBRANCHIA       ......        206 

5.  CEPHALOPODA   .......        208 

XVIII.  THE  BODY  CAVITY    .  .  .  .  .  .  .211 

XIX.  THE  NEPHRIDIA        .  .  ....        215 

A.  AMPHINEURA          .......        216 

B.  GASTROPODA  .  .  .  .  .  .  .217 

C.  SCAPHOPODA  .......         221 

D.  LAMELLIBRANCHIA  .  .  .  .  .  .  .221 

E.  CEPHALOPODA          .  .  .  .  .  .  .        222 

XX.  GENITAL  ORGANS      ...  ...        225 

A.  GENERAL     .  .  .'....  .  .  .  .        225 

B.  SPECIAL       ........         227 

XXI.  PARASITIC  GASTROPODA         ......        244 

XXII.  ATTACHED  GASTROPODA        ......        248 

XXIII.  ONTOGENY      ........        248 

A.  AMPHINEURA          .......        248 

B.  GASTROPODA  .......        252 

XXIV.  PHYLOGENY    ....  ...        268 

Review  of  the  most  Important  Literature         .  .  .  .269 

APPENDAGE, — RHODOPE  VERANII     .  .  .        281 


CHAPTER  VIII 

ECHINODERMATA 

Systematic  Review        .  .  .  .  .  .  .  .285 

CLASS  I.  HOLOTHURIOIDEA                          .....  285 

II.  ECHINOIDEA         .......  288 

III.  ASTEROIDEA             .......  295 

IV.  OPHIUROIDEA       .......  299 

V.  PELMATOZOA         .......  302 

1.  CRINOIDEA        .......  302 

2.  CYSTTDEA           .......  313 

3.  BLASTOIDEA      .  .  .  .  .  .  .314 

I.  GENERAL  MORPHOLOGY  or  THE  ECHINODERM  BODY          .            .  315 

II.  MORPHOLOGY  OF  THE  SKELETAL  SYSTEM                                        .  317 


CONTENTS  xiii 

PAGE 

INTRODUCTION           ....                                    •  317 

A.  THE  APICAL  SYSTEM  (Calyx)     .                                                .  319 

1.  ECHINOIDEA        ....                                                    .  319 

2.  ASTEROIDEA      .......  326 

3.  OPHIUROIDEA    .......  327 

4.  PELMATOZOA      .......  328 

(a)  CRINOIDEA   .......  328 

(6)  BLASTOIDEA  .......  330 

(c)  CYSTIDEA      .......  332 

B.  THE  ORAL  SYSTEM  OF  PLATES  .....  333 

C.  THE  PERISOMATIC  SKELETON     .....  337 

1.  HOLOTHURIOIDEA             ......  337 

2.  ECHINOIDEA      .......  338 

(a)  THE  NUMBER  OF  THE  VERTICAL  Rows  OF  PLATES         .  339 

(6)  THE  PORES  OF  THE  AMBTJLACRAL  SYSTEM            .            .  340 

(c)  THE  SYMMETRY  OF  THE  ECHINOID  SHELL            .            .  340 

(d)  THE  RELATION  OF  THE  AMBULACRAL  AND  INTERAMBU- 

LACRAL  PLATES  TO  THE  PERISTOME         .            .            .  344 

(e)  MANNER  IN  WHICH  THE  SKELETAL   PLATES   ARE  CON- 

NECTED       .......  345 

(/)  SPECIAL  MODIFICATIONS  OF  THE  AMBULACRA      .            .  346 

(g)  SPECIAL  MODIFICATIONS  OF  THE  INTERRADII        .            .  348 

(h)  FORM  OF  THE  PERISTOME  .....  349 

(i)  ORNAMENTATION      .            .            .            .            .             .  349 

(&)  MARGINAL  INCISIONS  OR  PERFORATIONS   .            .            .  349 

(1)  THE  PERIGNATHIC  APOPHYSIAL  GIRDLE   .            .            .  350 

3.  ASTEROIDEA      .......  351 

(a)  THE  AMBULACRAL  SKELETON         .  .  .  .351 

(b)  THE  INTERAMBULACRAL  SKELETON            .            .            .  353 

(c)  THE  ACCESSORY  SKELETAL  SYSTEM            .            .            .  354 

(d)  COMPARISON  OF   THE   PERISOMATIC   SKELETON   OF    THE 

ASTEROIDEA  WITH  THAT  OF  THE  ECHINOIDEA     .            .  355 

4.  OPHIUROIDEA    ......:  355 

(a)  SKELETON  OF  THE  ARMS    .....  355 

(b)  THE  ORAL  SKELETON           .....  358 

5.  CRINOIDEA        .......  362 

(a)  THE  PERISOMATIC  SKELETON  OF  THE  CALYX       .            .  362 

a.  THE  APICAL  CAPSULE  OR  DORSAL  CUP           .            .  367 

b.  THE  TEGMEN  CALYCIS  .....  369 

(b)  THE  BRACHIAL  SKELETON  .....  370 

(c)  THE  STEM  (COLUMNA)          .....  373 

(d)  THE  MANNER  OF  CONNECTION  BETWEEN  THE  SKELETAL 

PIECES         .......  376 

(e)  THE  NERVE  CANALS  OF  THE  ARMS  AND  OF  THE  APICAL 

CAPSULE      .......  377 


xiv  COMPARATIVE  ANATOMY 

PAGE 

(/)  THE  WATER  PORES            .....  377 

6.  BLASTOIDEA      .......  379 

(a)  THE  AMBULACRAL  SKELETON         ....  379 

(6)  THE  STEM    .......  384 

7.  CYSTIDEA          .......  384 

D.  THE  SPINES  AND  THEIR  DERIVATIVES — THE  SPHJERIDIA  AND 

THE  PEDICELLARI^G           .                                    ...  387 

E.  THE   MASTICATORY  APPARATUS   OF  THE  ECHINOIDEA.      (Aris- 

totle's Lantern)      .......  400 

F.  THE  CALCAREOUS  RING  OF  THE  HOLOTHURIOIDEA          .            .  403 

G.  FURTHER  DEPOSITS  OF  CALCAREOUS  MATTER      .            .            .  405 
H.  CONCLUDING  REMARKS  ON  THE  SKELETON          .            .            .  405 

III.  THE  OUTER  MORPHOLOGY  OF  THE  HOLOTHURIOIDEA         .            .  406 

IV.  THE    POSITION   AND   ARRANGEMENT   OF   THE   MOST    IMPORTANT 

ORGANS  IN  THE  RADII     ......  409 

V.  THE  INTEGUMENT      .......  414 

VI.  THE  WATER  VASCULAR  SYSTEM      .  .  .  .  .416 

A.  THE  MADREPORITE  AND  STONE  CANAL    .  .  .  .417 

B.  THE  WATER  VASCULAR  RING        .....  423 

C.  THE  RADIAL  CANALS,    THE   CANALS   OF  THE  TENTACLES  AND 

TUBE-FEET,  ETC.    .......  426 

D.  THE  AMBULACRAL  APPENDAGES    .            ...            .            .  431 

VII.  THE  COSLOM    ........  436 

A.  THE  BODY  CAVITY             .                        ....  437 

B.  THE  BRACHIAL  CAVITIES  ......  440 

C.  THE  PERICESOPHAGEAL  SINUS         .....  441 

D.  THE  PERIANAL  SINUS        .            .            .            .            .            .  444 

E.  THE  AXIAL  SINUS  .......  444 

F.  THE  AXIAL  ORGAN             .                                    ...  445 

G.  THE  CHAMBERED  SINUS     .  .  .  .  .  .446 

VIII.    THE   PSEUDOH^EMAL   SYSTEM                    .                 .                 .                 .                 .  447 

IX.  THE  EPINEURAL  SYSTEM       .                                    ...  448 

X.  THE  BLOOD  VASCULAR  OR  LACUNAR  SYSTEM          .            .            .  449 

XI.  THE  NERVOUS  SYSTEM          ......  453 

A.  THE  SUPERFICIAL  ORAL  SYSTEM  .....  454 

B.  THE  DEEPER  ORAL  NERVOUS  SYSTEM      ....  458 

C.  THE  APICAL  OR  ABORAL  NERVOUS  SYSTEM          .            .            .  459 

D.  THE  THIRD  NERVOUS  SYSTEM  OF  THE  CRINOIDEA  461 


CONTENTS  xv 

PAGE 

XII.  THE  SENSORY  ORGANS          ......  462 

A.  THE  AMBULACRAL  APPENDAGES  AS  SENSORY  ORGANS    .            .  462 

B.  NERVE  ENDINGS  IN  THE  INTEGUMENT      ....  466 

C.  AUDITORY  ORGANS,  ORGANS  OF  ORIENTATION      .            .            .  468 

D.  EYES            ........  468 

XIII.  THE  BODY  MUSCULATURE     .  .  .  .  .  .470 

A.  HOLOTHURIOIDEA      .......  471 

B.  ECHINOIDEA             ....                         .            .  471 

C.  ASTEROIDEA                   .......  472 

D.  OPHIUROIDEA          .......  474 

E.  CRINOIDEA  ........  474 

XIV.  THE  ALIMENTARY  CANAL     ......  474 

A.  GENERAL  REVIEW  .......  474 

B.  HOLOTHURIOIDEA    .......  476 

C.  ECHINOIDEA             .......  479 

D.  CRINOIDEA  ........  481 

E.  ASTEROIDEA            .......  483 

F.  OPHIUROIDEA          .......  485 

XV.  RESPIRATORY  ORGANS           ......  485 

A.  THE  (INNER)  RESPIRATORY  TREES  OF  THE  HOLOTHURIOIDEA     .  487 

B.  REVIEW  OF  THE   RESPIRATORY  ORGANS   OF  THE   ECHINODER- 

MATA          ........  487 

XVI.  THE  CUVIERIAN  ORGANS  OF  THE  HOLOTHURIOIDEA           .            .  488 

XVII.  EXCRETION      ........  489 

XVIII.  THE  SACCULI  OF  THE  CRINOIDEA    .....  489 

XIX.  GENITAL  ORGANS       .......  490 

A.  GENERAL  MORPHOLOGY      ......  490 

B.  HOLOTHURIOIDEA    ....                        .  491 

C.  ASTEROIDEA             ....                         .  492 

D.  OPHIUROIDEA          .......  494 

1.  THE  BURBLE      .......  494 

2.  THE  GENITAL  APPARATUS       .....  495 

E.  ECHINOIDEA            .......  498 

F.  CRINOIDEA  ........  500 

G.  ORIGIN  OF  THE  SEXUAL  PRODUCTS           ....  501 
H.  HERMAPHRODITISM  IN  ECHINODERMS       .            .            .            .501 

1.  CARE  OF  THE  BROOD  AND  SEXUAL  DIMORPHISM      .            .  502 

XX.  CAPACITY  FOR  REGENERATION  AND  ASEXUAL  REPRODUCTION       .  504 

XXI.  ONTOGENY      ........  506 

A.  THE  VARIOUS  LARVAL  FORMS  OF  THE  ECHINODERMATA  506 


xvi  COMPARATIVE  ANATOMY 

PAGE 

B.  ONTOGENY  OF  THE  HOLOTHURIOIDEA        .  .510 

C.  ONTOGENY  OF  THE  ECHINOIDEA    .....        519 

D.  ONTOGENY  OF  THE  ASTEROIDEA    .  .  .        524 

E.  ONTOGENY  OF  THE  OPHIUROIDEA  .  .  .         532 

F.  ONTOGENY  OF  THE  CRINOIDEA      ...  .        533 

XXII.  PHYLOGENY    .  .  545 

Review  of  the  most  Important  Literature    ....         551 


CHAPTER   IX 

ENTEROPNEUSTA  , 

I.  OUTER  ORGANISATION  .......  562 

II.  THE  BODY  EPITHELIUM           ......  563 

III.  THE  NERVOUS  SYSTEM            .            .            .            .            .            .  564 

IV.  THE  SENSORY  ORGANS             ......  565 

V.  THE  ALIMENTARY  CANAL        ......  565 

VI.  THE  CCELOMIC  SACS  AND  THE  BODY  MUSCULATURE.            .            .  571 
VII.  THE  "HEART  VESICLE"         ....                        .578 

VIII.  THE  LIMITING  MEMBRANES,  THE  PROBOSCIDAL  SKELETON,  AND  THE 

BRANCHIAL  SKELETON      .  .  .  .  .  .579 

IX.  THE  BLOOD  VASCULAR  SYSTEM          .....        581 

X.  THE  GONADS     ......  .585 

XI.  ONTOGENY        ........        586 

XII.  PHYLOGENY      ........        591 

Literature      .  ...  .  .  .  .  .         595 

APPENDAGE  TO  THE  ENTEROPNEUSTA 
I.  CEPHALODISCUS  .......        596 

II.  RHABDOPLEURA  .  ....        600 

Literature      .  .  .  .  .  .  .  .602 

INDEX  .  .  603 


CHAPTEE   VII 
SIXTH  RACE  OK  PHYLUM  OF  THE  ANIMAL  KINGDOM 

MOLLUSCA. 

THE  Mollusca  are  essentially  bilaterally  symmetrical  animals  with 
unsegmented  bodies.  The  ventral  wall  is  thick  and-  muscular,  and 
forms  a  foot  which  is  used  for  locomotion,  and  assumes  the  most 
varied  shapes.  A  fold  of  the  body  wall  forms  a  circular  mantle,  which 
hangs  down  round  the  body,  enclosing  a  space  which  is  called  the 
mantle  or  pallial  cavity.  This  cavity  is  originally  deepest  and 
most  spacious  posteriorly,  and  contains,  at  the  sides  of  the  median 
anus,  symmetrically  grouped,  the  two  gills  and  the  renal  and 
genital  apertures.  The  dorsal  portion  of  the  animal  is  generally 
developed  into  a  visceral  dome  or  sac,  and  is  protected  down 
to  the  edge  of  the  mantle  by  a  shell.  The  mouth  lies  at  the 
anterior  end  of  the  body  and  leads  into  a  pharynx,  Avhich  is  usually 
provided  with  jaws  and  a  rasp-like  organ  called  the  radula.  The 
mesenteron  or  mid-gut  is  supplied  with  a  large  digestive  gland  (liver). 
The  secondary  coelom  (enclosed  by  its  own  walls)  is  reduced,  but 
always  persists  as  a  pericardium.  The  blood  vascular  system  is  open, 
and  generally  to  a  great  extent  lacunar.  The  heart  is  dorsal  and 
arterial,  and  was  primitively  provided  with  two  symmetrical  auricles. 
The  nephridia  were  originally  paired,  and  in  open  communication 
with  the  pericardium.  The  central  nervous  system  consists  of  paired 
cerebral,  pleural,  pedal,  and  visceral  ganglia.  The  Mollusca  are  either 
sexually  separate  or  hermaphrodite.  The  gonads  are  usually  single, 
with  paired  or  unpaired  ducts.  In  the  course  of  development  a 
modified  Trochophora  arises  from  the  gastrula ;  this  is  the  Yeliger 
larva,  typical  of  the  Mollusca. 

These  general  characteristics  of  the  Molluscan  body  have  to  be  modified  for  each 

class.     In  each  class   there  are    series  of  forms   which   deviate   from   the  typical 

organisation  in  some  one  important  point,  or  in  several.     The  shell  may  disappear, 

and  so  may  the  mantle.     Either  one  or  both  of  the  gills  or  ctenidia  may  be  lost, 

VOL.  II  B 


COMPARATIVE  ANATOMY 


CHAP. 


and  new,  morphologically  different  respiratory  organs  may  be  substituted.  The 
visceral  dome  may  be  flattened  down,  and  the  foot  become  rudimentary  or  disappear. 
Teeth  of  all  kinds  may  be  wanting.  The  complex  of  the  sub-pallial  organs  may  be 
so  displaced  as  to  lie  anteriorly,  thereby  causing  a  very  pronounced  asymmetry  of 
the  whole  organism.  But  the  typical  Molluscan  characteristics  are  never  so  entirely 
obscured  that  the  members  of  the  race  cannot  be  recognised,  on  the  one  hand  by 
means  of  transition  forms  leading  to  well-known  Molluscan  types,  and  on  the  other 
by  their  developmental  history. 

The  Molluscs  are  divided  into  the  five  following  classes  : — 


I.  Amphineura. 
III.  Scaphopoda. 


II.  Gastropoda. 
IV.  Lamellibranchia. 
V.  Cephalopoda. 


Systematic  Review. 

CLASS  I.  Amphineura. 

Bilaterally-symmetrical  Molluscs.     The  nervous  system  consists  of  two  lateral 
and  two  ventral  nerve   trunks,  bound   together  by  numerous   commissures,    and 


FIG.  1.— Chiton,  from  life  (after  Pretre,  in  the  Voyage  de  V'Astroldbe). 

provided  with  ganglion  cells  throughout  their  whole  length  ;  these  pass  anteriorly 
into  the  cerebral  ganglion.     Special  sensory  organs  are  reduced.     Marine. 


ORDER  1.  Placophora  (Polyplacophora)  sive  Chitonidse. 

On  the  dorsal  side  there  are  eight  consecutive  shelly  plates  overlapping  like  the 
tiles  on  a  roof.  There  is  a  distinct  snout.  The  branchiae  are  numerous,  and  are 
arranged  in  two  longitudinal  rows,  one  on  each  side  in  the  groove  between  the  foot 
and  mantle.  The  foot  (except  in  Chitonellus)  is  strongly  developed,  with  a  large  flat 


VII 


MOLLUSCA— SYSTEMATIC  REVIEW 


sole  for  creeping  or  for  attachment.  The  sexual  ducts  and  the  nephridia  are  paired. 
The  sexes  are  separate.  The  heart  is  provided  with  two  auricles.  Radula  (3  +  1), 
(2  + 1),  (1  + 1  + 1),  (1  +  2),  (1  +  3).  Chiton  (Fig.  1),  Chitonellus. 

ORDER  2.  Aplacophora  sive  Solenogastres.1 

The  body  is  almost  cylindrical,  and  generally  vermiform.  There  is  no  shell. 
The  much  thickened  cuticle  contains  calcareous  spicules.  The  foot  is  rudimentary, 
a  mere  ridge  being  left,  and  the  mantle  cavity  is  reduced  to  a  groove  at  the  sides  of 
this  ridge,  and  a  cavity  (cloaca)  at  the  posterior  part  of  the  body,  into  which  the 
intestinal  canal  and  nephridia  open,  and  iu  which  are  found,  when  present,  the 
rudimentary  gills.  The  nephridia  serve  as  ducts  for  the  genital  products. 

Family  1.  Neomeniidae. 
The  foot  is  a  longitudinal  ridge,  which  rises  from  the  base  of  a  niedio-ventral 


FIG.  -2. — Proneomenia  Sluiteri,  two-thirds  natural  size.  A,  From  the  right  side ;  B,  from 
beneath  ;  o,  mouth  ;  d,  cloaca. 

longitudinal  furrow.     This  family  is  hermaphrodite.     Proneomenia  (Fig.   2),  Neo- 
menia,  Lepidomenia,  Dondersia. 

Family  2.  Chsetodermidae. 

The  foot  and  the  pedal  furrow  are  quite  degenerated.  The  sexes  are  separate. 
Chcetoderma. 

CLASS  II.  Gastropoda  (Cephalophora).     Snails. 

The  body  is  asymmetrical.  The  head,  which  carries  tentacles  and  eyes,  is 
generally  distinct  from  the  body.  The  foot  is  well  developed— usually  with  a  flat  sole 
for  creeping.  The  large  protruding  visceral  dome  may  be  flattened  down  secondarily 
in  all  the  groups.  It  is  covered  by  a  shell,  consisting  of  a  single  piece,  into  which  the 
animal  can  withdraw.  In  all  divisions,  however,  though  rarely  among  the  Proso- 


1  Simroth,  in  the  new  edition  of  Bronu's  Ktassen   und  Ordnunyen  des  ThierreiclieS) 
vol.  iii.,  1893,  divides  the  Solenogastres  as  follows  :— 

Sub-Order.  Fain. 

Chaatodermatina  ....        Chsetodermatida?. 

Neomeniidae. 
Proneomeniidae. 
Dondersiidae. 


Xeomeniiua 


Parameniida?. 


OF   THF 

UNIVJL, 


COMPARATIVE  ANATOMY 


CHAP. 


—  6 


FIG.  3.-Margarita  Groenlandica  (Trochid,  after 
Pelseneer).  1,  Head  ;  2,  anterior  epipodial  lobes  ; 
3,  foot  ;  4,  pigmented  prominence  at  the  base  of  the 
epipodial  tentacles  (5)  ;  6,  visceral  dome. 


branchia,  this  shell  may  become  more  or  less  rudimentary  (generally  in  connection 
with  the  reduction  of  the  visceral  dome). 

The  pallial  complex  becomes  shifted  forward  on  to  the  right  (seldom  the  left) 

side,  or  along  this  side  so  as  to  lie  quite 
anteriorly.  The  visceral  dome  and 
shell  (with  some  exceptions)  are  spirally 
coiled. 

In  all  except  the  lowest  Proso- 
brancMa,  the  asymmetry  is  evidenced 
by  the  disappearance  of  one  gill,  of  one 
kidney,  and  of  one  auricle. 

The  radula  is  rarely  wanting. 

ORDER  1.  Prosobranchia. 

The  pleuro-  visceral  connectives  are 
crossed.  The  mantle  complex  is  twisted 

round  to  the  front  side  of  the  visceral 
domei  Jn  mogt  formg  there  ig  on] 

one  gA  placed  anteriorly  to  the  heart, 
and  in  the  heart  the  auricle  lies  anter- 
iorly to  the  ventricle.  The  Proso- 

branchia are  chiefly  marine,  and  are  sexually  separate.     The  foot  is  generally  pro- 

vided with  an  operculum  for  closing 

the  aperture  of  the  shell.     A  shell  is 

wanting  only  in  Titiscania,  a  genus 

of  the  Neritacea. 

Sub-Order  1.  Diotocardia. 

The  heart  has  two  auricles  (except- 
ing in  Docoglossa).  There  are  two 
kidneys.  Instead  of  the  pedal  ganglion 
of  other  Gastropoda,  there  are  two 
longitudinal  nerves  in  the  foot,  sup- 
plied with  ganglia  and  connected  with 
one  another  by  numerous  commissures. 
The  gills  are  feathered  on  two  sides, 
their  points  projecting  freely.  The 
epipodium  is  well  developed,  and 
there  is  a  circle  of  more  or  less 
numerous  tentacles  around  the  base 
of  the  foot.  Proboscis,  penis,  and 
siphon  are  all  wanting. 

a.  Zeugobranchia  (Rhipidoglossa, 
Aspidobranchia).—  Two  gills  ;    both 
auricles  well  developed.     Heart  tra- 
versed  by   the   rectum.     Shell    with       FlG.  4._patella   vulgata  (from   beneath,   after 
marginal  cleft,  or  with  apical  perfora-    Lankester).  «,  Tentacle  ;  d,  efferent  branchial  vessel 
tion  or  with  a  row  of  perforations.    c>  free  edse  of  the  she11  ;  e>  free  edse  of  the  mantle 
Generally   without    operculum.    *-*>  "^  "»*'•   ?,  afferent  branchial    reweta 
*  *  i  i      /i  branchial  lamellae;  h,  one  of  the  afferent  vessels 

Marine.       Fam.     ffahotidce,     radula    i?  spaces  between  the  shell  muscles  ;  b,  foot. 
ool.(5.1.5)loo,    Fissurellidce    (Fissu- 
rella,  rad.  ool.(4.1.4)l.oo,  with  secondarily  symmetrical  shell.    Emarginula,  Scutum 


vii  MOLLUSC  A— SYSTEMATIC  REVIEW  5 

=  Par>nophonis),  Plev rotomaridce  (Pleurotomaria,  Stissurella,  Polytrcmaria),  Bellero- 
phontidce  (exclusively  fossil). 

b.  Azygobranchia.  —  One  gill,  homologous  with  the  left  gill  of  the  Zeugo- 
branchia.  Right  auricle  ending  blindly.  Heart  perforated  by  the  rectum.  Fam. 

Turbonidce,   rad.  a>0.  (5.1.5.  )0.oo ,   Trochidce  (Fig.  3)  St&matiidce,  Neritopxidce,  rad. 

ool. (2.0.2.)!.  oo  ,  marine,  Neritidce,  rad.  ool.(3.1.3.)l.»  (marine,  but  along  the  shore 
able  to  live  out  of  water),  Neritince  (marine  and  fresh-water).  The  Hydrocoenidce,  rad. 

ool.(l.l.l.)l.o),  and   Jfelicinidce,  rad.   oo  1. (4.1.4.)!. oo ,  have  no   gills  but  a  lung 

resembling  that  of  the  Pulmonata.     The  Helidnidce  are  terrestrial. 

c.  Docoglossa. — Heart  with  one  auricle,  and  not  perforated  by  the  rectum.     Left 
kidney  shifted  to  the  right  side  of  the  pericardium.      Visceral  dome  and   shell 
secondarily  symmetrical,  the  latter  usually  cup-like.     Operculum  wanting.    Marine. 


Fi<~;.  5. — Phorus  exutus  (after  Lankester).  a,  Proboscidal  snout  or  rostrum  ;  b,  tentacle  ; 
• .  eye  ;  d,  foot ;  e,  metapodium  with  operculum /. 

1.  Left  true  ctenidium  present.     Acmaeid.ee,  rad.  1.2.(1.0.1.)2.1.;  with  numerous 
accessory  gills   in   the   mantle    furrow:    Scurria ;  —  without   such   gills:    Acmaea 
(Tectura). 

2.  True  ctenidia  altogether  wanting,  accessory  gills  very  numerous  in  the  mantle 
furrow.— Fam.  Patellidce  (Fig.  4),  rad.  3.1. (2.0.2.)!. 3. 

3.  Xeither  ctenidia  nor  accessory  gills  found  (Lcpetidce),  rad.  2.0.1.0.2. 

Sub-Order  2.  Monotocardia  (Pectinibranchia). 

Heart  with  one  auricle.  A  single  true  ctenidium  feathered  on  one  side,  the  point 
not  projecting  freely  (except  in  Valvata).  Pedal  nerve  trunks  a  rare  exception,  pedal 
ganglia  the  rule.  Only  one  kidney.  Siphon  and  penis  generally  present.  Epi- 
podium  weakly  developed  or  wanting.  The  Monotocardia  are  very  numerous  and 
are  chiefly  marine. 

a.  Architaenioglossa.  — Pedal  nerve  trunks.  In  Cypraea  (and  in  other  forms  ?)  a 
rudiment  of  the  right  auricle  persists.  Fam.  Cypraeidce,  rad.  3.1.1.1.3,  Paludinidce 
(fresh-water),  Cydvphoridce,  (terrestrial,  pulmonate). 


COMPARATIVE  ANATOMY 


CHAP.  VII 


b.  Taenioglossa. — Typical  radula,  2.1.1.1.2.  Semiproboscidifera.  Fam. 
Naticidce  (Fig.  98,  p.  107),  Lamellaridce.  Rostrifera.  Fam.  Falvatidce  (fresh-water), 
Ampullaridce  (fresh  -  water),  Littorinidce,  Cyclostomidce  (terrestrial),  Planaxidce, 
Hydrobiidce  (fresh- water),  Aciculidce  (terrestrial),  Truncatellidce  (partly  terrestrial), 
Hipponycidce,  Capulidce,  Calyptraeidce,  Pseudomelanidce,  Melanidce,  Cerithiidce, 


FIG.  6.— Rostellaria  rectirostris  (after  Owen).    «,  Snout ;  b,  tentacle  ;  c,  stalked  eye  ;  d,  foot ; 
e,  inetapodium  with  operculuin  /;  h,  beak  (for  the  siphon). 

Vermetidce,  Turritellidce,  Xenophoridce  (Fig.  5),  Sir  uthiolar  idee,  Chenopidce, 
Strombidce  (Fig.  6).  Proboscidifera  holostomata.  Fam.  Scalaridce,  rad.  ooQco  ; 
Solaridce,  rad.  ooQoo  ;  Pyramidellidce,  rad.  0;  Eulimidce,  rad.  0.  Proboscidifera 
siphonostomata.  Fam.  Colombellinidce,  Tritoniidce,  Cassidiidce  (Fig.  7),  Doliidce. 


Fie.  7.— Cassis  suclosa  (after  Poli).  a,  Shell;  &,  beak;  c,  siphon;  d,  head;  g,  proboscis;  e, 
eye  ;  /,  tentacle  ;  h,  foot ;  i,  operculuin. 

Janthinidse,  rad.  ocO  oo .  Heteropoda  (marine  Taenioglossa,  with  foot  transformed 
into  a  perpendicular  rowing  fin).  Fam.  Atlantidce  (Fig.  8),  Pterotrachaeidce 
(Fig.  9). 

c.  Stenoglossa.  — Normal  rad.  1.1.1.  Rachiglossa.  Fam.  Turbinellidce, 
Fusidce,  Mitridce,  Buccinidce,  Muricidce,  Purpuridcc,  Ifaliadca,  Cancellariidce, 
Volutidce,  Olimdce,  Marginellidce,  Harpidce.  Toxiglossa.  Fam.  Plcurotomidce. 
Terebridce,  Conidce. 


FIG.  S.— Atlanta  Peronii  (after  Gegenbaur).  a,  Pharynx  ;  b,  buccal  ganglion  ;  c,  tentacle  ;  d, 
eye ;  e,  cerebral  ganglion ;  /,  aorta  cephalica  ;  [/,  pleuro-visceral  connective  ;  h,  columellar  muscle  ; 
i,  fc,  osphradiuni ;  /,  vagina  ;  m,  ctenidium  ;  n,  anus ;  o,  uterus ;  p,  nephridium ;  q,  aorta  cephalica  ; 
r,  auricle ;  s,  ventricle ;  t,  aorta  visceralis ;  u,  digestive  gland  (liver) ;  v,  ovary ;  w,  stomach ;  x, 
pedal  ganglion  ;  y,  operculum ;  2,  metapodium  ;  1,  sucker  of  the  fin-like  foot  (rudimentary  sole) ;  2, 
foot ;  3,  auditory  organ  ;  4,  oesophagus  ;  5,  snout ;  6,  salivary  gland. 

21 


FIG.  9.— Pterotrachea  (Firola)  coronata  (after Leuckart).  a,  Pharynx;  b,  proboscidal  snout ; 
c,  eye ;  d,  cerebral  ganglion ;  e,  pedal  ganglion ;  /,  pedal  artery ;  g,  intestinal  canal ;  h,  pleuro- 
visceral  connective ;  i,  parieto-visceral  ganglion  ;  t,  osphradium  ;  7,  ventricle  ;  m,  auricle ;  ?z, 
anus ;  o,  ctenidium  ;  p,  metapodium ;  q,  appendage  ;  r,  aorta  cephalica  ;  5,  nerve  running  to  the 
metapodium  ;  f,  artery ;  n,  foot ;  r,  common  pedal  artery ;  u%  cephalic  artery ;  a-,  auditory  organ  ; 
y,  buccal  ganglion. 


COMPARATIVE  ANATOMY 


CHAP. 


OHDER  2.  Pulmonata. 

The  pleuro-viseeral  connectives  are  not  crossed.  The  ctenidmm  has  disappeared 
from  the  mantle  complex  and  is  replaced  by  a  lung,  or  respiratory  vascular  network, 
on  the  inner  surface  of  the  mantle.  The  pallial  organs  lie  primitively  to  the  right, 
anteriorly  on  the  visceral  dome.  The  edge  of  the  mantle,  with  the  exception  of  a 
branchial  aperture  on  the  right,  unites  with  the  integument  of  the  neck.  In  terres- 
trial Pulmonata  the  visceral  dome  is  often 
flattened  down  and  the  shell  becomes  rudi- 
mentary (Slugs).  The  operculum  is  always 
wanting.  The  heart  has  one  auricle,  which 
almost  always  lies  anteriorly  to  the  ventricle. 
The  Pulmonata  are  hermaphrodites  with  herma- 
phrodite glands  or  ovotestes,  and  complicated 
efferent  ducts.  They  are  either  terrestrial  or 
fresh -water. 


\^~~r 


FIG.  10.  —  Amphipeplea  leuconensis 
(after  Adams),  a,  Lobe  of  the  mantle 
bent  back  over  the  shell ;  b,  portion  of 
the  shell  uncovered  ;  c.  foot. 


Sub-Order  1.  Basommatophora  (fresh-water). 

Eyes  at  the  bases  of  the  non-invaginable 
optic  tentacles.     Genital  apertures  separate,  to 

the  right  anteriorly,  the  male  in  front  of  the  female.     Fam.  Limnceidce,  (Limncea, 
Amphipeplea  [Fig.  10],  Physa  [Fig.  11],  Planorbis,  Ancylus),  Auriculidce. 


FIG.  11.— Physa  fontinalis  (after  L.  Reeve),  a,  Mantle  lobes  folded  back  over  the  shell ;  b, 
evaginated  penis. 

Eyes  at  the  tips  of  the  optic  tentacles  ;  tentacles  invaginable. 

Sub-Order  2.   Stylommatophora. 

a.  Monogonopora.—  With  a  single  genital  aperture  to  the  right.  Fam.  Helicidse 
(Helix  [Fig.  12,  A],  Avion  [Fig.  12,  D\  Bulimus}.  Testacellidse  (Daudebardia 
[Fig.  12,  E\,  Testacella  [Fig.  12,  C].  Limacidse  (Ariophanta,  Limax,  Vitrina, 
Zonitcs,  Helicarioii).  Bulimulidae  (Fig.  13),  Pupidse  (Buliminus,  Pupa,  Clausilia}, 
Succineidse. 

&.  Digonopora.  — Shell-less  snails  with  separate  male  and  female  genital  apertures, 
the  male  anterior,  the  female  at  the  posterior  end  of  the  body,  both  to  the  right. 
Pallial  complex  at  the  posterior  end  of  the  body,  lung  cavity  reduced.  Fam.  Vagi- 
nulidae  (terrestrial),  Oncidiidse  (marine  or  amphibious)  ;  respiration  partly  by  means 
of  dorsal  branchial  appendages. 


MOILUSCA—SYSTEMA  TIC  REVIEW 


Fit;.  I*.— A,  Helix  pomatia ;  B,  Daudebardia  (Helicophanta)  brevipes :  C,  Testacella  halio- 
tidea ;  A  Arion  ater  :  s,  shell,  in  D  shield  (from  Lankester). 


~^~\// 


];••.— Peltella  va.\liolum(BuUmnUd,  after  Ferussac). 


10 


COMPARATIVE  ANATOMY 


CHAP. 


ORDER  3.  Opisthobranchia. 

The  pleuro- visceral  connectives  do  not  cross.1  There  is  one  auricle  placed  behind 
the  ventricle.  A  shell  is  sometimes  present,  more  frequently  wanting.  An 
operculum  is  rarely  found.  Respiration  by  means  of  true  ctenidia,  or  of  adaptive 
gills,  or  through  the  skin.  The  visceral  dome  is  very  often  levelled  down.  Herma- 
phrodites with  ovotestes.  Marine. 

Sub-Order  1.  Tectibranchia. 

The  pallial  complex  is  to  the  right  of  the  body,  and  is  more  or  less  covered  by 
the  mantle  fold  belonging  to  that  side.  One  true  ctenidium  (viz.  that  which  was 
originally  the  right)  is  always  retained  in  the  mantle  cavity,  but  is  often  very 
incompletely  covered  by  the  mantle.  The  visceral  dome  tends  to  disappear.  A 
shell  is  always  present,  but  tends  to  become  rudimentary.  Generally  with  para- 
podia,  and  mantle  lobes  covering  the  shell. 

A.  Reptantia. 

a.  Cephalaspidse. — With  frontal  or  cephalic  disc.  Fam.  Actseonidse  (with 
operculum),  Scaphandridse,  Bullidse  (Sulla,  Acera),  Gastropteridae  (Fig.  14), 
Philinidae.  Doridiidse. 

6.  Anaspidse. — Head  without  frontal  disc  ;  four  triangular  or  ear-like  tentacles. 
Fam.  Aplysiidse  (Ajjlysia,  Dolabella,  Notarchus). 


FIG.  14.  —  Gastropteron      Meckelii,         FIG.  15.— Pleurobranchus  aurantiacus,  with  internal 

with   internal    shell    (after  Vayssiere).  shell  (after  Leuckart'S  Wandta.feln),  seen  from  the  right 

1,  Cephalic  shield  (frontal  disc) ;  2,  para-  side,     ft,  Rhinophores  ;  b,  labial  sail ;  c,  genital  aperture  ; 

podium;  3,  ctenidium,  left  almost  un-  d,  nephridial  aperture  (?);  e,  ctenidium  ;/,  anus, 
covered  by  the  rudimentary  mantle  fold  ; 
4,  flagellum  =  appendage  of  the  mantle 
fold. 

c.  Notaspidae. — Head  short,  with  or  without  tentacles.  Large  dorsal  disc 
(notseum)  in  or  on  which  a  shell  may  lie.  Fam.  Pleurobranchidse  (Pleurobranchus 
[Fig.  15],  Pleurobranchcca,  Oscainius),  Umbrellidae  (Umbrella,  Tylodina),  Peltidse. 

B.  Natantia  sive  Pteropoda.2 

These  formerly  constituted  a  separate  class  of  the  Molluscs,  but  are  now  recog- 
nised to  be  Tectibranchia  adapted  to  a  free-swimming  pelagic  life.  The  parapodia  of 
the  Tectibranchia  develop  as  fins  or  wing-like  swimming  organs. 

1  Except   in  Actceon,  which   is  streptoueurous,  and  thus  forms  a  connecting  link 
between  the   Opisthobranchia  and    Pulmonata   on   the  one  hand,    and  the  remaining 
Gastropods  on  the  other  [Bouvier  and  Pelseneer],  v.  Nat.  Sci.,  July  1893. 

2  The  classification  of  the  Opisthobranchs,  which  places  the  Pteropoda  thecosomata 
with  the  Cephalaspidse,  and  the  Pteropoda  gymnosomata  with  the  Anaspidse,  is  accepted 
on  p.  110  and  elsewhere. 


VII 


MOLLUSCA— SYSTEMATIC  REVIEW 


11 


a.  Pteropoda  thecosomata.  —  These  are  nearly  related  to  the  Cephalaspidea, 
and  possess  a  mantle,  mantle  cavity,  and  shell.  The  head  is  not  distinct,  and  has 
only  one  pair  of  tentacles.  The  fins,  at  their  anterior  edges,  are  fused  over  the  mouth; 
the  anus  lies  to  the  left.  Fam.  Limacinidse.  An  external  calcareous  shell,  with 
left-handed  or  sinistral  twist,  and  a  spiral  operculum.  Anus  to  the  right  (Linia- 
[Fig.  16],  Peraclis).  Fam.  Cavoliniidse.  External  symmetrical  shell  (Clio, 
Cavolinia).  Fam.  Cymbuliidse.  Internal  cartilaginous  shell  (Cymbulia,  Cymbuli- 
opsis,  Gleba).  The  Thecosomata  feed  chiefly  on  small  Protozoa  and  Algae. 

CL 


1 


FIG.  Iti. — Limacina  Lesueuri  (dorsal  aspect, 
after  Pelseneer).  1,  Penis  ;  2,  fin  (parapo- 
dium) ;  3,  seminal  furrow  ;  4,  mantle  process 
("balancer");  5,  visceral  dome;  6,  head  with 
two  tentacles  and  the  seminal  furrow  3. 

FIG.  17.  -Pneumoderma  (diagram  from  the 
right,  after  Pelseneer).  1,  right  evaginated 
process  bearing  hooks  (hook  sac) ;  2,  proboscis  ; 
3,  right  buccal  tentacle  ;  4,  position  of  the  right 
nuchal  tentacle  ;  5.  right  tin  (parapodium) ;  6, 
seminal  furrow  ;  7,  genital  aperture ;  8,  position 
of  the  jaw  ;  9,  ventral  proboscidal  papilla  ;  10, 
right  buccal  appendage  provided  with  suckers  ; 
11.  head  ;  12,  aperture  for  penis ;  13,  right  an- 
terior pedal  lobe  ;  14,  anus  ;  15,  posterior  pedal 
lobe ;  16,  ctenidium  ;  17,  posterior  adaptive 
gill ;  d,  v,  o,  p  denote  dorsal,  ventral,  anterior, 
and  posterior. 


17 


b.  Pteropoda  gymnosomata.  —  These  are  nearly  related  to  the  Anaspidae. 
They  have  no  mantle,  mantle  cavity,  nor  shell.  The  head  is  distinct,  and  carries 
two  pairs  of  tentacles.  The  fins  are  separate  ;  the  anus  lies  to  the  right.  Fam. 
Pneumodermatidse.  One  ctenidium  to  the  right  (Dcxiobranch<ea,  Spongiobranchcea, 
Pif  nrnod' rma  [Fig.  17]).  In  the  last  two  genera  there  is  an  adaptive  posterior  gill 
as  well.  Fam.  Clionopsidae  and  Notobranchseidse.  Xo  ctenidium,  but  a  posterior 
adaptive  gill.  Fam.  Clionidse.  Xeither  ctenidium  nor  adaptive  gill.  All  Gymnoso- 
mata are  carnivorous,  feeding  principally  on  Thecosomata. 

Sub-Order  2.  Ascoglossa. 

This  sub-order  is  characterised  by  the  fact  that  the  worn-out  teeth  of  the  long 
narrow  radula,  which  consists  of  a  single  row  of  dental  plates,  are  preserved  in  a  sac 


12 


COMPARATIVE  ANATOMY 


CHAP. 


at  its  anterior  end.  No  jaws.  The  anus  almost  always  dorsal.  Except  in  the 
Steganobranchia,  the  disappearance  of  the  mantle  and  its  cavity  is  accompanied  by 
the  disappearance  of  the  single  ctenidium  of  the  Tectibranchia. 

Section  1.  Steganobranchia. — With  mantle,  cavity,  and  ctenidium  to  the  right ; 
with  a  shell  and  parapodia.  Fam.  Oxynoidea  (Oxynoe,  Lobiger}. 

Section  2.  Cirrobranchia. — Leaf-  or  club-shaped  processes  found  laterally  on  the 
back.  Fam.  Hermseidse,  Phyllobranchidse. 

Section  3.  Pterobranchia. — The  sides  of  the  body  produced  into  lobes,  in 
which  the  branches  of  the  glands  of  the  mid -gut  spread  out.  Fam.  Elysiadse, 
Placobranchidse. 

Section  4.  Abranchia. — Neither  ctenidium,  nor  dorsal  appendages,  nor  leaf-like 
lateral  expansions  of  the  body.  Respiration  through  the  skin.  The  body  is  almost 
like  that  of  a  Planarian.  Fam.  Limapontiidae. 

Sub-Order  3.  Nudibranchia. 
Without  mantle  fold,  shell,  or  ctenidium.     Jaws  almost  always  found.     Radula 


FIG.  IS.— Aeolis  rufibranchialis  (right  aspect,  after  Alder  and  Hancock),  a,  Eye ;  6,  oral 
tentacle ;  c,  cephalic  tentacle ;  d,  anus  ;  e,  genital  aperture  ;  /,  dorsal  respiratory  appendages 
(cerata). 

generally  well  developed,   with  teeth  which  fall  off  and  are  lost*.     Adaptive  gills 
very  variously  developed,  but  occasionally  wanting. 


V 

11  12 

FIG.  19.  —  Phyllirhoe  bucephalum  (lateral  aspect,  after  Souleyet,  modified).  1,  Tentacle; 
2,  cerebral  ganglion ;  3,  stomach ;  4  and  12,  intestinal  cseca  (forming  the"  digestive  gland)  ; 
5  ventricle  ;  6,  auricle ;  7,  pericardial  aperture  of  the  kidney ;  8  kidney  ;  9,  external  aperture  of 
the  same  (on  the  right  side)  ;  10,  anus  (on  the  right  side) ;  11,  hermaphrodite  glands,  the  ducts  not 
drawn  ;  13,  genital  apertures  ;  14,  buccal  ganglion  ;  15,  salivary  glands. 

Section  1.  Holohepatica. — One  large  unbranched  hepatic  gland  (liver).     Fam. 


MOLLUSCA— SYSTEMATIC  REVIEW 


13 


Phyllidiidse.  Numerous  branchial  lamellae  lie  in  a  groove  which  encircles  the  body. 
Xo  jaws  and  no  radula.  Pharynx  transformed  for  sucking.  Fam.  Doridopsidse. 
Without  jaws  or  radula  ;  pharynx  adapted  for  sucking.  Branchial  rosette  round 
the  dorsal  anus.  Dorididaa  crypto- 
branchiatse.  The  branchial  rosette 
round  the  dorsal  anus  can  be  with- 
drawn into  a  cavity.  (Bathydoris. 
Archidoris,  Discodoris,  Diaulula,  Kent- 
roduris,  Platydaris,  Chromodoris,  etc.) 
Dorididae  phanerobranchiatse.  Bran- 
chial rosette  not  retractile.  (Gonio- 
doris,  Polycera,  Aca  nthodoris,  Idalia, 
Aiicula,  Euplocamus,  Triopa,  etc.) 

Section  2.  Cladohepatica. — Diges- 
tive glands  inore  or  less  broken  up  into 
separate  branched  canals  spreading 
widely  in  the  body.  Variously  formed 
dorsal  appendages  chiefly  connected 
with  respiration.  Anus  usually  to  the 
right.  Fam.  Aeolidiadse  (AcvUdiu 
[Fig.  18],  Bcryhia,  Tcrgip:s,  Galcuia, 
Coryptclta,  JRizzolia,  Faccllina,  Flabel- 
lina,  Fiona,  Glaacuy,  Janus,  Nero). 
Fain.  Tethymelibidse,  without  radula 
(Tethys,  Metibc).  Fams.  Lomanotidse, 
Dotonidse,  Dendronotidse,  Bornellidse, 
Scyllaeidse,  Phyllirhoidse  (Fig.  19  ; 
marine  free  -  swimming  animals  with 
narrow  laterally  -  compressed  body, 


without  foot  or  respiratory  append- 
ages). Fam.  Pleurophyllidiidae.  Xu- 
merous  branchial  lamella  arranged  in 
a  single  row  on  each  side  along  a  tentacle  shield ;  7,  foot, 
furrow  between  the  dorsal  shield  and  the  foot  (Fig.  20). 
Tritoniadse  (Tritonia, 


2-JI 


FIG.  -20.— Pleurophyllidia  lineata  (from  below, 
after  Souleyet).  1,  Genital  apertures ;  2,  branchial 
leaflets  ;  3,  anus ;  4,  pedal  gland ;  5,  mouth ;  6, 


Fam.  Pleuroleuridse, 


CLASS  III.  Scaphopoda. 

The  body  is  symmetrical,  and  elongated  dorso-ventrally.  The  mantle  is  a  tubular 
sac  with  a  narrow  dorsal  and  a  wider  ventral  aperture.  Posteriorly,  the  mantle 
cavity  reaches  to  the  apical  (dorsal)  aperture.  The  shell  forms  a  high  tubular  cone, 
and,  like  the  mantle,  has  a  small  apical  and  a  larger  ventral  aperture.  Cteuidia  are 
wanting  ;  the  kidneys  are  paired.  The  vascular  part  of  the  circulator}'  system  is 
reduced  to  a  ventricle  ;  without  auricles.  The  sexes  are  separate.  There  are  no 
special  ducts  for  the  sexual  products,  which  are  ejected  through  the  right 
kidney.  The  mouth  lies  at  the  end  of  a  barrel-shaped  snout,  and  is  surrounded  by 
a  circle  of  leaf-like  appendages.  At  the  base  of  this  snout  there  are  numerous 
filamentous  appendages,  which  can  be  protruded  through  the  lower  aperture  of  the 
shell  and  mantle.  The  foot  is  ventrally  elongated.  A  radula  is  found.  Limicolous. 
Marine.  Fam.  Dentalium  (Fig.  101,  p.  113).  The  foot  is  relatively  short ;  it  is  shaped 
somewhat  like  an  acorn,  with  a  conical  central  portion  and  two  lateral  lobes. 
Siphonodentalium.  The  foot  is  long  and  worm-like,  but  broadens  out  at  the  end 
into  a  disc  edged  with  papilke. 


14 


COMPARATIVE  ANATOMY 


CHAP. 


CLASS  IV.    Lamellibranchia  (Pelecypoda,  Bivalva,  Acephala,  Aglossa).    Mussels. 

The  body  is  symmetrical  and  more  or  less  transversely  flattened  ;  it  has  two 
large  lateral  leaf-like  mantle  lobes,  enclosing  a  spacious  mantle  cavity  large  enough 
to  contain  the  foot,  which  is  usually  hatchet-  or  wedge-shaped.  The  shell  consists 
of  two  lateral  valves  connected  together  only  at  the  dorsal  hinge.  It  is  closed  by  means 
of  two  adductor  muscles  passing  transversely  from  one  valve  to  the  other  (Dimyaria) ; 
occasionally  the  anterior  adductor  degenerates  and  only  one  remains  (Monomyaria). 
On  each  side  in  the  mantle  cavity  there  is  a  ctenidium.  There  are  no  jaws,  no 
pharynx,  no  radula,  no  tentacles,  and  no  distinct  head.  The  kidneys  and  genital 
organs  are  paired,  and  the  latter  either  have  separate  ducts  or  eject  their  products 
through  the  nephridia.  The  heart  has  two  auricles.  At  each  side  of  the  mouth 
there  are  two  oral  lobes.  Either  sexually  separate  or  hermaphrodite.  They  live 
in  salt  or  fresh  water,  and  are  either  limicolous  or  attached. 

ORDER  1.  Protobranchia. 

The  gills  with  two  rows  of  leaflets,  in  the  posterior  part  of  the  mantle  cavity  ; 
they  correspond  in  all  respects  with  the  ctenidia  of  the  Zeugobranchia,  their  ends 


FIG.  21.— Nucula  nucleus,  left  aspect  after  removal  of  the  left  valve  and  mantle '.(after 
Pelseneer).  a,  Anterior  adductor ;  b,  anterior  retractor  of  the  foot ;  c,  elevator  of  the  footed,  genital 
mass  ;  e,  hypobranchial  gland  ;  /,  posterior  retractor  of  the  foot ;  g,  posterior  adductor  ;  /i,  cteni- 
dium ;  i,  mantle  cavity  ;  k,  creeping  sole  of  the  foot  (I) ;  m,  oral  lobes  (labial  palps)  with  posterior 
appendages  n  and  o. 

project  freely  backward  into  the  cavity.  The  foot  has  a  sole  for  creeping.  The 
pleural  ganglion  can  be  distinguished  from  the  cerebral.  Fam.  Nuculidse  (Nucula 
[Fig.  21],  Leda,  Yoldia,  Solenomyidoe). 


ORDER  2.  Filibranchia. 

The  branchial  leaflets  of  the  ctenidium  have  become  lengthened  out  into  long 
filaments  hanging  far  down  into  the  mantle  cavity.  Each  is  in  two  parts,  the  proxi- 
mal descending  and  the  distal  ascending  (cf.  Fig.  88  B).  Fam.  Anomiidse:  mantle  open 


VII 


MOLLUSCA— SYSTEMATIC  REVIEW 


15 


without  siphons  ;  Monomyarian.  Foot  small ;  body  and  shell  asymmetrical.  Attached. 
Branchial  filaments  entirely  free  (Anomia,  Placuna}.  Fam.  Arcidae  :  the  branchial 
filaments  of  eacli  row  connected  by  ciliated  discs  ;  Dimyarian.  Xo  siphons.  Foot 
large  (Arcfi,  Pedunculus).  Fam.  Trigoniidse  :  ctenidia  like  those  of  the  Arcidce; 


FIG.  22.— Mytilus  edulis  (after  Meyer  and  Mobius),  left  aspect,  with  extended  foot  attaching  a 
byssus  thread  ;  d,  byssus  threads  ;  a,  exhalent  aperture  (anal  siphon) ;  b,  fringed  edge  of  the  in- 
halent  mantle  aperture  ;  c,  object  to  which  the  animal  is  attached. 

Dimyarian.  No  siphons  (Trigonia}.  Fam.  Mytilidse  (excluding  Aviculidce) :  cten- 
idia connected  by  means  of  non  -  vascularised  trabeculfe.  The  anterior  adductor 
is  smaller  than  the  posterior  (Heterornyarian).  "With  siphons.  Foot  long.  (Mytilus 
[Fig.  22],  Mod  I' ilfi,  Lithodomus  [boring  mussel],  Modiolaria). 

ORDER  3.  Pseudolamellibranchia. 

The  consecutive  ctenidial  filaments  of  each  row  are  connected  by  means  either 
of  ciliated  discs  or  of  vascularised  trabeculee  ;   and  the  ascending  and  descending 


16  COMPARATIVE  ANATOMY  CHAP. 

portions  of  each  filament  are  similarly  united  (cf.  Fig.  88.  p.  92).     Fam.  Pectinidse: 


FIG.  23. — Pecten  Jacoboeus.  ventral  aspect,  shell  opened.  The  mantle  cleft  is  seen  between 
the  fringes  of  the  mantle,  which  are  beset  with  numerous  tentacles  and  eyes  (after  Leuckart  and 
Nitsche,  Zool.  Wandtafeln). 


Pi    PV 


FIG.  24.— Anatomy  of  the  Oyster  (Ostrea  edulis),  right  aspect  (after  Mobius,  Leuckart,  and 
Nitsche,  Zool.  Wandtafdii).  br,  Gills  ;  PH,  posterior  mantle  nerve  ;  x,  xi,  apertures  of  the  cavities 
between  the  fused  plates  of  the  two  left  gills  ;  M,  large  adductor  muscle  ;  a,  anus  ;  Mm,  posterior 
portion  of  the  adductor  muscle  ;  Pa,  mantle  ;  P,  pericardium  ;  V,  heart ;  go,  gonad  (hermaphrodite 
gland) ;  d,  intestinal  canal ;  I,  digestive  gland  (liver) ;  o,  mouth  ;  os,  osj,  oral  lobes  (labial  palps)'of 
the  left  side  ;  Cg,  cerebral  ganglion  ;  n,  kidney ;  bn,  branchial  nerve  ;  Vg,  visceral  ganglion  ;  P1;  abdo- 
minal process  ;  Pwj,  nerve  of  the  pallial  edge  ;  m,  stomach,  with  the  apertures  of  the  digestive  gland. 

Monomyarian  with  mantle  entirely  open,  and  eyes  at  its  edge.     Without  siphons. 
Foot  small   and   linguiform.     Valves  of  the  shell  equal  or  unequal.     Capable  of 


MOLLUSC  A— SYSTEMATIC  REVIEW 


swimming.  (Pecten  [Fig.  23],  Chlamys).  Fam.  Aviculidae :  Monomyarian  or  Hetero- 
myarian  without  siphons.  Valves  equal  or  unequal  (Avicula  [Meleagrina],  Malleiis, 
Vulsclla,  Perna,  Inoceramus,  Pinna,  Meleagrina  margaritifera,  pearl  mussel). 
Fam.  Ostreidse  :  Monomyarian  without  foot,  with  completely  open  mantle,  without 
siphons.  Valves  unequal,  the  left  valve  attached  to  the  substratum.  (Ostrca : 
oyster  [Fig.  24]). 

ORDER  4.  Eulamellibranchia. 

The  gills  no  longer  consist  of  distinct  filaments.  On  the  contrary,  the  filaments 
in  each  row  and  the  two  parts  of  each  filament  are  so  connected  by  means  of 
vascularised  trabeculre  or  sutures  as  to  form  together  a  lamella  or  trellis -work. 
There  are,  on  either  side,  two  such  branchial  lamellae  (hence  the  name  of  Lamclli- 
branchia],  which  in  fact  correspond  with  the  two  rows  of  leaflets  of  the  typical 
ctenidium.  This  order  includes  the  majority  of  the  Lamellibranchia. 

Sub-Order  1.  Submytilacea. 

Branchial  lainelLe  smooth.  The  mantle  edges  usually  grown  together  only 
between  the  inhalent  and  the  exhalent  apertures.  Dimyarian.  Fam.  Carditidse  : 
with  open  mantle  and  large  foot  (Cardita,  Venericardia}.  Fam.  Lucinidae  : 
with  simple,  and  as  a  rule  single,  siphonal  aperture.  Foot  often  vermiform.  Fam. 


no 


FIG.  25.— Anatomy  of  Unio  (Margaritana)  margaritiferus,  left  aspect  (after  Leuckart  and 
Nitsche).  o,  Mouth ;  eg,  cerebral  ganglion ;  MI,  anterior  adductor  muscle ;  ce,  oesophagus  ;  Z, 
digestive  gland  (liver) ;  HO.  nephridial  aperture  ;  to,  aperture  of  the  digestive  gland  in  the  stomach 
m  ;  Aa,  anterior  aorta  ;  H,  nephridium,  the  outlines  given  in  dotted  lines  ;  V,  heart ;  r,  proctodseum ; 
Ap,  posterior  aorta  ;  Af2,  posterior  adductor  ;  a,  anus  ;  Vg,  visceral  ganglion ;  Br,  gill ;  Bk,  mantle 
cavity ;  go,  gonad  and  ducts  goi ;  Pg,  pedal  ganglion  ;  p,  foot.  The  arrows  mark  the  direction 
of  the  inhalent  and  exhalent  streams  of  water. 

Erycinidae  :  mantle  closed  except  at  the  two  siphonal  and  the  pedal  apertures. 
Foot  long.  (Erycina,  Kellya,  Lascea,  Lepton,  Galeomma. )  Fam.  Crassatellidae  : 
mantle  open  without  siphons.  Foot  moderately  developed.  Fam.  Cyrenidse  :  mantle 
open,  two  siphons.  Foot  large.  In  fresh  or  brackish  water.  "  (Cyrena,  Corbicula, 
SpJwerium,  Pisidium,  Galatea.}  Fam.  Dreissensiidse  (fluvial).  Fam.  Unionidse:  fresh- 
water ;  foot  large,  hatchet-  or  wedge-shaped,  two  simple  siphonal  apertures  or  clefts, 
mantle  open  (Unio  [Fig.  25],  Painter's  Mussel  ;  Anodonta,  pond  Mussel ;  Mutela}. 
VOL.  II  C 


18  COMPARATIVE  ANATOMY  CHAP. 

Sub-Order  2.  Tellinacese. 

Dimyarian  with  completely  separate  siphons.  Foot  large.  Gills  smooth.  Fam. 
Tellinidse  ( Tellina).  Fam.  Donacidse  (Donax),  Mactridse  (Mactra). 

Sub-Order  3.  Veneracea. 

Dimyarian  with  somewhat  folded  branchial  lamellae.  Siphons  separate,  and  foot 
large.  Fam.  Veneridae  (Venus,  Meretrix  [Cytherea],  Tapes).  Fam.  Petricolidse : 
boring  muscles. 

Sub-Order  4.  Cardiacea. 

Dimyarian  or  Monomyarian.  Branchial  lamelke  much  folded.  Mantle  closed 
except  at  the  two  siphonal  and  one  pedal  apertures.  Fam.  Cardiidse  :  Dimyarian. 

710, 


FIG.  26.— Anatomy  of  Cardium  tuberculatum,  left  aspect  (after  Grobben,  Leuckart,  and 
Nitsche,  Zool.  Wandtafeln).  p,  Foot ;  go,  gonad  ;  S,  shell ;  Pa,  mantle ;  os,  labial  palps  ;  o,  mouth; 
MI,  anterior  adductor  muscle ;  ce,  oesophagus ;  m,  stomach  ;  I,  digestive  gland ;  d,  intestinal 
canal ;  go2,  genital  aperture  ;  noi,  pericardial  aperture  of  the  kidney  ;  V,  ventricle  ;  At,  auricle  ;  P, 
pericardium  ;  no,  aperture  of  the  kidney  in  the  mantle  cavity  ;  n,  kidney ;  JVf2,  posterior  adductor ; 
Bl,  point  of  concrescence  of  the  right  and  left  ctenidia  behind  the  foot ;  a,  anus  ;  Ak,  anal  chamber 
of  the  mantle  cavity  j  with  anal  siphon  As ;  Bk,  branchial  chamber  of  the  same  cavity  with 
branchial  siphon  Bs  ;  Br,  ctenidium. 

(Cardium[~Fig.  26].)  Fam.  Chamidae :  Dimyarian.  Valves  of  shell  unequal.  (Chama, 
Diceras,  Requienia.}  To  these  the  fossil  forms  Monopleuridce,  Caprinidce,  Hip- 
Eadiolitidce.  Fam.  Tridacnidse  :  Monomyarian.  (Tridacna,  Hippopus.) 


Sub-Order  5.  Myacea. 

Dimyarian  with  folded  branchial  lamellae.  Tendency  towards  concrescence  of  the 
edges  of  the  mantle  folds.  Siphons  very  long  and  foot  large.  Fam.  Psammobiidae  : 
pedal  cleft  of  the  mantle  still  very  large  (Psammobia).  Fam.  Mesodesmatidse, 


MOLLUSCA— SYSTEMATIC 


UNIVI 

CtU'r.. 


Lutrariidae,  Myidae  (Afy",  Corbula}.  Fam.  Glycymeridae  (Glycymeris,  Saxicava 
[boring  mussels]).  Solenidse :  shell  with  anterior  and  posterior  cleft  ;  foot  very 
large  (Solawcui-tus,  CultcUus,  Ensis,  Solen). 

Sub-Order  6.  Pholadacea. 

Dimyarian  with  closed  mantle  and  well-developed  siphons.     Foot  varies,  and  is 
M,         Ifz?  ffa  jft 

j/^^Li  / 


FIG.  27.— Anatomy  of  Pholadidea,  left  aspect  (after  Egger).  Lettering  as  before.  In  addition, 
AJXJ,  Npp,  anterior  and  posterior  nerves  of  the  mantle  edge  ;  mo,  anterior  aperture  of  mantle  ;  Ks, 
sac  of  the  crystalline  stylet  ;  Ki-,  branchial  vein ;  ol,  anterior  upper  mantle  lobe  ;  Rpp,  posterior 
retractor  of  the  foot ;  Ss,  partition  between  the  two  siphons ;  Ms,  accessory  adductor ;  mb,  intestinal 
caecum  ;  x,  pericardial  section  of  the  kidney,  which  opens  into  the  pericardium  by  means  of  the 
renal  funnel  at  »/. 


M, 


HFC 


FIG.  -23.—  Anatomy  of  Jouannetia  Cumingii,  left  aspect  (after  Egger).    Lettering  as  in 

last  figure. 

sometimes  rudimentary.     Shell  open,  often  having  accessory  pieces   added   to   it. 
Fam.  Pholadidse:  boriag  mussels  (Pholas,  Pholadidea  [Fig.  27],  Jouannetia  [Fig.  28], 


20 


COMPARATIVE  ANATOMY 


CHAP. 


FIG.  29.— Teredo  Navalis  in  its  boring, 
ventral  aspect  (after  Meyer  and  Motoius). 
The  centre  is  omitted,  the  calcareous  tube 
is  for  the  most  part  uninjured. 


FIG.  30.— Shell  of  Aspergillum  (Bre- 
chites)  vaglniferum,  dorsal  view,  a,  An- 
terior; p,  posterior;  d,  right;  s,  left;  1, 
siphonal  aperture  of  the  pseudoconch ;  2, 
pseudoconch  (calcareous  tube) ;  3,  true  shell 
embedded  in  the  pseudoconch ;  4,  anterior 
aperture  of  the  pseudoconch. 


VII 


MOLLUSCA— SYSTEMATIC  REVIEW 


21 


Xylophaga).     Fam.  Teredinidae  :  boring  mussels  ( Teredo  [Fig.  29]).     Fam.  Clava- 
gellidse  (Clavagella,  Brechites  [Aspergillum,  Fig.  30]). 

Sub-Order  7.  Anatinacea. 

Mantle  to  a  great  extent  closed.     With  siphons  and   foot.     Hermaphrodite. 
Fam.  Pandoridse,  Lyonsiidse,  Anatinidae  (Anatina,  Thrada). 

ORDER  5.  Septibranchia. 
The  ctenidium  on  each  side  is  transformed  into  a  muscular  septum  pierced  by 

A 

d 


FIG.  31.— Soft  body  of  Silenia  Sarsii  (Cuspidaria),  after  Pelseneer.  A,  Left  aspect  after 
removal  of  the  mantle  ;  B,  ventral  aspect  after  removal  of  most  of  the  mantle  ;  a,  p,  anterior  and 
posterior  ;  d,  v,  dorsal  and  ventral ;  d,  s,  right  and  left ;  1,  anterior  adductor  ;  2,  mouth ;  3,  anterior 
group  of  branchial  slits;  4,  hepatic  mass  ;  5,  branchial  septum  ;  6,  posterior  group  of  branchial  slits  ; 
7,  posterior  adductor ;  8,  anal  siphon ;  9,  siphonal  tentacles  ;  10,  valve  of  the  branchial  or  inhalent 
aperture  ;  11,  point  where  the  free  mantle  edges  limiting  the  pedal  aperture  fuse ;  12,  median  group 
of  branchial  slits  ;  13,  free  edges  of  mantle  ;  14,  foot ;  15,  posterior  labial  palps  ;  16,  anterior  labial 
palp. 

slits,  which  divides  the  mantle  cavity  into  two  chambers,  one  lying  above  the  other. 
Hermaphrodite.     Fam.  Poromyidae,  Cuspidaridse  (Fig.  31  A  and  B). 


CLASS  V.— Cephalopoda  (Cuttlefish). 

Body  symmetrical  with  high  visceral  dome.  The  mouth  is  surrounded  by 
tentacles  or  prehensile  arms,  which  may  be  considered  as  portions  of  the  foot 
developed  round  the  mouth.  Another  portion  of  the  foot  forms  the  siphon.  In 


22 


COMPARATIVE  ANATOMY 


CHAP. 


the  posterior  mantle  cavity  there  are  two  or  four  ctenidia.  The  heart  has  two 
or  four  auricles,  and  there  are  two  or  four  kidneys.  Gonad  unpaired,  with  single 
or  paired  ducts.  The  sensory  organs  are  highly  developed,  especially  the  eyes, 
which  lie  anteriorly  and  laterally  on  the  "head"  (Kopffuss).  The  jaws  and 
radula  are  powerful.  There  is  sometimes  a  shell,  either  external  or  internal.  An 
ink-bag  is  generally  present.  The  Cephalopoda  are  large,  highly-developed  marine 
carnivora.  Dioecious. 

ORDER  1.  Tetrabranchia. 

An  external  chambered  shell,  the  animal  inhabiting  the  last  (and  largest) 
chamber.  It  is  symmetrical,  and  exogastrically  coiled.  The  mouth  is  surrounded 
by  numerous  tentacles  without  suckers,  which  rise  from  large  lobes  and  can  be 


FIG.  32.— Nautilus  Pompilius,  after  Owen.  Median  section  of  shell,  a,  Cephalic  hood; 
b,  tentacles ;  c,  infundibulum  (siphon) ;  d,  eye  ;  e.  projection  caused  by  nidaniental  gland ;  /,  point 
of  attachment  of  the  adductor  muscle ;  g,  upper  portion  of  the  visceral  dome ;  h,  last  (inhabited) 
chamber  of  the  shell ;  i,  anterior  lobe  of  the  mantle  ;  t,  last  chamber  but  one ;  I,  siphuncle. 

retracted  into  special  sheaths.  There  are  four  ctenidia,  four  auricles,  and  four 
kidneys.  The  siphon  consists  of  two  lateral  lobes  distinct  from  one  another,  which 
by  the  apposition  of  their  free  edges  form  a  tube.  Without  ink-bag.  The  eyes  are 
simple  pits.  The  only  living  form  is  the  Nautilus,  radula  2.2.1.2.2  (Fig.  32). 
The  two  large  divisions  of  this  order,  Nautiloidea  and  Ammonitidea, 1  occur  as 
fossils. 

1  The  Ammonitidea,  owing  to  the  uncertainty  concerning  their  anatomy,  are  by 
many  authorities  arranged  in  a  separate  order,  "Arnmonea,"  and  placed  between  the 
other  two. 


VII 


MOLL USGA  —SYSTEMATIC  REVIEW 


23 


FIG.  33.—  Spirula  prototypes,  right  aspect  (after  Chun  and  Owen), 
from  Leuckart  and  Nitsche,  ZooL  Wandtafeln.  Both  portions  of  the 
shell  are  visible,  the  inner  portion  seen  through  the  mantle.  The  eye 
should  be  placed  more  anteriorly  on  the  "  head"  (Koptfuss). 


FIG.  34.— Loligo  vulgaris  (after  D'Orbigny).  A,  Dorsal  (physiologically  ventral)  view;  B, 
anterior  (physiologically  dorsal)  view.  Of  the  five  pairs  of  anus,  the  fourth  are  seen  to  be 
developed  as  long  prehensile  tentacles  ;  the  eyes,  the  edge  of  the  mantle,  the  fins,  and  the 
chromatophores  in  the  skin  are  depicted. 


24 


COMPARATIVE  ANATOMY 


CHAP. 


ORDER  2.  Dibranchia. 

The  shell  is  either  internal,  rudimentary,  or  altogether  wanting.  When 
present  it  is  endogastrically  coiled.  There  are  two  ctenidia,  two  auricles,  and  two 
kidneys.  The  mouth  is  surrounded  by  eight  or  ten  sucker-bearing  prehensile  arms. 
The  free  edges  of  the  two  lobes  which  form  the  siphon  have  grown  together.  The 
eyes  are  vesicular.  An  ink-bag  is  present. 


Sub-Order  1.  Decapoda. 

Shell  internal  and  often  rudimentary.  There  are  ten  arms,  the  fourth  pair  being 
developed  into  long  prehensile  tentacles,  which  can  be  withdrawn  into  special  cephalic 
cavities.  The  Decapoda  are  good  swimmers  ;  their  bodies  are  elongated  dorso- 
ventrally,  and  provided  with  lateral  fins.  The  oviduct  is  unpaired.  Fam.  Spirulidse  : 
internal  shell  spirally  (endogastrically)  coiled.  Spirula  (Fig.  33).  Fam.  Belem- 
nitidse  :  fossil  forms  with  internal  chambered  shell,  usually  long  and  straight 
(Belemnites,  Spirulirostra,  Belemnoteuihis).  Fam.  Oigopsidse  ( Ommastrephes,  radula 
3.1.3,  Loligopsis,  Crancliia,  Chiroteuthis,  Owenia,  ThysanoteutMs,  Onychoteuthis, 
Ommatostrephes).  Fam.  Myopsidse  (Rossia,  Sepiola,  Sepiadarium,  Idiosepion,  Loligo 
[Fig.  34],  SepioteutMs,  Beloscpia  [fossil],  Sepia,  radula  3.1.3). 


Sub-Order  2.  Octopoda. 

Without  shell  or  "guard  "  (rostrum)  ;  eight  arms  ;  without  specialised  prehensile 
tentacles.     Body  thick,  generally  without  fins,   and  little  adapted  for  swimming. 


FIG.  35.— Female  Argonauta,  in  the  swimming  position,  right  aspect  (after  Lacaze-Duthiers). 
1,  Uncovered  part  of  the  shell ;  2,  the  right  arm  of  the  first  (anterior)  pair,  with  its  lobe-like  expan- 
sion (sail)  3,  covering  a  large  part  of  the  shell ;  4,  fourth  arm ;  5,  third  arm ;  6,  siphon ;  7,  eye ; 
8,  jaw;  9,  second  arm.  The  second,  third,  and  fourth  arms  are  stretched  backwards  inside  the 
shell. 

Oviducts  paired.  Fam.  Cirrhoteuthidse  :  with  fins.  Fam.  Philonexidse  :  Argonauta 
(Figs.  35,  36,  and  200,  p.  243).  Female  with  external  unchambered  shell.  Philonexis, 
Tremoctopus.  Fam.  Octopodidse  (Octopus,  radula  1.3.1.  [Fig.  37],  Eledone), 


MOLLUSC  A—  SYSTEMATIC  REVIEW 


25 


FIG.  36.— Female  of  Argonauta  Argo  (after  Verany).  Second,  third,  and  fourth  pairs  of  arms 
stretched  downwards,  a,  Siphon ;  b,  eye ;  c,  first  pair  of  arms,  covering  with  its  sail  d  nearly  the 
whole  shell  e. 


— 


FIG.  37.— Octopus  vulgaris,  after  Merculiano  (in  "A< 
swimming  position ;  below,  quiescent,  watching  for  prey. 


itanum").     Above,  in 


26 


COMPARATIVE  ANATOMY 


CHAP. 


I.  Organisation  of  the  Primitive  Mollusc. 

The  hypothetical  primitive  Mollusc,  reconstructed  from  the  results 
of  morphological  research,  may  be  described  as  follows  : — 

The  body  is  bilaterally  symmetrical  and  dorsally  arched;  its 
anterior  end  carries  the  mouth,  eyes,  and 
tentacles,  forming  a  distinct  head.  The 
ventral  side  forms  a  powerful  muscular 
foot,  distinct  from  the  rest  of  the  body, 
with  a  flat  sole  for  creeping. 

The  soft  integument  of  the  arched 
dorsal  side  forms  a  fold,  which  hangs 
down  all  round  the  body,  and  is  called 
the  mantle  or  pallium.  The  mantle 
encloses  a  circular  cavity,  the  mantle- 


renal  aperture ;  mJt, 
cteniclium ;  /,  foot. 


lantle  cavity;  vt, 


FIG.  38.  —  Hypothetical  Primitive 
Mollusc,  diagrammatic,  left  aspect,  o, 
Mouth  ;  fc,  head ;  sm,  shell  muscle ;  oso, 
upper  aperture  of  the  shell;  a,  anus;  n,  or  palllal  Cavity,  which  Surrounds  the 

body,  and  communicates  freety  with  the 
surrounding  medium  between  the  free 
edge  of  the  mantle  and  the  foot.  The  dorsal  integument  of  the  body 
and  of  the  mantle  secretes  a  closely-applied  shell,  which  consists  of  a 
chitinous  matrix  (conchyolin)  in- 
ter-stratified with  deposits  of  car- 
bonate of  lime.  This  shell  repeats 
the  form  of  the  dorsal  surface,  and 
is  thus  bilaterally  symmetrical  and 
arched.  Such  a  shell  detached  and 
turned  over  would  resemble  a  cup 
or  plate.  Since  the  dorsal  shell 
covers  the  whole,  or  at  any  rate 
the  greater  part  of  the  body,  it 
forms  a  protection  for  it  and  at 
the  same  time  plays  the  part  of  a 
skeleton,  to  which  the  muscles  run- 
ning more  or  less  dorso-ventrally 
into  the  foot  and  head,  can  be 
firmly  attached. 

The   mantle  is  of  special   im- 
portance as  a  protective  structure.      Fl(;.  39. -Hypothetical  Primitive  Mollusc, 

Apart    from   the    fact    that  its  edge    from  above,     o,  Mouth ;  ulc,  «?/>/,  ulp,  primitive 

secretes  the  greater  part  of  the  shell 
substance,  and  in  this  way  adds  to 
the  shell  as  the  animal  grows,  it 
covers  the  delicate  gills,  which 
thus  also  share  the 
afforded  bv  the  shell. 


left  cerebral  pleural  and  pedal  ganglia ;  iilpa, 
urpa,  primitive  left  and  right  parietal  ganglia ; 
nla,  primitive  left  auricle  ;  iios,  uros,  primitive 
left  and  right  osphradia  (Spengel's  organ) ;  itlct, 
urct,  primitive  left  and  right  ctenidia  (gills) ;  nib, 
base  of  the  mantle  ;  mr,  edge  of  the  mantle ;  m, 
protection  mantle  cavity ;  r,  visceral  ganglion ;  re,  ventricle ; 

Analogous 


arrangements  are  to  be  found  in  other  divisions  of  the  animal  kingdom, 


vii  THE  HYPOTHETICAL  PRIMITIVE  MOLLUSC  27 

the  dorsal  fold  or  carapace  which,  in  the  higher  Crustacea,  covers 
the  branchial  cavity,  and  the  operculum  of  Fishes.  The  relations 
existing  between  the  branchiae,  the  mantle,  and  the  shell  in  the 
Mollusca  are  of  the  highest  importance ;  these  organs  should  always 
be  regarded  as  essentially  interdependent  structures. 

The  branchiae  lying  in  the  mantle  cavity  are  paired  and  symme- 
trical. It  may  be  left  an  open  question  whether  the  primitive  Mollusc 
possessed  more  than  one  pair  of  gills.  If  only  one,  we  must  suppose 
that  one  gill  lay  on  each  side  of  the  mantle  cavity  posteriorly ;  if  more 
than  one.  that  there  was  a  row  of  branchiae  on  each  side. 

Each  gill  is  feather-like,  with  a  shaft  and  two  rows  of  very 
numerous  leaflets.  The  shaft  stands  out  freely  from  the  body  in  the 
mantle  cavity.  Close  to  the  base  of  each  gill,  a  sensory  organ,  con- 
sidered to  be  olfactory,  and  called  the  osphradium,  is  found.  Such 
a  gill  with  an  osphradium  at  its  base  has  a  very  definite  morphological 
value ;  in  order  to  distinguish  it  from  analogous  though  not  homologous 
respiratory  organs  found  in  certain  Mollusca,  it  has  been  named  a 
ctenidium. 

The  head  is  provided  with  one  pair  of  tentacles  and  one  pair  of 
eyes.  The  mouth  lies  anteriorly  and  ventrally.  The  remaining  open- 
ings of  the  inner  organs  lie  posteriorly  above  the  foot ;  the  anus  in  the 
middle  line,  and  on  each  side,  between  it  and  the  ctenidium  (supposing 
that  there  is  only  one  pair  of  ctenidia),  an  aperture  for  the  sexual 
organs,  and  another  for  the  kidney  (nephridium).  These  five  apertures 
are  covered  by  the  mantle,  and  thus  lie  in  the  mantle  cavity.  We 
have  thus,  to  recapitulate,  in  the  posterior  part  of  the  mantle  cavity 
two  ctenidia,  two  osphradia,  and  five  apertures,  the  median  anus,  and 
the  paired  symmetrical  sexual  and  renal  apertures.  These,  taken 
together,  form  what  is  known  as  the  pallial  complex. 

The  inner  organisation  may  thus  be  briefly  described. 

The  intestinal  canal.  The  mouth  leads  to  a  muscular  pharynx,  with 
horny  jaws.  At  its  base  lies  a  chitinous  rasp-like  ribbon  called  the 
tongue  or  radula,  which  carries  numerous  consecutive  transverse  rows 
of  sharp  chitinous  teeth.  Paired  salivary  glands  enter  the  pharynx, 
which  passes  into  an  oesophagus,  which  latter  leads  into  the  mid- 
gut.  This,  which  we  will  suppose  to  be  more  or  less  coiled,  runs 
right  through  the  body,  passing  posteriorly  into  a  very  short  hind- 
gut,  which  opens  outward  through  the  median  anus.  The  mid-gut 
has  large  paired  glandular  divertieula  (mesenteric  gland,  diges- 
tive gland,  hepatopancreas,  liver). 

Musculature. — The  muscles  of  the  foot  are  powerful,  and  are 
adapted  for  the  creeping  movement.  There  are,  in  addition,  muscles 
running  from  the  inner  surface  of  the  shell  into  the  foot  and  head 
(eolumellar  or  shell  muscles),  and  special  muscles  for  the  different 
organs. 

Nervous  system.  —  Two  well  -  developed  cerebral  ganglia  lie 
dorsally  in  the  head,  and  are  connected  by  means  of  a  short  cerebral 


28  COMPARATIVE  ANATOMY  CHAP. 

commissure,  which  runs  over  the  oesophagus.  Each  cerebral  ganglion 
gives  rise  to  two  powerful  nerve  trunks  which  are  provided  along 
their  whole  length  with  ganglion  cells ;  there  are  thus  two  pairs  of 
nerve  trunks  running  right  through  the  body  longitudinally.  One  pair, 
the  pedal  cords,  run  right  and  left  in  the  foot ;  the  other  pair,  the 
visceral  cords,  which  lie  more  dorsally  and  are  more  deeply  embedded 
in  the  body,  run  through  the  body  cavity.  The  two  visceral  nerves 
are  connected  posteriorly. 

If  we  leave  the  Amphineura  and  Diotocardia  out  of  the  question, 
the  following  modified  sketch  of  the  Molluscan  nervous  system  holds 
good.  Two  cerebral  ganglia,  two  pedal  ganglia,  two  pleural 
ganglia  lying  at  the  sides  of  the  pharynx,  two  visceral  ganglia 
lying  posteriorly  in  the  body  cavity.  Giving  the  name  connectives 
to  such  nerves  as  unite  the  ganglia  of  one  side  of  the  body,  i.e.  dis- 
similar ganglia,  and  that  of  commissures  to  the  nerves  that  unite  the 
similar  ganglia  of  the  two  sides  of  the  body,  we  have  the  following 
system:  Commissures  are  found  —  (1)  between  the  two  cerebral 
ganglia  (over  the  fore-gut) ;  (2)  between  the  two  pedal  ganglia 
(under  the  fore-gut) ;  (3)  between  the  two  visceral  ganglia  (under 
the  hind-gut).  The  connectives  on  each  side  are:  (1)  the  cerebro- 
pedal  connective  ;  (2)  the  cerebropleural  connective ;  (3)  the  pleuro- 
pedal  connective ;  (4)  the  pleurovisceral  connective. 

There  is  a  secondary  eoelom  or  body  cavity  lined  with  endo- 
thelium,  which  has  at  least  two  divisions.  In  the  anterior  division, 
the  genital  chamber,  the  sexual  products  arise  from  the  endothelium  ; 
this  chamber  is  connected  by  means  of  two  canals,  the  genital  ducts, 
with  the  mantle  cavity.  In  the  posterior  chamber,  or  pericardium, 
lies  at  least  one  organ,  the  heart ;  this  chamber  is  connected  with  the 
mantle  cavity  by  means  of  two  nephridial  duets  or  vesicles. 

The  circulatory  system  is  partly  vascular  and  partly  lacunar. 
The  arterial  heart  lies  in  the  pericardium  above  the  hind-gut.  It 
consists  of  one  ventricle  and  two  lateral  auricles. 


II.    Review  of  the  Outer  Organisation  characterising  the  Chief 
Groups  of  the  Mollusea. 

Having  given  above  a  general  plan  of  the  morphology  of  the  Mollusea,  let  us 
now  see  how  far  the  various  groups  of  Molluscs  agree  with  this  description  in  their 
outer  organisation.  We  shall  at  first  only  mention  in  connection  with  each  group 
those  special  features  which  are  now  considered  to  be  typical  or  characteristic  of 
that  group.  In  other  words,  we  shall  again  give  a  general  scheme  of  the  outer 
organisation  of  each  class  of  the  Mollusea,  in  order  that  these  more  specialised 
schemes  may  be  compared  with  that  of  the  hypothetical  primitive  Mollusc  above 
described. 

Later  sections  will  deal  with  the  changes  which  the  separate  organs  undergo, 
not  only  in  the  different  classes,  but  within  one  and  the  same  class,  so  far,  that  is, 
as  these  modifications  bear  on  external  morphology. 


vii  MOLLUSC  A— OUTER  ORGANISATION  29 


A.  Plaeophora  OP  Polyplaeophora  (Chitonidse). 

The  body  of  the  Placophora  is  bilaterally  symmetrical,  and  dorso- 
ventrally  flattened ;  viewed  from  the  dorsal  or  ventral  surface  its 
shape  is  that  of  a  long  oval.  On  the  ventral  side  there  is  a  large 
muscular  foot  with  a  flat  sole,  the  outline  of  which  runs  very  nearly 
parallel  with  that  of  the  body.  In  front  of  the  foot,  and  also  on  the 
ventral  side,  there  is  a  distinct  snout  which  carries  the  mouth  in  the 
middle  of  its  ventral  surface.  There  are  no  eyes  or  tentacles  on  the 
head.  Between  the  mantle,  which  forms  the  outer  edge  of  the  body, 
and  the  body  and  head  it  covers,  there  is  a  deep  groove,  in  the  base  of 
which  lie  numerous  lancet-shaped  gills,  arranged  in  a  single  row  on  each 
side.  These  two  rows  of  gills  sometimes  approach  each  other  so  nearly 
both  anteriorly  and  posteriorly  that  there  is  an  almost  complete  circle 
of  gills  around  the  foot,  or  else  they  are  more  or  less  shortened,  and 
are  in  some  forms  so  reduced  as  only  to  occupy  the  posterior  third 
of  the  branchial  furrow.  The  anus  lies  posteriorly  in  the  median 
line,  ventrally,  immediately  behind  the  foot.  The  two  apertures 
of  the  nephridial  ducts  lie  in  the  branchial  furrow  on  each  side,  and 
slightly  in  front  of  the  anus.  The  two  genital  apertures  lie  imme- 
diately in  front  of  the  nephridial  apertures,  also  in  the  branchial 
furrow. 

The  median  dorsal  region  is  covered  by  eight  consecutive  imbri- 
cating calcareous  plates.  The  peripheral  dorsal  region,  between  the 
edge  of  the  body  and  these  shell  plates,  carries  calcareous  spicules, 
granules,  etc.  The  corresponding  peripheral  region  on  the  ventral 
side  forms  one  of  the  boundaries  of  the  branchial  groove,  and  may  be 
considered  as  the  mantle. 


B.  Aplaeophora,  Solenogastres. 

The  body  is  here  bilaterally  symmetrical  and  vermiform  ;  in  section 
it  is  round,  and  is  sometimes  long  and  thin,  at  others  short  and  thick. 
The  large  oral  aperture  lies  in  the  form  of  a  longitudinal  slit  on  the 
ventral  surface  of  the  anterior  end  of  the  body.  The  cloacal  aperture 
— or  common  opening  for  the  intestinal  canal  and  the  urogenital 
organs — lies  ventrally  at  the  posterior  end  of  the  body.  A  narrow 
median  ventral  groove  runs  forward  from  the  cloacal  aperture 
and  terminates  anteriorly  near  the  mouth.  In  the  base  of  this  pedal 
groove  rises  a  ciliated  ridge  or  fold  which  runs  along  its  whole 
length;  this  ridge,  in  cross  section,  is  triangular,  and  represents 
the  reduced  foot.  In  the  Chcetoderma  both  foot  and  pedal  groove 
are  wanting.  The  Solenogastres  have  no  distinct  compact  shell ; 
its  place  is  taken  by  calcareous  spicules  embedded  in  the  integu- 
ment. 


30 


COMPARATIVE  ANATOMY 


CHAP. 


C.  Gastropoda  (Cephalophora). 

Although  there  can  be  no  doubt  as  to  the  relationship  to  one 
another  of  the  Mollusca  grouped  together  'in  this  class,  it  is  almost 
impossible  to  give  a  general  scheme  of  the  outer  form  of  the  whole 
class.  The  greatest  variation  occurs,  the  body  being  sometimes  out- 
wardly bilaterally  symmetrical,  sometimes  in  a  high  degree  asym- 
metrical. Further,  forms  such  as  Fissurdla,  Oliva,  Turritella,  Cleodora. 
Pterotrachea,  Phyllirhoe,  Limax,  Pleurobranchus,  Thetys,  differ  so  greatly 
in  outward  appearance  that,  at  the  first  glance,  it  is  almost  impossible 
to  believe  that  they  are  related.  A  shell  may  be  present,  and  may  show 
the  most  marvellous  variation  in  form ;  or  it  may  be  rudimentary  or 
even  (in  adult  forms)  altogether  wanting.  The  foot  also  may  assume 
the  most  varied  forms,  or  may  be  entirely  wanting.  The  same  may 
be  said  of  the  mantle  fold,  the  gills,  etc. 

Setting  aside  those  forms  which  are  quite  one-sidedly  differenti- 
ated, it  may  be  said  in  general — (1)  that,  in  the  Gastropods,  the 
protective  shell  consists  of  one  piece,  and  follows  in  a  remarkable  way 
the  forms  assumed  by  the  body ;  (2)  that  the  dorsal  portion  of  the 
body,  which  contains  the  viscera,  becomes  constricted  almost  hernia- 
like  from  the  head  and  foot,  making  a  sac-like  protuberance  (visceral 
dome) ;  (3)  that,  for  the  diminution  of  its  surface,  this  dome  or  hump 
becomes  coiled  spirally,  the  shell  repeating  its  shape  ;  (4)  that  the  head 
and  foot,  which  project  through  the  aperture  of  this  shell  for  purposes 

of  locomotion,  can  be  withdrawn 
into  it.  The  large,  long  foot 
generally  has  a  flat  sole  for  creep- 
ing. The  head  is  distinct,  and 
provided  with  tentacles  and  eyes. 
At  some  part  of  the  body,  the  in- 
tegument of  the  visceral  dome 
forms  a  mantle  fold  which  hangs 
downwards,  covering  and  protect- 
ing the  respiratory  organs.  The 
outer  surface  of  this  mantle  takes 
part  with  the  rest  of  the  integu- 
ment of  the  visceral  dome  in  the 
formation  of  the  shell.  The  follow- 
ing are  more  special  descriptions  of 
the  outer  organisation  of  the  chief 
Gastropodan  groups. 


FIG.  40.— Diagram  of  the  Organisation  of  a 
Zeugobranchiate  Diotocardian.  a,  Anus  ;  ve, 
ventricle  ;  ula,  right  auricle  ;  nrct,  left  ctenidium  ; 
uros,  left  osphradiuin. 


1. 


Prosobranehia. 

The    large    visceral    dome    is 
coiled    spirally,  generally   to   the 
right  (dextrally),  the  shell  naturally  assuming  the  same  form.    The  well- 


VII 


MOLLUSCA— OUTER  ORGANISATION 


31 


ol 


developed  foot  has  a  flat  creeping  sole.  On  the  dorsal  side  of  the 
posterior  portion  of  the  foot,  the  metapodium,  there  is  a  calcareous 
plate,  the  opereulum,  which,  when  the  animal  withdraws  its  head  and 
foot,  closes  the  aperture 

of  the  shell.    The  mantle  ^     «•« 

fold  hangs  down  from 
the  anterior  side  of  the 
visceral  dome,  and  covers 
the  spacious  branchial  or 
mantle  cavity,  in  which 
lie  certain  organs  of 
special  morphological 
importance.  These, 

which  may  be  called  the 
mantle  or  pallial  organs, 
are,  in  such  forms  as 
may  be  considered  primi- 
tive, (1)  the  anus,  which 
lies,  not  posteriorly,  but 
on  the  anterior  side 
of  the  visceral  dome, 
shifted  forwards  to- 
wards the  mouth ;  (2) 
the  two  apertures  of  the 
paired  nephridia,  one  on 
each  side  of  the  anus;  (3) 
the  two  gills,  one  to  the 
left  and  one  to  the  right : 
(4)  the  two  osphradia 
near  the  bases  of  the  gills. 
In  most  Prosobranchia, 
however,  the  organs  just 
mentioned  as  paired  are 

unpaired;    only  the  gill,      Fio.4i.-DiagramofaProsobrancMateMonotocardian.  The 

nephl'idial  aperture,  and    outer  form,  shell,  mantle,  pallial  complex,  heart  and  pericardium , 

OSphradilim    tO    the    left    nervous  system  and  opereulum,  are  depicted.    Lettering  mostly 

as  in  Fig.  39.     In  addition  :  /,  foot ;  si,  siphon  ;  sup,  sub,  supra  - 

01  tne  anUS  being  re-  ailc^  sub -intestinal  connectives;  op,  opereulum;  ot,  auditory 
tained,  while  the  hind-  organ  ;  p,  penis ;  sr,  seminal  groove ;  mh,  mantle  cavity ;  hy, 

gut  with  the  anus  moves  **??Z££f* '  6' male  genital  apertnre ;  r' rectum ;  au' 
to  the  right  side  of  the 

mantle  cavity.  The  single  genital  aperture  lies  on  the  right  side,  in 
the  head,  or  on  the  floor  of  the  mantle  cavity.  (In  the  Prosobranchia 
the  sexes  are  separate.)  The  abortion  of  one  of  each  of  these  originally 
paired  organs,  gills,  nephridia,  and  osphradia,  produces  a  very  striking 
asymmetry  of  the  whole  body.  The  name  Prosobranchia  indicates  the 
fact  that  the  gills  lie  in  front  of  the  heart. 


32 


COMPARATIVE  ANATOMY 


CHAP. 


2.  Pulmonata. 

Type :  Helix  pomatia. — The  visceral  dome  is  well  developed,  and 
protrudes  hernia-like  from  the  rest  of  the  body ;  it  is  dextrally  coiled, 

and  has  a  corresponding  shell.  The 
foot  is  large  and  long,  and  has  a 
flat  creeping  sole.  The  head  has 
;two  pairs  of  feelers,  one  of  which 
carries  the  eyes.  The  mantle  fold 
hangs  down  from  the  anterior  side 
of  the  visceral  dome,  and  covers  a 
spacious  mantle  cavity  (respiratory 
or  pulmonary  cavity).  The  free 
edge  of  the  mantle  fold  unites  with 
the  integument  of  the  neck  near  it, 
only  leaving  an  aperture  to  the 
right,  the  respiratory  aperture. 
This  aperture  serves  for  the  inhala- 
tion and  exhalation  of  the  air. 
The  anus  and  the  unpaired  nephri- 
dial  aperture  lie  close  to  the  re- 
spiratory aperture,  and  are  thus  on 
the  right  side.  There  are  no  gills 
in  the  mantle  cavity,  which  con- 
tains air.  Respiration  takes  place 
at  the  inner  surface  of  the  mantle 
fold,  in  which  runs  a  fine  network 
of  vessels  lying  in  front  of  the 
heart.  The  foot,  unlike  that  of 
the  Prosobranchia,  has  no  operculum.  There  is  a  common  genital 
aperture  on  the  neck,  to  the  right,  in  front  of  the  respiratory  cavity 
(the  Pulmonata  being  hermaphrodite).  Many  Pulmonata,  however, 
differ  greatly  in  their  outer  organisation  from  the  Helix  type. 


FIG.  42.— Diagram  of  a  Basommatophoran 
Pulmonate.  ul,  Respiratory  aperture ;  rgn,  vas- 
cular network  on  the  inner  surface  of  the  mantle. 
The  kidney  is  incorrectly  drawn.  Further  letter- 
ing as  in  Figs.  39  and  41. 


3.  Opisthobranehia. 

The  respiratory  organs  lie  behind  the  heart. 

(a)  Teetibranehia. — The  visceral  dome  is  usually  not  large.  It 
may  be  either  spirally  coiled  or  symmetrical,  and  is  covered  by  a 
variously  shaped  shell.  The  foot  is  large,  and  usually  has  a  flat 
sole  for  creeping.  The  head  is  variously  shaped,  and  often  carries 
tentacles  or  rhinophores,  and  unstalked  eyes.  The  small  mantle 
fold  hangs  down  from  the  right  side  of  the  visceral  dome,  and 
often  does  not  quite  cover  the  single  gill  lying  beneath  it.  The 
anus  lies  behind  the  gill,  more  or  less  removed  from  it.  The  Teeti- 
branehia are,  like  all  Opisthobranehia,  hermaphrodite ;  the  genital 


711 


MOLLUSCA— OUTER  ORGANISATION 


33 


and  nephridial  apertures  lie  on  the  right  side  of  the  body  in  front  of 
the  anus. 

(b)  Nudibranehia.  —  The  body  is  outwardly  symmetrical,  the 
visceral  dome  does  not  protrude  from  it,  but  is  closely  applied  to 
the  whole  length  of  the  foot,  from  which  it  is  often  not  distinctly 


FIG.  43.  —  Diagram  of  a  Tectibranchiate  Opistho- 
branchiate.  Lettering  as  before.  In  addition  :  gg,  genital 
ganglion ;  s,  shell ;  <? ,  female  genital  aperture  ;  Ipp,  rpp, 
left  and  right  parapodial  lobes,  that  on  the  right  laid  back. 


FIG.  44.  —  Dentalitun,  diagram- 
matic, left  aspect,  g,  Sexual  glands  ; 
M,  cephalic  tentacles  ;  other  letter- 
ing as  before. 


differentiated.  The  foot  has  a  flat  creeping  sole.  There  is  no  distinct 
mantle  fold,  no  gill  corresponding  with  that  of  the  Tectibranchia, 
and  no  shell.  The  head  carries  tentacles  or  rhinophores,  and  sessile 
eyes.  The  anus  lies  either  dorsally  in  the  median  line,  or  laterally  to 
the  right.  The  genital  and  renal  apertures  lie  to  the  right  in  front 
of  the  anus.  The  gills,  which  vary  much  in  form,  number,  and 
arrangement,  are  found  dorsally  or  laterally,  and  are  not  homologous 
with  the  typical  Molluscan  ctenidia. 


VOL.  II 


34 


COMPARATIVE  ANATOMY 


CHAP. 


D.  Seaphopoda. 

The  body  is  symmetrical  and  long,  i.e.  the  visceral  sac  is  elongated 
dorso-ventrally,  and  is  completely  enveloped  in  a  tubular  mantle.  The 
mantle  cavity  lies  posteriorly,  and  is  prolonged  ventrally  far  enough  to 
allow  the  snout  and  retracted  foot  to  be  completely  concealed  in  it. 
Besides  the  large  ventral  aperture,  there  is  a  smaller  dorsal  aperture 
further  placing  the  mantle  cavity  in  communication  with  the  exterior. 
The  shell,  like  the  mantle,  is  tubular,  or  like  a  tapering  cone,  slightly 
curved  anteriorly.  It  has  two  apertures  corresponding  with  those  in 
the  mantle.  The  head  is  developed  into  a  barrel-shaped  snout,  and 
has  no  eyes.  The  mouth,  which  lies  at  its  ventral  end,  is  surrounded 
by  a  circle  of  leaf-like  tentacles.  At  the  base  of  the  snout  there  arise 
two  tassels  of  long  filamentous  contractile  tentacles,  which  hang  down 
into  the  mantle  cavity  and  can  be  projected  far  beyond  the  ventral  aper- 
ture. Behind  the  snout,  the  cylindrical  muscular  foot  rises  from  the 
body,  and  can  be  protruded  downwards.  There  are  no  gills.  The 

median  anus  lies  posteriorly  above 
the  foot.  The  two  nephridial 
apertures  are  at  the  sides  of  the 
anus.  There  are  no  special  genital 
apertures  (Figs.  44  and  101, 
p.  113). 

E.  Lamellibranehia. 

The  body  is  bilaterally  sym- 
metrical ;  somewhat  elongated 
(from  before  backward).  The 
integument  forms  leaf-like  mantle 
folds  to  the  right  and  to  the  left, 
which  at  their  bases  are  attached 
to  the  trunk  along  its  whole 
length,  and  grow  down  ventrally. 
If  the  body  of  a  Lamellibranch, 
from  which  the  shell  has  been 
removed  (the  foot  bein°;  re- 

FIG.   45.  —  Transverse   section   of   Anodonta  j\  v       •         j  r  i_       •  j 

Cjg&xa,  (ordinary  freshu-ater  mussel)  (after  Howes),     tracted),  be  Viewed  trom  the  Side, 
lg,  Ligament ;  ty,  typhlosolis  ;  kb,  pericardial  gland 
(Keber's  organ);   re,  kidney  (glandular  portion); 
sbc,  chambers  at  the  bases  of  the  gills  ;  yd,  genital 


ibc 


l-F 


the  outline  will  be  found  to  be 
formed,   dorsally,   by  the   dorsal 


ducts  ;  brli,  brl2,  outer  and  inner  branchial  lamellae  ;     median     line    of     the    body  ; 


terioiiy,  posteriorly,  and  Ventrally 
,1         <•  i  f    ,  -, 

the    fr6e    edg6    °f   the    mantle 


ibc,  mantle  cavity  ;  s,  shell  ;  s1;  edge  of  the  shell  ; 

/i,  foot  ;  pm,  pallial  muscle  ;  i,  intestine  ;  pli,  right 

mantle  fold  ;  ^Z,  gonad  ;  r,  rectum  ;  q>,  cerebro- 

pedal  connective  ;  relf  non-glandular  vestibule  of    fold.       The    two  mantle    f  olds  en- 

kidney  ;  re,,  renal  aperture  ;  pc,  pericardium.  cloge     a    gpace    whoge     transverse 

axis  is  always  markedly  shorter  than  either  its  dorso-ventral  or  its 
longitudinal  axis,  i.e.  the  animal  with  its  mantle  is  laterally  compressed. 


VII 


MOLLUSC  A— OUTER  ORGANISATION 


35 


Projecting  into  the  mantle  cavity,  there  is  a  large  muscular  process  of 
the  body,  the  foot,  which  is  directed  downward  and  somewhat  forward, 
and  can  be  protruded  between  the  free  edges  of  the  mantle.  This  foot 
is  also  laterally  compressed.  In  certain  cases  which,  though  excep- 
tional, deserve  special  mention,  its  free  end  is  flattened,  and  it  thus  has 
a  flat  sole.  The  outer  surface  of  the  trunk  and  mantle  folds  secretes 
a  bivalve  shell  which  covers  the  whole  body.  One  valve  lies  to  the 
right,  the  other  to  the  left  of  the  median  plane,  and  the  two  are  exactly 
alike.  Each  valve  repeats  the  outline  of  its  own  side  of  the  trunk 
with  its  mantle  fold. 

The  two  valves  articulate  dorsally,  and  are  open  anteriorly,  ventrally, 
and  posteriorly.      Two   strong  muscles  (adductors)  run  transversely 


FIG.  4o.  —Anatomy  of  Unio  (Margaritana)  margaritiferus,  left  side  (after  Leuckart  and  Nitsche). 
o,  Mouth  ;  Cg,  cerebral  ganglion  ;  MI,  anterior  adductor  muscle  ;  a>,  oesophagus  ;  I,  digestive  gland 
(liver) ;  HO,  nephridial  aperture  :  lo,  apertures  of  the  digestive  gland  in  the  stomach  m  ;  Aa,  anterior 
aorta  ;  ?i,  nephridium,  the  outline  given  in  dotted  lines  ;  I",  heart ;  r,  hind-gut ;  Ap,  posterior 
aorta ;  J/2.  posterior  adductor ;  a,  anus ;  Vg,  visceral  ganglion ;  Br,  gill ;  Bk,  mantle  cavity ;  go, 
gonads  with  genital  duct  goi ;  Pg,  pedal  ganglion  ;  p,  foot.  The  arrows  indicate  the  direction  of 
the  inhalent  and  exhalent  streams  of  water. 

from  one  valve  to  the  other.  Their  contraction  serves  to  shut  the 
shell  completely.  One  of  these  muscles  lies  anteriorly,  the  other 
posteriorly.  Their  points  of  attachment  produce  impressions  on  the 
inner  surface  of  the  shell,  which  are  always  distinctly  visible  when 
the  shell  is  removed. 

The  mouth  lies  below  the  anterior  adductor,  between  it  and  the 
anterior  base  of  the  foot.  The  anus  lies  behind  the  posterior  adductor. 
There  is  no  distinct  head.  Xear  each  side  of  the  mouth,  the  body 
carries  two  leaf -like  processes,  the  oral  lobes  or  labial  palps.  At  the 
line  of  insertion  of  the  foot  in  the  mantle  cavity,  a  longitudinal 
ridge  rises  on  each  side  in  the  middle  and  posterior  regions  of  the 
body  ;  this  carries  two  rows  of  long  branchial  leaflets.  There  is  thus, 


36 


COMPARATIVE  ANATOMY 


CHAP. 


on  each  side  of  the  mantle  cavity,  one  plumose  gill,  the  shaft  of  which 
is  attached  lengthwise  to  the  body  (Figs.  45,  46,  etc.). 

In  various  divisions  of  the  Lamellibranchia,  the  outer  organisation 
deviates  very  greatly  from  the  above. 


F.  Cephalopoda. 

The  body  is  bilaterally  symmetrical.     The  visceral  dome  is  large 

and  often  much  elongated  dorso-ventrally.     The  head  is  more  or  less 

distinct,  and  is  surrounded  by  the  foot, 
<*•  which  is  transformed  in  a  peculiar  man- 

ner. The  foot  has,  in  fact,  grown  round 
the  head,  and  has  developed  numerous 
differently -shaped  processes  (arms  and 
tentacles)  arranged  in  a  circle  round  the 
mouth;  these  serve  principally  for  seizing 
and  holding  prey.  In  viewing  the  body 
of  a  Cephalopod,  it  must  be  remembered 
that  the  apex  of  the  visceral  dome  (which 
a  casual  observer  might  take  to  be  the 
posterior  end  of  the  body)  is  really  the 
highest  dorsal  point,  while  the  head  and 
its  arms  lie  lowest.  We  may  thus  dis- 
tinguish, both  in  the  visceral  dome  and 
in  the  transformed  foot  which  has  been 
combined  with  the  head,  and  drawn  out 
into  tentacles,  an  anterior  and  a  posterior 
part  (which  to  a  casual  observer  would 
seem  upper  and  lower  parts),  and  a  right 
and  a  left  side.  This  at  first  sight  seems 
a  paradox  to  those  not  acquainted  with 
the  comparative  anatomy  of  the  Mollusca, 
since  the  normal  position  in  the  water  of 
certain  well-known  Cephalopods  does  not 
agree  with  it.  A  Sepia,  for  example, 
swims  or  lies  at  rest  in  such  a  way  that 
the  strongly  pigmented  anterior  side  of 
the  visceral  dome  and  of  the  "  head " 
(Kopffuss)  is  uppermost,  and  the  posterior 
side  lowermost.  The  accompanying  dia- 
gram illustrates  the  strict  morphological 

position  of  the  tody,  which  alone  concerns  the  comparative  anatomist 

(Fig.  47). 

On  the  right  and  left  of  the  "  head  "  there  is  a  highly-developed 

eye,  and  near  it  an  olfactory  pit. 

The  mantle  fold  hangs  down  posteriorly  from  the  visceral  dome, 

covering  a  spacious  mantle-  or  respiratory  cavity,  which  communicates 


FIG.  47.— Diagram  of  Sepia,  median 
section  from  the  left  side,  v,  Ventral 
(physiologically  anterior);  d,  dorsal 
(physiologically  posterior);  an,  anterior 
(physiologically  upper) ;  po,  posterior 
(physiologically  lower) ;  1,  2,  3,  4,  5,  the 
five  arms  of  the  left  side ;  au,  eye ;  co, 
internal  shell;  go,  gonad;  d,  pigment  gland 
=  ink-bag  ;  m,  stomach  ;  n,  kidney  ;  ct, 
ctenidium  ;  a,  anus  ;  mh,  mantle  cavity  ; 
in,  siphon.  The  arrows  indicate  the 
direction  of  the  respiratory  current. 


vii  MOLLUSOA— OUTER  ORGANISATION  37 

with  the  exterior  at  the  free  edge  of  the  mantle  fold,  above  the 
"head."  Within  the  mantle  cavity  there  are  two  or  four  gills, 
arranged  symmetrically,  the  median  anus,  and  the  apertures  of  the 
sexual  and  excretory  organs.  Two  symmetrical  lobes  are  found  on 
the  posterior  lower  side  of  the  visceral  dome;  the  edges  of  these 
are  apposed  in  such  a  way  as  to  form  a  tube,  the  funnel  or  siphon, 
one  aperture  of  which  lies  in  the  mantle  cavity,  while  the  other 
protrudes  from  the  mantle  cleft.  The  respiratory  water  enters  the 
mantle  cavity  through  the  mantle  cleft,  and  escapes  through  the 
siphon.  The  faBcal  masses,  waste  and  sexual  products,  and  the 
secretion  of  the  ink-bag  also  leave  the  body  through  the  siphon. 
Originally,  no  doubt,  all  Cephalopoda  possessed  a  shell  which  covered 
the  whole  visceral  dome  as  well  as  the  mantle  fold.  In  recent 
Cephalopods  the  shell  is  rarely  developed  in  this  way ;  it  is  often 
rudimentary,  and  may,  indeed,  be  altogether  wanting.  Recent 
Cephalopods  fall  into  two  entirely  distinct  divisions,  the  Tetra- 
branchia  and  the  Dibranchia. 

The  Tetrabranehia  (Nautilus,  Fig.  48). 

These  have  a  shell  coiled  anteriorly  (exogastrically)  in  the  plane 
of  symmetry,  and  divided  by  septa  into  consecutive  chambers. 
The  animal  occupies 
the  last  and  largest 
chamber ;  the  others 
contain  gas. 1  The  septa 
separating  the  consec- 
utive chambers  are 
pierced  in  the  middle 
to  allow  of  the  passage  * 
of  a  siphunele,  which 
runs  through  all  the 
compartments,  and  is 
attached  to  the  visceral 
dome.  That  portion  of 
the  foot  which  sur- 
rounds the  mouth  is 
produced  into  numerous  Fic  4S._Diagram  of  Nautilus>  Mt  ^  rt,  Ventral .  do> 

tentacles,  Which  Can  be    dorsal ;  m,  anterior ;  hi,  posterior  ;  /,  foot  (tentacles  and  siphon) ; 
retracted      into     Special    snl>  she11  muscle  ;  ct,  ctenidia  ;  mh,  mantle  cavity  ;  a,  anus ;  s, 
i        ,r  shell ;  si,  siphunele  ;  a«,  eye  ;  o,  mouth. 

The  anterior  portion  of  the  foot,  which  lies  in  front  of  and  over 
the  head,  is  widened  out  into  a  concave  lobe,  the  hood ;  this  is  applied 
to  the  outer  surface  of  the  occupied  chamber  of  the  shell  anteriorly, 
and,  when  the  tentacles  are  withdrawn,  can  close  its  aperture.  The 
hood  carries  two  tentacles,  and  on  each  side  of  the  head  there  is  an  eye. 

1  Or  water  ;  c.  Ford's  Introduction  to  Brit.  Mus.  Cat.,  Fossil  Cephalopoda,  1889. 


38  COMPARATIVE  ANATOMY  CHAP. 

Above  the  head,  the  mantle  fold  encircles  the  whole  body.  It  is  short 
at  the  sides,  but  anteriorly  it  forms  a  large  lobe  which  is  folded  back 
over  the  shell  in  the  way  shown  in  Fig.  32,  p.  22.  Posteriorly,  the 
mantle  covers  a  very  deep  cavity  which  contains  the  whole  posterior  side 
of  the  visceral  dome.  The  siphon  consists  of  two  entirely  distinct 
lateral  lobes  (epipodial  lobes),  whose  free  edges  overlap  in  such  a 
manner  as  to  form  a  tube,  open  above  and  below.  As  we  shall  see 
later,  this  siphon  is  a  part  of  the  foot.  Deep  down  in  the  mantle 
cavity,  two  pairs  of  pinnate  gills — a  lower  and  an  upper  pair — spring 
from  the  visceral  dome.  Nine  apertures  of  inner  organs  are  also 
found  in  this  cavity ;  a  single  median  anal  aperture,  and  four  paired 
apertures,  viz.  one  pair  of  genital,  two  pairs  of  nephridial,  and  one 
pair  of  viscero  -  pericardial  apertures.  The  position  of  these  is 
depicted  in  Figs.  78  and  79,  p.  82. 

The  Dibranehia. 

With  one  exception,  viz.  the  female  Argonawta,  which  has  an 
external  unchambered  shell,  the  Dibranehia  either  have  an  internal 
shell  lying  on  the  anterior  side  of  the  visceral  dome,  covered  by  an 
integumental  fold,  or  no  shell  at  all.  The  visceral  dome  is  sometimes 
compact  and  pouch-like  (in  reptant  animals,  Fig.  37),  sometimes,  in 
the  good  swimmers,  much  elongated  dorso-ventrally,  produced  dorsally 
to  a  point,  and  flattened  antero-posteriorly  (Fig.  34).  In  the  latter 
case,  the  body  is  further  generally  encircled  by  a  fin-like  integumental 
fold,  which  marks  the  limit  between  the  anterior  and  posterior  sides  of 
the  visceral  dome. 

The  "  head  "  is  usually  distinct  from  the  visceral  dome,  and  carries 
to  the  right  and  left  the  well -developed  eyes.  The  mouth  is  sur- 
rounded by  eight  or  ten  arms  for  seizing  prey ;  these  are  provided 
with  suckers  on  their  lower  adoral  sides. 

The  mantle  fold  covers  nearly  the  whole  posterior  surface  of  the 
visceral  dome,  and  thus  encloses  a  very  deep  and  spacious  cavity. 
Laterally  and  anteriorly  to  the  visceral  dome,  the  mantle  fold  is 
continued  as  a  narrow  border  which,  immediately  above  the  "head," 
covers  a  shallow  groove  or  furrow. 

The  two  lateral  lobes  which  form  the  siphon  of  the  Tetrabranchia 
have  in  the  Dibranehia  grown  together  at  their  free  edges,  and  form 
a  tube  open  at  each  end.  There  are  only  two  gills  in  the  mantle 
cavity,  one  right,  and  one  left.  Near  the  upper  siphonal  aperture  in 
the  mantle  cavity  lie  the  anus,  and  the  genital  and  nephridial  apertures 
as  well  as  that  of  the  ink-bag.  Details  as  to  the  arrangement  and 
number  of  these  apertures  will  be  given  further  on. 


vii     MOLLUSCA— INTEGUMENT,  MANTLE,  VISCERAL  DOME    39 


III.  The  Integument,  the  Mantle,  and  the  Visceral  Dome. 

The  whole  body  is  covered  by  a  single  layer  of  epithelium,  which, 
in  parts  not  protected  by  the 
shell,  may  be  more  or  less 
ciliated.  This  layer  is  very 
rich  in  glands  which  are 
almost  exclusively  unicellular ; 
some  of  these  lie  in  the  epi- 
thelium itself,  while  some 
have  sunk  into  the  subjacent 
tissue,  their  ducts,  however, 
passing  between  the  epithelial 
cells.  3 

The  layer  immediately 
beneath  this  body  epithelium 
is  called  the  corium,  and  con- 
sists of  connective  tissue  and 
muscle  fibres.  It  is,  how- 
ever, not  distinctly  marked 

off  from  the  tissues  beneath 

j  {-  FIG.  49.— Section  of  the  integument  of  Daudebardia 

„,  .  rufa  (after  Plate).  1,  Epithelium  ;  2,  3,  9,  various  forms 

I  he  pigment  IS  almost  of  unicellular  glands  ;  4,  globular  pigment  cells ;  5,  7, 

always    found    in   the  Cells    of  unpigmented  cells  of  the  connective  tissue  ;  6,  muscle 

the   subepithelial    connective  JJ^^^^^ISSr^0^^^*^ 
tissue. 

A.  Placophora.     (Cf.  the  sketch  of  the  Outer  Organisation,  p.  29.) 

The  Chiton  is  provided  dorsally  with  eight  consecutive  shell-plates  (Fig.  1,  p.  2), 
which  overlap  in  such  a  manner  that  the  posterior  edge  of  each  plate  covers  the  anterior 
edge  of  the  next.  These  plates  are  bilaminar.  The  outer  and  upper  layer  which  forms  the 
dorsal  surface  is  called  the  tegmentum,  the  lower  hidden  layer  the  articulamentum. 
As  a  rule,  the  tegmentum  of  the  anterior  plate  only  is  as  large  as  the  articulamentum 
beneath  it ;  in  the  other  plates,  the  latter  is  the  larger  and  projects  beyond  the 
former  laterally  and  anteriorly.  These  projecting  parts  of  the  articulamentum, 
called  apophyses,  slide  under  the  plate  next  in  order  anteriorly.  Between  these 
two  layers,  tissue  is  found,  which  is  a  continuation  of  the  dorsal  integument. 
The  tegmentum  is  penetrated  by  canals  of  various  sizes,  which  open  at  its 
surface  through  characteristically  arranged  pores.1  The  tegmentum  consists  of  a 
horny  or  chitinous  substance,  which  may  be  considered  as  a  cuticular  formation, 
impregnated  with  calcareous  salts.  The  articulamentum  is  compact  and  free  from 
canals  ;  it  contains  little  organic  substance,  and  much  calcareous  salt.  It  alone 
answers  to  the  shell  of  other  Molluscs,  while  the  tegmentum  must  be  considered  as 
a  calcined  cuticle  covering  the  true  shells  (the  articulamenta)  as  a  continuation  of 
the  cuticle  of  the  zone  which  encircles  the  eight  shell-plates.  This  zone  carries 


1  On  the  relation  of  these  canals  and  pores  to  peculiar  sensory  organs  and  eyes  on 
the  shell  of  the  Chiton,  cf.  section  on  Sensory  Organs,  p.  166. 


40 


COMPARATIVE  ANATOMY 


CHAP. 


chitinous  or  calcareous  spines,  seise,  scales,  granules,  etc.,  varying  in  number  and 

arrangement  according  to  the  genus  and  species. 

Each  spine,  as  a  rule,  arises  as  a  globular  vesicle  within  an  epithelial  papilla  and 

above  a  very  large  formative  cell  (Fig. 
50).  As  it  grows,  it  is  pushed  upwards  by* 
the  newly  -  forming  cuticular  layers.  The 
formative  cell  at  its  base  persists,  but  remains 
connected  with  the  epithelial  papilla  only  by 
a  protoplasmic  process  which  continually 
lengthens,  and  may  surround  itself  with  a 
nucleated  sheath.  In  fully-developed  spines, 
the  remains  of  this  cell  are  still  found  as  a 
small  terminal  swelling  (Endkolbchen). 

There  are,  however,  spines  and  specially 
flat  scale-  or  plate-like  calcareous  formations 
in  the  integument  which  do  not  arise  from 
single"  large  formative  cells,  but  are  probably 
produced  by  several  cells  in  the  base  of  an 
epithelial  papilla. 

Just  as  we  have  recognised  the  tegmentum 
covering  the  articulamenta  to  be  merely  a 


FIG.  50.— A,  B,  C,  Three  stages  in  de 
velopment  of  a  spine  in  the  Chiton  (after 


Blumrich),  diagrammatic,  st,  Spine ;  bz,  its  special  portion  of  the  general  cuticle,  so  we 
formative  cell ;  e,  epithelium ;  c,  thick  may  further  recognise  in  the  articulamenta 
cuticle  secreted  by  the  epithelium;  el;  the  homologues  of  the  calcareous  spines, 
terminal  swelling  (Endkolbchen)  =  remains 
of  the  formative  cell. 

ment   of    the    mantle.      The   articulamenta 


scales,  etc.,  which  are  developed  in  the  integu- 
ment  of    the    mantle.       The   articu 
would  thus  be  nothing  more  than  very  large  and  expanded  calcareous  scales. 


y*  ^ 

FIG.  51.— Transverse  section  through  a  Chiton  near  the  nephridial  'apertures,  highly 
diagrammatic  (after  Sedgwick),  somewhat  modified.  1,  Pericardium ;  2,  ventricle  ;  3,  auricle ; 
4,  branchial  "vein";  5,  branchial  groove  (mantle  cavity);  0,  gill  (ctenidium) ;  7,  foot;  8,  pleuro- 
visceral  connective;  9,  branchial  "artery";  10,  secondary  coelom ;  11,  intestine;  12,  posterior 
portion  of  the  gonad  lying  below  the  pericardium ;  13,  14,  the  two  posterior  branches  of  the 
nephridium,  one  of  which  (13)  opens  into  the  branchial  groove  (at  16),  the  other  (14)  being  connected 
in  a  way  not  here  depicted  with  the  pericardium  ;  15,  pedal  nerves. 

This  view,  finally,  leads  to  the  conclusion  that  the  shell  (if  it  may  here  be  so 


vii    MOLLUSC  A— INTEGUMENT,  MANTLE,  VISCERAL  DOME    41 


called)  of  the  Molluscs  originally  consisted  of  isolated  calcareous  spicules  or  spines, 
which  were  enclosed  in  a  thick  cuticle,  and  projected  from  the  same  as  in  the 
Proneomenia,  Neomenia,  etc.  (v.  below). 

In  Cryptochiton  the  shell  is  internal,  i.e.  it  is  entirely  covered  by  a  fold  of  the 
integument,  which  grows  over  it  from  all  sides.  It  consists  exclusively  of  the 
articulamentum,  since  the  whole  dorsal  integument  is  covered  by  an  even  cuticle, 
which  therefore  forms  no  tegmentum. 

The  only  part  of  a  Chiton  which  can  be  called  the  mantle  fold  is  the  marginal 
zone  of  the  body,  the  ventral  side  of  which  encircles  the  head  and  foot  and  forms 
the  lateral  boundary  of  the  branchial  groove  or  furrow.     Just  as  the  dorsal  side  of 
this  mantle,  which  is  called 
the  zone,  carries  large 
spines,  setae,  or  scales,  so 
may  the  under  surface  be 
covered  with  small  closely- 
crowded  spines.     The  rest 
of  the  integument  is  bare, 
being  merely  covered  with 
a  simple  epithelium. 

The  genus  Chitonellus 
is  of  great  importance  in 
comparing  the  outer  or- 
ganisation of  the  Placo- 
phora  with  that  of  the 
Soleitor/'.'.xf /•••*.  The  body 
is  not  dorso-ventrally  flat- 
tened, as  in  the  Chiton. 
but  nearly  cylindrical ;  the 
ventral  surface,  however, 
is  flattened  (Fig.  52),  and 
has  a  median  longitudinal 
groove.  The  foot  is  not 
externally  visible,  but  can 


FIG.  52.— Transverse  section  of  Chitonellus,  diagrammatic, 
adapted  from  figures  by  Pelseneer  and  Blumrich.  g,  Shell  (articu- 
lamentum) ;  go,  gonad ;  i,  intestine  ;  ab,  vb,  branchial  arteries  and 
veins ;  pv,  pleuro-visceral  nerves ;  x,  latero-ventral  thickening  of 
the  cuticle  ;  p,  foot ;  ct,  ctenidium  ;  pn,  pedal  nerve  ;  h,  digestive 
gland  (liver) ;  c,  secondary  ccelom ;  ao,  aorta. 


be  discovered,  much  reduced,  in  the  base  of  the  median  groove,  itself  possessing 
a  ventral  median  groove  representing  a  narrow  contracted  sole.  The  flat  ventral 
surface  is  therefore  the  mantle.  In  the  narrow  cleft  on  each  side,  between  mantle 
and  foot,  in  the  posterior  half  of  the  body,  lie  the  gills.  The  lateral  margin  of 
the  body  in  Chiton  is  represented  in  ChitoncUus  by  a  mere  blunted  ridge,  which 
is  almost  exclusively  caused,  as  may  be  seen  in  transverse  sections,  by  a  great 
thickening  of  the  cuticle. 

B.  Solenogastres. 

In  the  Solenogastres  (Aplacophora),  whose  outer  organisation  has  already  been 
sufficiently  described  (p.  29),  the  shell  is  altogether  wanting,  but  the  cuticle  secreted 
by  the  epithelium  over  the  whole  body  is  usually  exceedingly  thick  (Fig.  53).  It 
contains  calcareous  spicules,  which  sometimes  project  above  the  surface.  These,  like 
the  spines  of  the  Polypli'i.cophorn,  rise  from  cellular  cups,  which  are  connected  with 
the  basal  epithelium  of  the  cuticle  by  nucleated  stalks.  There  can  be  no  doubt 
that  the  spicules  are  formed  by  these  cups  and  nourished  by  them  during  growth. 
The  foot,  as  we  have  seen,  is  reduced  to  a  narrow  ciliated  longitudinal  ridge,  which 
rises  from  the  base  of  the  medio-ventral  groove.  The  term  mantle  is  here  inappli- 
cable, except  perhaps  to  the  integument  which  forms  the  lateral  boundary  of  this 
groove. 


42 


COMPARATIVE  ANATOMY 


CHAP. 


In  Chaetoderma  the  foot   finally  atrophies,  and   the  medio-ventral  groove  also 
disappears. 

The'long  series  of  undoubtedly  primitive  characteristics  in  these  two  groups — the 

Placophora  and  Solenogastres — obliges 
us  to  place  them,  as  \ve  shall  have 
repeatedly  to  point  out,  near  the  root 
of  the  Molluscan  phylum.  In  some 
points  the  Solenogastres  are  perhaps 
more  primitive  than  the  Polyplaco- 
phora,  and  the  vermiform  body,  the 
slight  development  of  the  mantle,  the 
foot  and  the  gills  have  been  thought 
to  be  primitive  characteristics.  More 
recently,  however,  it  has  been  main- 
tained, as  the  present  writer  thinks, 
with  justice,  that  these  conditions  are 
rather  the  result  of  secondary  adapta- 
tion to  a  limicolous  habit  of  life  (most 
Solenogastres  inhabiting  mud).  The 
shell,  mantle,  gills,  and  foot  are  such 
essential  characteristics  of  the  Mol- 
lusca  that  we  must  assume  their 
existence  in  the  racial  form. 

The    series    Chiton,    Chitoncllus, 
Ncomenia,  Chcetoderma  does  not,  there- 


FIG.  53. —Transverse  section  of  Proneomenia 
Slulteri  in  the  region  of  the  mid-gut.  1,  Mid-gut ; 
2,  rudimentary  "foot ;  3,  sepia  projecting  into  the  mid- 
gut  ;  4,  testicular  portion  of  the  gonad ;  5,  ovarial 


portion  of  the  same  ;  6,  thick  cuticle  secreted  by  the 
epithelium. 


fore,   illustrate   for  us   the  rise  and 
development    of   typical    Molluscan 
characteristics,  but  rather  their  progressive  obscuration  and  disappearance. 


C.   Gastropoda.     (Of.  Sketch  of  Outer  Organisation,  pp.  30-33.) 

Integument. 

The  free  edge  of  the  mantle,  which  takes  the  chief  part  in  the 
formation  and  growth  of  the  shell,  is  particularly  rich  in  mucous,  pig- 
ment, and  calcareous  glands. 

The  epithelium  is  ciliated  over  areas  of  varying  extent,  especially 
in  aquatic  Gastropods.  In  many  of  the  shell-less  Opisthobranchia  the 
whole  surface  of  the  body  is  ciliated. 

The  remarkable  marking  and  colouring  of  the  integument  especi- 
ally seen  in  the  Nudibranchia  are  caused  by  pigment  cells,  which  are 
more  often  found  in  the  cutis  than  in  the  epithelium. 

Where  there  is  no  firm  shell,  calcareous  granules  or  spicules  may 
be  found  scattered  throughout  the  cutis. 

In  several  Nudibranchia  stinging  cells  have  been  discovered  in 
the  integument. 

Mantle,  Visceral  Dome. 

The  mantle  fold  is,  as  a  rule,  well  developed  in  Gastropods,  and 
covers  a  spacious  pallial  cavity.  Whenever  the  fold  is  small  or  alto- 
gether wanting,  the  condition  is  secondary  rather  than  primitive. 


vii    MOLLUSCA— INTEGUMENT,  MANTLE,  VISCERAL  DOME    43 


1.  Prosobranehia. 

In  the  Prosobranchia,  the  mantle  fold  develops  on  the  anterior 
side  of  the  visceral  dome,  and  there  covers  a  spacious  cavity.  It 
further  usually  extends  like  a  narrow  collar  right  round  the  base  of 
the  visceral  dome. 

In  the  symmetrical  Fissurellidce,  the  mantle  cavity  is  short,  and  opens  out- 
wardly by  means  of  a  dorsal  aperture  through  the  mantle  fold,  which  corresponds 
with  the  perforation  at  the  apex  of  the  shell.  A  circular  fold,  provided  with  a 
highly  sensitive  fringe,  is  formed  by  the  mantle  around  the  aperture,  and  projects  for 
a  short  distance  beyond  the  perforation  in  the  shell.  The  water  needed  for  respira- 
tion passes  into  the  pallial  cavity  through  the  slit-like  aperture  at  the  free  edge  of 
the  mantle  fold,  over  the  nuchal  region,  and  flows  out  through  the  apical  aperture 
just  described.  This  aperture  also  serves  for  the  ejection  of  excretory  matter  from 
the  rectum,  which  lies  immediately  behind  it.  In  Rimula,  the  apical  apertures  in 
shell  and  mantle  have  moved  somewhat  forward,  and  lie  anteriorly  between  the 
apex  and  edge  of  the  shell.  In  Emarginula,  the  mantle  has  an  anterior  cleft,  the 
edges  of  which,  in  the  living  animal,  are  folded  in  such  a  way  as  to  form  a  tubular 
siphon,  which  can  be  protruded  through  the  marginal  cleft  of  the  shell.  In  Par- 
mophorus  there  is  no  second  opening  into  the  mantle  cavity,  but  the  lateral  edges 
of  the  mantle  are  very  much  widened,  and  bent  back  dorsally  over  the  outer  surface 
of  the  shell  in  such  a  way  as  to  cover  the  greater  part  of  it. 

In  Haliotis,  the  enormous  development  of  the  columellar  muscle  on  the  right 
side  confines  the  mantle  cavity  to  the  left.  The  mantle  fold  has  a  long  slit  reach- 
ing from  its  edge  to  the  base  of  the  pallial  cavity.  This  slit  lies  under  a  row  of 
perforations  in  the  shell  which  are  characteristic  of  Haliotis,  and  through  these  the 
respiratory  water  is  expelled.  In  the  spaces  between  the  consecutive  perforations,  the 
edges  of  the  mantle  cleft  are  apposed,  merely  separating  beneath  each  aperture  to 
allow  of  free  communication  between  the  cavity  and  the  exterior.  The  edges  carry 
three  tentacular  processes,  which  can  be  thrust  outward  through  the  perforations. 
The  anus  is  always  found  under  the  posterior  perforation.  The  edge  of  the  mantle 
surrounding  the  body  splits  into  two  narrow  lamellse,  which  bend  round  to  form 
a  groove  for  the  reception  of  the  edge  of  the  shell. 

The  Trochida',  Turbinidce,  Neritidce,  and  nearly  all  Monotocardia  have  no  second 
aperture  and  no  mantle  cleft. 

In  Docoglossa  (Patella,  etc.)  the  mantle  forms  a  circular  fold  round  the  visceral 
dome,  which  is  in  the  form  of  a  blunt  cone.  It  covers  the  edge  of  the  almost 
circular  broad -soled  foot.  The  mantle  is  broadest  anteriorly,  where  it  covers  the 
head  and  neck,  i.e.  the  pallial  cavity  or  groove  is  here  deepest. 

The  visceral  dome,  in  the  Monotocardia,  is  almost  always  distinctly  constricted 
at  the  base,  and  spirally  coiled.  The  pallial  cavity  occupies  its  typical  position. 
In  many  Monotocardia,  the  free  edge  of  the  mantle  fold  is  prolonged  on  the  left  side, 
projecting  forward,  sometimes  to  a  great  extent ;  the  lower  edges  of  this  projecting 
fold  bend  round  towards  each  other  to  form  a  tube  or  semi-cylindrical  channel, 
which  is  called  the  siphon.  Through  this  siphon,  the  water  needed  for  respiration 
flows  into  the  mantle  cavity.  It  can  generally  be  told,  by  the  shape  of  the  shell,  if 
there  is  a  siphon  or  not,  since  most  Monotocardia  which  possess  one  have  either  a 
notch  in  the  edge  of  the  shell  at  the  columella,  or  a  process  called  the  canal  or  beak, 
at  this  same  point,  which  encloses  the  siphon.  The  length  of  this  latter  canal  need 
not,  however,  correspond  with  that  of  the  siphon. 


44 


COMPARATIVE  ANATOMY 


CHAP. 


The  Monotocardia  have  even  been  grouped,  according  to  the  presence  or  absence 
of  a  siphon,  into  the  Siphoniata  or  Siphonostomata,  and  the  Asiphoniata  or  Holo- 

stomata  ;  but  this  classification  is  artificial, 
since  siphons  are  sometimes  present  and 
sometimes  absent  in  forms  which  are  un- 
doubtedly nearly  related. 

In  most  Monotocardia,  the  shell  is  not 
outwardly  covered  by  the  mantle,  but  in 
some  groups,  the  edges  of  the  mantle  bend 
back  over  the  shell,  and  finally  grow  over 
it  .to  ^such  an  extent  as  to  unite  above  it. 
The  external  shell  in  such  cases  becomes 
an  internal  shell. 

In  the  Harpidce  among  the  Rhacliiglossa, 
the  mantle  bends  back  over  the  columellar 
lip  of  the  shell.  In  the  Margimllidm,  it 
covers  a  large  part  of  the  outer  surface,  and 
the  same  is  the  case  in  Pynda  among  the 
Taenioglossa,  in  most  Gyprceidcc  and  in  the 
Lamellaridcc.  In  Lamellaria,  the  shell  is 
cqmpletely  grown  over  by  the  mantle.  In 
Stilifer  among  the  Eulimidce  also,  the  shell 
is  more  or  less  covered  by  the  mantle. 

The  edge  of  the  mantle  may  be  fringed 
or  notched,  or  (Cypraeidce)  provided  with 
wart -like,  tentacular,  or  branched  ap- 
pendages. 


e.. 


2.  Pulmonata. 


In  the  Pulmonata,  the  arrange- 
ments of  the  mantle  fold  and  visceral 
dome  and  of  the  shell,  which  is  in- 
timately connected  with  them,  are  of 
great  interest.  We  have,  on  the  one 
hand,  forms  such  as  Helix,  with  large 
FIG.  54.  —  Testaceiia  haiiotidea  (after  protruding  spi  rally -coiled  visceral 

Lacaze-Duthiers).    A,  right  view ;  b,  enormous    dome  and  }  mantle  f  ^  enclosing 

pharynx  evaginated  through  the  buccal  cavity,  __;*._  ^  ^ 

carrying  on  its  surface  the  radula  (a) ;  c,  open- 
ing of  the  pharynx  into  the  oesophagus ;  d, 
position  of  the  genital  aperture  ;  e,  latero-dorsal 
groove  along  the  body ;  /,  latero- ventral  groove ; 
g,  mantle,  rudiment  of  the  visceral  dome.  B, 
dorsal  view  :  a,  b,  the  two  pairs  of  tentacles  ; 


c,   the  latero-ventral  groove  ;    <?,   the   latero- 
dorsal  groove ;  e,  shell. 


a  spacious  cavity ;  on  the  other, 
forms  such  as  Oncidium,  without 
distinct  visceral  dome  or  mantle  fold 
and  without  shell.  Between  these 
two  extremes  there  are  numerous 


transition  forms ;  indeed,  complete 
series  of  such  forms  may  be  found 

even  within  some  of  the  natural  divisions  of  the  Pulmonata.     The 

following  are  a  few  characteristic  types. 

Helix  (Figs.  12  A,  p.  9  ;  72,  p.  75). — The  visceral  dome  is  large  and  spirally 
coiled,  and  is  covered  by  a  spiral  shell  sufficiently  large  to  shelter  with  ease  the  whole 
body.  The  mantle  fold  covers  a  cavity  lying  anteriorly  to  the  visceral  dome  (pul- 
monary cavity).  Its  free  thickened  glandular  edge  unites  with  the  nuchal  integument 


vii    MOLLUSCA— INTEGUMENT,  MANTLE,  VISCERAL  DOME 


near  it  in  a  way  characteristic  of  the  Pulmonata,  leaving  only  one  aperture,  the 
respiratory  aperture — on  the  right.  (In  Pulmonata  whose  shells  have  the  sinistral 
twist,  the  respiratory  aperture  lies  to  the  left. )  The  apertures  of  the  hind-gut  and 
excretory  organ  are  close  to  the  respiratory  aperture,  through  which  their  excreta 
have  to  pass  out. 

In  many  species  of  the  genus  Vitrina,  the  shell  cannot  contain  the  whole  animal. 
The  mantle  fold  projects  in  front  of  the  shell,  and  has  a  process  which  is  bent  back 
over  the  shell,  and  is  used  for  cleansing  it. 

In  Daudebardia  (Helicophanta)  (Fig.  12  B,  p.  9)  the  visceral  dome  and  shell  are, 
in  comparison  with  the  rest  of  the  body,  much  smaller  than  in  Vitrina.  The  animal 
cannot  be  sheltered  by  the  shell.  The  visceral  dome  begins  to  be  levelled  down  to  a 
certain  extent,  disappearing  into  the  dorsal  surface  of  the  foot.  It  lies  far  back 
on  the  body,  the  respiratory  aperture  being  on  its  right  side. 

A  somewhat  similar  arrangement  is  found  in  the  genus  Homalonyx,  in  which  the 
low  visceral  dome  lies  on 
the  centre  of  the  back. 
The  respiratory  aperture 
lies  to  the  right  at  the  edge 
of  the  mantle.  The  edge 
of  the  flat  ear-shaped  shell 
is  fixed  into  the  mantle 
fold.  Daudebardia  and 
Ho'nialonyx  begin  to  look 
like  slugs. 

In   Testacella  (Figs.    54 


and  55)  a  visceral  dome 
hardly  exists.  The  only  re- 
mains of  it  is  a  small  mantle 
at  the  dorso-posterior  end 

of  the  body,  which  is  from  the  right  (after  Lacaze-Duthiers).  The  shell  is  removed  to 
covered  by  an  ear  -  shaped  show  the  rudimentary  visceral  dome,  a,  latero-dorsal  groove ;  ft, 
shell  Beneath  the  mantle  latero-ventral  groove  ;  c,  end  of  the  muscle  attached  to  the  shell ; 

e,  mantle  edge  of  the  visceral  dome  ;  g,  respiratory  aperture, 
lies   a   reduced   respiratory 

cavity.  The  respiratory  aperture  lies  to  the  right  posteriorly,  beneath  the  edge  of 
the  shell.  The  viscera  lie  dorsally  on  the  foot. 

The  common  terrestrial  snails  Umax,  and  Arion  (Fig.  12  D,  p.  9)  resemble 
Testacella  in  the  reduction  of  the  visceral  dome,  but  in  them  the  mantle  or  so-called 
shield  which  takes  its  place  lies  anteriorly  behind  the  head.  At  its  right  edge  lies 
the  respiratory  aperture.  In  Li  max  there  is  a  small  round  rudimentary  shell  which 
is  internal,  i.e.  it  is  entirely  enveloped  in  or  overgrown  by  the  mantle  fold.  In 
Arion  this  shell  is  represented  by  isolated  calcareous  granules.  In  Onchidium  and 
Vnyinulus  there  is  no  trace  of  a  visceral  dome,  nor,  in  the  adult,  of  a  shell.  The 
visceral  dome  has  to  a  certain  extent  spread  out  over  the  whole  dorsal  surface  of 
the  foot,  and  has  disappeared.  There  is,  further,  no  outwardly  recognisable  mantle 
fold  distinct  from  the  rest  of  the  dorsal  integument.  A  longitudinal  furrow  still 
divides  the  dorsal  part  of  the  body  from  the  foot.  The  respiratory  aperture  with 
the  anus  lie  posteriorly  in  the  median  line. 

In  the  genus  Physa  (Fig.  11,  p.  8),  the  edge  of  the  mantle  takes  the  form  of 
lobe-like  or  finger-shaped  processes,  which  bend  back  over  the  shell,  and  can  be 
applied  to  its  outer  surface.  In  Amphipeplea  (Fig.  10,  p.  8)  the  mantle  is  much 
widened  and,  when  bent  back  over  the  shell,  covers  all  but  an  oval  spot  on  the 
dorsal  side  of  the  last  coil. 

The  dorsal  integument  of  the  Onchidia  has  wart-like  protuberances  or  (in  Peronia) 


h  b 

FIG.  55.— Testacella  haliotidea,  posterior  portion  of  the  body 


46  COMPARATIVE  ANATOMY  CHAP. 

branched  appendages.  These  are  richly  supplied  with  blood-vessels,  and  serve  for 
respiration.  In  Peronia  there  are  besides  these  also  dorsal  prominences  which  carry 
eyes. 

The  dorsal  integument  projects  all  round  the  body  above  the  foot,  and  thus 
forms,  as  in  Chiton,  a  peripheral  zone,  which  is  ventrally  separated  from  the  foot 
by  a  groove.  In  Oncidiella  the  edge  of  this  zone,  i.e.  the  lateral  edge  of  the  body, 
is  dentate  or  fringed. 

3.  Opisthobranehia. 

The  typical  outer  organisation  of  the  Gastropoda  here  suffers 
even  more  varied  and  thorough  modification  than  in  the  Pulmonata. 
We  have,  on  the  one  hand,  forms  with  head,  foot,  visceral  dome,  shell, 
mantle  and  gill ;  on  the  other,  forms  which  possess  none  of  these 
organs  and  nevertheless  are  both  Gastropods  and  Opisthobranehia.  In 
one  principal  division  of  this  order,  the  Palliata  or  Tectibranchia,  the 
mantle  fold  is  retained  on  the  right  side  of  the  body,  and  partially 
covers  a  typical  Molluscan  ctenidium ;  in  other  divisions  both  mantle 
and  ctenidia  are  wanting.  We  do  not  here  apply  the  term  mantle  to 
the  fold  or  edge  of  the  dorsal  integument  which  surrounds  the  body 
at  the  part  where  the  head  and  foot  take  their  rise  ;  such  an  edge  is 
more  or  less  developed  in  most  Opisthobranehia  and  distinctly  marks 
off  the  foot  and  head  from  the  rest  of  the  body  or  back.  The  mantle 
here  means  only  the  broader  fold  which  covers  the  mantle  cavity,  in 
which  lies  a  typical  molluscan  gill.  The  edge  of  the  mantle  never 
forms  a  distinct  siphon  in  the  Opisthobranehia,  though  there  is  an 
approach  to  such  a  structure  in  the  Ringiculidce. 

(a]  Tectibranchia. 

(a)  fteptantia. — In  this  division  we  have,  on  the  one  hand,  forms  which  still 
have  a  distinctly  projecting  visceral  dome,  whose  integument  secretes  a  coiled  shell, 
into  which  the  whole  body  can  be  withdrawn.  On  the  other  hand,  forms  occur  in 
which  the  flattened  visceral  dome  has  spread  out  over  the  whole  dorsal  surface  of 
the  foot,  the  shell  being  rudimentary  and  internal.  Examples  of  the  former  are 
found  in  the  Cephalaspidce,  e.g.  the  Actaeonidce,  Tornatinidce,  and  some  Scaphandridce 
(Atys,  Cylichna,  Amphisphyra),  a  few  Bullidce  (Bulla),  and  the  Ringiculidce. 

In  Scaphander  among  the  Scaphandridce,  and  Acera  among  the  Bullidce,  the  body 
cannot  be  completely  withdrawn  into  the  shell. 

In  the  Cephalaspidce,  to  which  so  far  reference  has  been  made,  the  shell  is 
external. 

In  Gastropteron  the  mantle  is  rudimentary,  and  is  provided  posteriorly  with  a 
filiform  appendage.  It  covers  a  delicate  membranous  internal  shell,  into  which  the 
body  cannot  be  withdrawn.  The  same  is  the  case  in  Philine  and  Doridium,  where 
there  is  also  a  delicate  internal  shell  covering  only  a  small  portion  of  the  viscera  ;  this 
shell,  in  Doridium,  is  produced  in  the  form  of  two  lobes,  the  one  to  the  left  ending 
in  a  filiform  process. 

The  visceral  dome  in  the  Anaspidce  is  small  as  compared  with  the  size  of  the 
animal,  but  rises  distinctly  above  the  rest  of  the  body,  and  is  covered  by  a  thin 
inconspicuous  shell.  The  mantle  and  shell  often  only  partially  cover  the  gill.  In 
Aplysia,  the  shell  is  internal,  i.e.  it  is  entirely  overgrown  by  the  mantle  ;  in  Dola- 
bella,  this  enveloping  overgrowth  is  not  quite  complete,  as  a  circular  median  dorsal 


vii    MOLLUSCA— INTEGUMENT,  MANTLE,  VISCERAL  DOME    47 


aperture  is  left,  through  which  the  dorsal  surface  of  the  shell  is  visible.     The  mantle 
in  Dolabella  forms  a  small  anal  siphon  posteriorly. 

Notarchus  has  a  microscopically  minute  shell.     In  certain  species  of  this  genus, 
the   integument    forms    protuberances   or   delicately 
branched  appendages.  A 

In  the  Oxynoidea,  the  shell  is  only  partially  covered 
by  the  mantle,  and  is,  further,  much  too  small  to 
shelter  the  body. 

Among  the  Notaspidcc,  the  Umbrellidce  have  a 
small  flattened  cap-like  visceral  dome  lying  upon  the 
massive  foot.  The  visceral  dome  is  surrounded  by  a 
mantle  fold  which,  on  the  right  side,  covers  the  gill. 
The  integument  of  the  dome  and  mantle  is  covered 
by  a  flattened  disc-shaped  shell. 

In  Pleurobranchia,  the  visceral  dome  is  relatively 
large.  The  right  and  left  margins  project  as  short 
mantle  folds,  but  there  are  no  such  folds  to  the  front 
and  back,  so  that  at  these  latter  parts  the  flattened 
visceral  dome  is  not  distinct  from  the  rest  of  the  body. 
In  Pleurobranchus,  the  integument  of  the  flattened 
visceral  dome  broadens  out  into  a  large  fleshy  disc 
which  projects  on  all  sides  beyond  the  large,  broad - 
soled  foot ;  its  margin  (mantle  fold)  is  separated  from 
the  foot  by  a  deep  continuous  groove  running  right 
round  the  body  ;  in  this  groove,  to  the  right,  lies  the 
large  gill,  while  in  Pleurobranchus  a  small  flat  internal 
shell,  thin  and  membranous,  is  still  found  ;  in  related 
forms  this  may  be  wanting.  The  dorsal  integu- 
ment is  often  strengthened  by  a  layer  of  calcareous 
granules. 


Natantia. 


FIG.  56.— Diagrammatic  trans- 
verse sections  of  Gastropods,  to 
illustrate  the  arrangement  of  the 
shell  (black,  1),  visceral  dome  and 
Pteropoda  Thecosomata.—  The  Limacinidce  have    mantle  (dotted,  2),  and  foot  (streak- 
a  well -developed   visceral   dome  and  corresponding    ed' 3)'    A,  Prosobranchiate  with 

*       .  .  8    outer  shell  and  epipodmm  (4).    B, 

shell,  with  simstral  twist ;  the  shell  can  be  closed  by  Tectibranchiate  with  lobes  (6)  of 
means  of  a  typical  operculum.  The  mantle  fold  covers  the  mantle  turned  back  over  the 
a  cavity  which  lies  anteriorly  to  the  visceral  dome,  outer  surface  of  the  shell.  Dorsally 
The  anus  is  to  the  right.  The  animal  can  with-  the  shell  is  still  uncovered;  5,  para- 
i  •  A.  -j.  -i  n  T  it.  n  T  ••?  J.T.  i  podia:  7.  ctenidium.  C,  Tecti- 

draw  into  its  shell.     In  the   Cavohnnda  the  dome    {,ranchia'te  with  interiial  shell,  i.e. 
and  shell  are  bilaterally  symmetrical,  not  twisted,  and    completely  overgrown  by  the  lobes 
the  body  can  be  entirely  hidden  within  the   shell,    of  the  mantle. 
The    mantle    cavity    here    lies    posteriorly    to    the 

visceral  dome,  on  what  is  usually  called  its  lower  side.  The  symmetrical  shell  of 
the  Cymbuliidce  does  not  correspond  with  the  shell  of  other  Thecosomata  ;  it  is  a 
cartilaginous  "  pseudoconch  "  covered  with  body  epithelium.  In  the  Cymbuliidce 
the  mantle  cavity  also  lies  posteriorly.  We  shall  return  later  to  the  varying 
position  of  this  cavity  among  the  Thecosomata. 

The  mantle,  in  the  genus  Cavolinia,  shows  peculiarities  which  can  best  be  described 
in  connection  with  the  shell.  In  the  latter,  two  surfaces  are  distinguished,  a  slightly 
arched  anterior  surface  (usually  described  as  the  upper),  and  an  arched  posterior 
surface.  The  anterior  surface  projects  forwards  and  downwards  beyond  the 
posterior  for  a  third  of  its  length.  The  shell  has  three  slit-like  apertures,  one 


48  COMPARATIVE  ANATOMY  CHAP. 

anterior  and  ventral,  through  which  the  fin-like  processes  of  the  foot  can  be  protruded, 
and  two  lateral  apertures  stretching  far  up,  so  that  the  shell  appears  almost  bivalve. 

At  these  lateral  slits,  which  admit  water  to  the  mantle  cavity,  the  mantle  bends 
round  on  to  the  outer  surface  of  the  shell,  covering  the  greater  part  of  it ;  and,  at 
the  upper  angles  of  the  slits,  has  two  freely  projecting  processes. 

Pteropoda  Gymnosomata.— In  these,  the  long  outwardly  symmetrical  body  is 
naked  and  without  a  mantle,  and  the  foot,  which  is  much  reduced,  is  found  on 
the  ventral  side  of  the  most  anterior  part  of  the  body. 

(ft)  Ascoglossa  and  Nudibranchia. 

In  mature  Ascoglossa  and  Nudibranchia,  with  the  single  exception  of  the  Stegano- 
branchia,  a  shell  is  always  wanting,  as  also  a  distinctly  demarcated  visceral  dome. 
The  latter,  indeed,  spreads  out  over  the  whole  dorsal  surface.  The  dorsal  integu- 
ment, nevertheless,  forms  a  circular  fold  (mantle  fold)  separated  from  the  foot  by  a 
groove  sometimes  deep,  sometimes  shallow  ;  but,  except  in  the  Phyllidiidas,  no  gills 
lie  in  this  groove.  Where  this  groove  has  nearly  disappeared,  the  animals  strongly 
resemble  Planaria. 

Phyllidiidse.— In  these,  the  mantle  fold  is  distinct,  and  carries  on  its  lower 
surface,  to  the  right  and  left,  a  row  of  branchial  leaves,  herein  recalling  Patella  and 
Chiton. 

The  genus  Dermatobranchus,  which,  judged  by  its  organisation,  belongs  here, 
has,  however,  no  gills. 

Dorididse. — The  dorsal  integument  (notaeum),  which  here  covers  the  body  like 
a  shield,  being  generally  distinctly  demarcated  from  the  foot  and  the  head,  contains 
numerous  calcareous  particles,  which  give  it  a  firmer  consistency.  Anteriorly,  there 
are  two  feeler-like  processes,  the  rhinophores,  which  can  generally  be  withdrawn 
into  special  sheaths  or  pits;  these  are  not  to  be  confounded  with  the  tentacles. 
The  anus  lies  in  the  median  line,  generally  behind  the  middle  of  the  body,  and 
is  surrounded  by  an  ornamental  circlet  of  pinnate  gills.  The  noteeum  is  often 
covered  with  prominences,  and  in  some  genera  the  margin  carries  variously  shaped 
processes. 

Cladohepatica. — Here  there  are  no  anal  gills.  The  dorsal  integument  has 
variously  formed  and  variously  arranged  appendages  ;  these  may  be  conical,  club- 
or  finger-shaped,  lobate  or  branched  ;  they  are,  for  the  most  part,  very  striking  in 
colour  and  appearance.  Sacs  of  nematocysts  are  generally  found  at  their  tips,  and 
<53eca  of  the  intestinal  canal  (branches  of  the  digestive  gland)  penetrate  them.  These 
dorsal  appendages,  which,  like  the  rest  of  the  body,  are  ciliated,  have,  at  least 
partly,  a  respiratory  function.  In  many  forms  they  easily  fall  off,  and  are  later 
regenerated  (Fig.  18,  p.  12). 

Many  Cladohepatica  have  a  certain  external  likeness  to  Planaria  with  dorsal 
papillae  (Thysanozoon),  but  this  likeness  is  still  more  marked  in  the  following 
family  : — 

Ascoglossa. — Anal  gills  and  also,  as  a  rule,  dorsal  appendages  are  here  wanting. 
The  whole  body  is  naked  and  ciliated.  The  back  is  indistinctly  demarcated  from 
the  head. 

Phyllirhoe. — This  Nudibranchiate  genus,  of  all  Opisthobranchia,  shows  least  of 
the  typical  external  organisation  of  the  Mollusca.  The  body  here  is  naked  and 
laterally  compressed,  with  sharp  dorsal  and  ventral  edges.  It  has  neither  foot 
nor  gills  (Fig.  19,  p.  12). 


vii    MOLLUSC  A— INTEGUMENT,  MANTLE,  VISCERAL  DOME    49 


D.  Seaphopoda.     (Cf.  Review  of  Outer  Organisation,  p.  34.) 

E.  Lamellibranehia. 

From  each  side  of  the  body  there  typically  hangs  a  large  leaf-like 
mantle  fold  of  the  same  shape  as  the  shell-valve  formed  by  it.  These 
mantle  folds  project  beyond  the  body  in  front,  below,  and  behind,  and 
enclose  a  mantle  cavity  which  everywhere,  except  dorsally,  opens 
outward  by  means  of  the  slit  left  between  the  edges  of  the  folds. 
This  large  single  cleft  serves  for  the  admission  of  nourishment  and 
water  into  the  mantle  cavity,  and  for  the  expulsion  of  the  excreta, 
genital  products,  and  respired  water;  through  it  also  the  foot  is 
protruded.  Such  a  primitive  mantle  is  thus  completely  open,  its 
simple  edges  (i.e.  without  folds,  papillae,  tentacles,  or  eyes)  are  quite 
free,  coalescing  nowhere. 

The  above  serves  for  a  description  of  the  mantle  of  Nucula — 
one  of  the  Protobranchia — and  must  be  considered  as  the  primitive 
arrangement. 

In  most  Lamellibranehia,  however,  special  differentiations  of  the 
margin  of  the  mantle  occur ;  these  take  the  form  of  folds,  thickenings, 
protuberances,  papillae,  tentacles,  glands,  eyes,  etc.,  and  this  is  the 
case  both  in  forms  which  have  an  open  mantle  and  in  those  in  which 
the  mantle  is  partially  closed. 

The  partial  closing  of  the  mantle  is  brought  about  by  the  con- 
crescence at  one  or  more  points  of  the  free  edges  of  the  mantle 
folds. 

A.  A  completely  open  mantle,  i.e.  one  single  large  cleft  entirely  separating  the 
edges  of  the  mantle,  is  found : 

(a)  Among  the  Protobranchia  in  Nucula. 

(6)  Among  the  Filibranchia  in  the  Anomiidce,  Articles,  Trigoniidce,  and  a  few 
Mijtilidce  (Pinna). 

(c)  In  all  Pseudolamcllibranchia  except  Meleagrina. 

(d)  Among  the  Eulamellibranchia,  only  in  a  few  species  of  Crassatella. 

B.  The  mantle  folds  of  the  two  sides  grow  together  at  one  point.— In  this 
case  the  point  of  concrescence  almost  always  lies  high  up  posteriorly  ;  and  marks 
off  a  small  aperture  from  the  originally  simple  cleft.     This  aperture,  occurring  on 
a  level  with  the  anus,  forms  the  exhalent  or  anal  aperture  of  the  mantle.     Its  edge 
may  be  more  or  less  prolonged  posteriorly  to  form  an  anal  siphon,  which  can  be 
protruded  beyond  the  valves  of  the  shell. 

At  a  point  a  little  below  this  exhalent  aperture,  the  mantle  edges  usually  become 
applied  to  one  another,  although  no  concrescence  takes  place.  Above  this  point, 
between  it  and  the  anal  siphon,  they  separate  to  form  an  inhalent  or  branchial 
aperture.  The  edges  of  this  aperture  also  may  be  produced  posteriorly  into  a 
branchial  siphon,  which,  however,  in  this  case,  has  a  cleft  extending  along  the 
whole  of  its  lower  side,  which  is  a  continuation  of  the  large  cleft  of  the  mantle.  A 
branchial  siphon  formed  in  this  way,  by  mere  apposition  of  the  mantle  edges,  is  found 
in  the  genus  Malletia  among  the  Protobranchia. 

VOL.  II  E 


50 


COMPARATIVE  ANATOMY 


CHAP. 


An  anal  aperture,  separated  by  a  point  of  concrescence  from  the  large  mantle  cleft, 
is  found  in  the  following  Lamellibranchia  : 
(a)  Among  the  Protobranchia  in  Malletia. 
(6)  Among  the  FilibrancMa  in  most  MytUidcB. 

(c)  Among  the  Pseudolamellibranchia  in  the  Aviculidce  (genus  Meleagrina). 

(d)  Among    the   Eulamellibranchia,,  in   the    Carditidoe   (Venericardia,    Cardita 
Milneria),  the  Astartidce,  and  most  Crassatellidcje  ;  among  the  Oyrenidce,  in  the  genus 
Pisidium ;  among  the   Unionidce  in  the   Unionince  (Unio,  Anodonta)  ;  and  among 
the  Lucinacea,  in  Cryptodon  Moseleyi. 

In  Solenomya,  among  the  Protobranchia,  the  two  mantle  edges  grow  together 
only  at  one  point,  but  to  such  an  extent  as  to  close  the  whole  posterior  half  of  the 


FIG.  57.— Diagrams  to  illustrate  the  various  ways  in  which  concrescence  of  the  mantle 
and  formation  of  siphons  take  place  in  the  Lamellibranchia.  The  foot  (7)  protruded  forward 
through  the  mantle  cleft ;  A,  mantle  completely  open  ;  B,  mantle  open,  but  with  its  edges  applied 
to  one  another  at  two  points,  thus  giving  rise  to  incompletely  separated  anal  and  respiratory 
cavities  ;  C,  edges  of  the  mantle  grown  together  at  one  point  (1),  the  anal  or  exhalent  aperture  of 
the  mantle  (4)  is  separated ;  D,  edges  grown  together  at  two  points  (1,  2),  the  branchial  or  inhalent 
aperture  (5)  is  also  separated,  the  mantle  has  three  apertures  ;  E,  mantle  closed  by  the  extension 
of  the  place  of  concrescence  (2),  three  limited  apertures  remain,  viz.  the  anal,  branchial,  and  pedal 
apertures — the  first  two  are  produced  into  siphons;  F,  a  third  concrescence  (3)  takes  place. 
Mantle  with  four  apertures  (4,  5,  6a,  6&),  the  most  anterior  (6&)  for  the  protrusion  of  the  foot.  The 
siphons  have  united. 

ventral  mantle  opening.  In  this  way  the  mantle  cleft  is  divided  into  two  ;  the  anterior 
aperture  serves  for  the  protrusion  of  the  foot,  while  the  posterior  serves  at  the  same 
time  as  inhalent  (branchial)  and  exhalent  (anal)  aperture.  Solenomya  is  the  only 
bivalve  in  which  this  arrangement  is  found. 

C.  The  mantle  folds  grow  together  at  two  points,  thus  forming  three 
apertures. — This  condition  arises  in  consequence  of  the  complete  separation  (through 
concrescence)  of  the  branchial  aperture  from  the  rest  of  the  large  anterior  mantle 
cleft.  The  anal  and  branchial  apertures  may  remain  as  slits,  or  may  be  produced 
into  longer  or  shorter  anal  and  branchial  siphons.  The  large  anterior  and  ventral 
mantle  cleft  serves  for  the  protrusion  of  the  foot,  and  is  called  the  pedal  cleft. 
These  two  points  of  concrescence  are  found : 

(a)  Among  the  Protobranchia,  in  Yoldia  and  Lcda. 


viz    MOLLUSC  A— INTEGUMENT,  MANTLE,  VISCERAL  DOME    51 

(b)  In  most  Eulamellibranchia,  viz.  in  most  Lucinidce,  most  Cyrenidce ;  among 
the   Unionidce,  in  the  Mutelince,  in  the  Donacidce,  Psammobiidce,  Tellinidce,  Scrobi- 
culariidce  ;  among  the  Vcneracea,  in  the  Vcneridce,  in  the  Cardiidce,  the  Mactridce, 
Mcsodcsniat.idce,  and  the  Solenidce  (excepting  Solen  and  Lutraria). 

(c)  In  all  Septibranchia  (Poromyia,  Cuspidaria). 

In  the  above  forms  the  mantle  is  still  wide  open,  i.e.  the  points  of  concrescence 
are  small  and  local.  But  these  points  may  become  lines  of  concrescence  of  con- 
siderable length.  In  the  CJiamacea,  for  example,  and  especially  in  the  Tridacnidce 
among  the  Eulamellibranchia,  the  three  apertures  of  the  mantle  are  found  at  con- 
siderable distances  from  one  another,  being  divided  by  long  intervals  where  the 
edges  have  grown  together. 

In  some  groups  of  Lamellibrauchia,  the  concrescence  between  the  anal  and 
branchial  apertures  or  siphons  remains  short,  i.e.  the  one  aperture  lies  directly 
below  the  other,  but  in  such  cases  the  edges  anterior  to  the  branchial  aperture 
grow  together  to  a  greater  extent,  so  that  the  pedal  aperture  becomes  reduced  to 
a  small  anterior  fissure.  In  this  condition  the  mantle  is  closed.  Such  a  mantle  is 
found  : 

Among  the  Eulamellibranchia  in  the  Modiolarca,  Dreissensia,  Petricola,  all 
PholadidcG  (Pholas,  Pholadidea,  Jouannetia,  in  which  the  pedal  aperture  is  said 
to  close  entirely  in  old  animals,  Xylophaga,  Martesia) ;  in  the  Teredinidce,  and 
among  the  PandoridcB,  Pandora,  the  Vcrticordiidce  and  Ly&nsiidce  (Anatinacea). 

D.  There  are  some  Lamellibranchia  with  closed  mantle,  in  which  a  fourth 
aperture  is  added  to  the  three  found  in  the  above  groups,  the  mantle  thus  having 
three  points  of  concrescence.  The  fourth  aperture  is  always  small,  and  is  found 
between  the  pedal  and  branchial  apertures  ;  it  probably  corresponds  with  a  rudi- 
mentary fissure  for  the  byssus. 

This  arrangement  is  found  in  the  Eulamellibranchia ;  among"  the  Solenidce,  in 
Sole/i  and  Lutraria ;  among  the  Pandoridce,  in  Myochama  ;  in  Glycymeris ;  among 
the  Anatinacea,  in  the  genus  Thracia ;  in  the  Pholadomyidce  and  the  ClavageZlidce 
(Clavagella  and  Brechites  [Aspergillum'])  ;  and,  finally,  in  Lyonsia  norvegica. 

The  anal  aperture  is  often  and  the  branchial  aperture  nearly  always 
fringed,  or  in  various  ways  edged  with  protuberances,  papillae,  or  ten- 
tacles, and  this  is  the  case  whether  these  apertures  are  found  on  the 
edge  of  the  mantle  or  at  the  ends  of  (longer  or  shorter)  siphons. 

The  siphons  can  be  contracted  and  extended,  and  either  wholly 
or  partly  withdrawn  into  the  shell,  by  means  of  special  muscles. 
These  muscles  are  attached  on  the  inner  surface  of  the  shell-valves  to 
the  right  and  left  posteriorly,  and  their  line  of  attachment  forms  the 
pallial  sinus,  which  will  be  described  later  on. 

The  length  of  the  siphons  varies  greatly.  Specially  long  siphons  are  found  in 
the  Mactridce,  Donacidce,  Psammobiidce,  Tellinidce,  Scrobiculariidce,  many  Veneracea 
and  Cardiidce,  the  Mesodesmatidce,  Lutraria,  the  Pholadidce,  Teredinidce,  Anatinidce, 
and  Clavagclliilo:. 

The  siphons  may  be  separated  throughout  their  whole  length,  and  often  diverge 
one  from  the  other  (e.g.  Galatea  among  the  Cyrenidce,  the  Donacidce,  Psammobiidce, 
TcUinidce,  Scrobicularidcc  (Fig.  58),  Mesodesmatidce,  Phancs,  etc.). 

In  other  forms  they  coalesce  along  their  entire  length  ;  they  may  even  look  like 
a  single  tube,  which  is,  however,  always  internally  di \dded  into  an  upper  (anal)  and 
a  lower  (branchial)  channel.  This  common  siphon  is  sometimes  protected  by 
a  special  sheath  of  ,epidermis,  particularly  in  those  forms  in  which  it  cannot  be 


52  COMPARATIVE  ANATOMY  CHAP. 

withdrawn  into  the  shell.  Siphons  united  throughout  their  whole  length  are  found 
in  the  Mactridce,  a  few  Veneracea,  Lutraria,  Solenocurtus,  Solen,  the  Pholadidce, 
many  Anatinidce,  and  the  Clavagellidce. 

In  some  cases,  siphons  which  are  united  for  some  distance  at  the  base,  separate 
near  their  ends  and  even  diverge,  e.g.  in  Petricola  among  the  Vemracca,  Teredo,  etc. 

The  two  siphons  are  often  of  unequal  length.  In  the  Modiolaria  (Mytilidw)  only 
one,  the  anal,  is  developed,  while  the  branchial  aperture  remains  unseparated  from 
the  large  mantle  cleft.  The  reverse  is  the  case  in  Dreissensia  and  Scrobicularia, 
where  the  branchial  siphon  is  much  longer  than  the  anal. 

The  siphons  are  sometimes  provided  with  valves  ;  these  occur  more  often  in  the 
anal  than  in  the  branchial  siphon. 

Significance  of  the  development  of  the  Anal  and  Branchial  Apertures 
and  Siphons. 

Most  Lamellibranchia  inhabit  mud  or  sand,  into  which  they  sink  the  anterior  part 
of  the  body,  burrowing  by  means  of  the  protrusible  foot.  The  water  necessary  for 
bathing  the  gills  and  for  respiration  can  only  be  received  into  and  expelled  from  the 
mantle  cavity  through  the  cleft  at  the  posterior  end  of  the  body  which  projects  above 
the  mud.  The  fcecal  masses  from  the  anus  near  this  point  must  also  here  be  ejected 
from  the  cavity.  The  development  of  localised  inhalent  and  exhalent  apertures  is 
explained  by  the  fact  that  a  constant  regulated  stream  of  water  into  and  out  of  the 
mantle  cavity  is  necessary  both  for  respiration  and  for  conducting  particles  of  food 


Fio.  58.— Scrobicularia  piperata  buried  in  mud.    The  inhalent  siphon  takes  in  mud  as 
nourishment ;  the  anal  siphon  stands  erect  (after  Meyer  and  Mobius). 

to  the  mouth.  The  most  advantageous  point  for  the  exhalent  aperture  is  obviously 
directly  behind  the  anus. 

Siphons  attain  development  in  consequence  of  the  habit  of  life  of  many  bivalves, 
which  bury  themselves  deep  in  mud,  sand,  wood,  and  even  rock.  By  means  of  their 
siphons  they  can  still  remain  connected  with  the  water  which  bathes  the  surface  of 
their  place  of  concealment,  and,  as  long  as  the  animals  remain  undisturbed,  a  con- 
stant current  enters  the  mantle  cavity  through  the  branchial  and  leaves  it  through 
the  anal  siphon. 

Where  the  mantle  folds  have  grown  together  to  a  large  extent  (closed  mantle)  the 
siphons  are  always  well  developed.  Such  closing  of  the  mantle  is  found  principally 
among  bivalves  which  bore  into  wood,  clay,  rock,  etc.,  and  in  which  the  foot  of  the 
adult  is  weakly  developed,  or  altogether  rudimentary.  The  degeneration  of  the  foot 
leads  to  the  shortening  of  the  pedal-  aperture  which  originally  served  for  its  pro- 
trusion. 

The  mantle  is  found  completely  open  with  only  slightly  developed  anal  and 
branchial  apertures  or  none  at  all,  in  bivalves  which  do  not  burrow,  but  live 
surrounded  by  water,  either  attached  to  the  bottom  or  lying  freely  on  it. 


vii    MOLLUSC  A— INTEGUMENT,  MANTLE,  VISCERAL  DOME    53 

In  such  animals  the  surrounding  water  can  circulate  through  the  usually  open 
mantle  cleft  and  the  mantle  cavity.  We  here  find  protuberances,  papillae,  tentacles, 
etc.,  carrying  sensory  organs,  all  along  the  free  edges  of  the  mantle,  whereas,  in 
bivalves  which  inhabit  mud  or  bore  into  wood,  rock,  etc.,  such  organs  are  mostly 
found  massed  together  round  the  edges  of  the  branchial  and  anal  apertures. 

The  Edge  of  the  Mantle. 

The  edge  of  the  mantle  often  forms  a  number  of  diverging  folds,  which  in  trans- 
verse section  look  like  finger-shaped  processes.  The  outermost  fold  is  always  applied 
to  the  shell.  The  edge  of  the  mantle  may  also  be  beset  with  one  or  more  rows  of  pro- 
tuberances, papillae,  or  tentacles,  and  often  contains  unicellular  or  multicellular 
glands,  mucous  glands,  and  others  which  have  been  considered  to  be  poison  glands  for 
protective  purposes.  Tactile  sensory  cells  are  very  common.  Eyes  are  rarely  developed 
on  the  edge  (cf.  section  on  the  Sensory  Organs). 

In  the  Pectinidce,  Spondylidce,  and  Limidce,  the  inner  fold  of  the  mantle  has  a 
somewhat  broad  border,  which,  when  the  shell  is  open,  projects  from  the  mantle 
towards  the  median  line  of  the  body  (Fig.  23,  p.  16).  The  free  opposite  edges  of 
these  folds  (flaps,  or  curtains),  springing  from  right  and  left,  may  meet  in  such  a 
way  as  to  shut  off  the  central  part  of  the  mantle  cavity  even  while  the  shell  is  open, 
apertures  only  remaining  anteriorly  and  posteriorly. 


F.  Cephalopoda. 

Integument. 

The  integument  of  the  Cephalopoda  consists  of  an  outer  cylindrical 
epithelium,  and  a  subjacent  cutis  in  the  form  of  thick  connective  tissue. 
In  this  cutis,  not  far  removed  from  the  epidermis,  and  above  a  layer 
of  connective  tissue  plates  (which  are  refractive  and  often  shimmer  like 
silver),  there  are  large  pigment  cells  or  ehromatophores  which,  by  their 
alternate  contraction  and  expansion,  bring  about  the  well-known  changes 
of  colour  in  these  animals. 

These  ehromatophores  are  single  cells  containing  yellow,  brown,  black,  violet  or 
carmine  pigment,  either  as  fluid  or  in  small  granules.  The  layers  containing  them 
are  either  single  or  double  ;  in  the  latter  case,  the  colour  of  the  pigment  in  the  one 
layer  of  ehromatophores  differs  from  that  of  the  ehromatophores  in  the  other. 
Radial  fibres,  arising  from  the  surrounding  connective  tissue,  are  attached  to  each 
chromatophore,  round  that  equator  which  lies  parallel  to  the  integument.  The 
ehromatophores  are  enveloped  in  a  special,  possibly  elastic,  membrane,  and  when 
contracted  are  almost  globular  ;  the  pigment  corpuscles  are  then  crowded  together. 
The  ehromatophores  expand  equatorially,  diminishing  the  distance  between  their 
poles,  i.e.  they  become  much  flattened.  In  this  condition,  they  may,  further,  throw 
out  fine  branches,  the  pigment  granules  being  thus  spread  out  over  a  large  surface. 
It  was  formerly  believed  that  the  expansion  of  the  ehromatophores  was  caused  by  the 
contraction  of  the  radial  fibres,  which  were  thought  to  be  muscular,  but  more  recent 
investigations  have  shown  the  fibres  to  be  of  the  nature  of  connective  tissue.  The 
changes  of  colour,  which  are  of  great  physiological  and  biological  interest,  and  which 
are  partially  under  the  control  of  the  animal,  are  brought  about  by  the  alternate 
contraction  and  expansion  of  these  variously  coloured  chromatophores 


54  COMPARATIVE  ANATOMY  CHAP. 


Mantle,  Visceral  Dome. 

Some  of  the  most  important  points  connected  with  the  mantle  and  visceral  dome 
have  already  been  mentioned  (pp.  36-38). 

In  Nautilus,  the  body  is  attached  right  and  left  to  the  inner  surface  of  the 
shell  of  the  last  or  inhabited  chamber  by  powerful  muscles,  which  may  make 
a  slight  impression  on  the  shell.  Between  the  points  of  attachment  of  these 
lateral  muscles,  the  integument  of  the  visceral  dome  coalesces  with  the  inner  surface 
of  the  shell  of  the  inhabited  chamber  in  a  narrow  circular  zone,  so  that  the  gas,1 
enclosed  in  the  upper  chambers  of  the  shell  cannot  escape.  While  the  integu- 
ment and  mantle  beneath  this  zone  of  concrescence  (i.e.  towards  the  free  aperture 
of  the  last  chamber)  are  rough,  fleshy,  and  muscular,  the  integument  of  that 
portion  of  the  visceral  dome  which  lies  above  the  zone  and  is  applied  to  the  last 
septum  is  delicate  and  soft.  The  siphuncle,  which  arises  at  the  dorsal  end  of  the 
visceral  dome  and  passes  through  all  the  septa,  is  membraneous  and  hollow  and  filled 
with  blood.  It  is  said  to  communicate  with  the  pericardium.  In  the  female  Nau- 
tilus, the  nidamental  gland  (see  Genital  Organs,  p.  241)  lies  in  the  free  mantle  fold, 
near  the  point  at  which  it  separates  from  the  visceral  dome.  "VVe  thus  have  parts 
which  usually  lie  in  the  visceral  dome  wandering  into  the  mantle  fold. 

Among  the  Dibranchia,  which  are  good  swimmers,  fins  are  found.  In  the 
Octopoda,  which  are  distinguished  by  the  round,  compact  form  of  the  visceral  dome, 
these  are  wanting,  except  in  the  remarkable  genus  Cirrhoteuthis.  Fins  are  universal 
among  the  Decapoda,  and  vary  much  in  form,  size,  and  arrangement. 

In  Sepia  (Fig.  80,  p.  83)  and  Sepioteuthis,  the  fins  are  inserted  on  the  lateral 
edges  of  the  body,  along  the  whole  height  (length)  of  the  visceral  dome,  forming  the 
boundary  between  the  anterior  and  posterior  (physiologically  the  dorsal  and  ventral) 
surfaces  of  the  latter.  In  Eossia,  Sepiola,  and  Sepioloidea  they  'are  almost 
semicircular,  and  are  like  distinct  appendages  situated  on  the  anterior  surface  of 
the  dome,  about  half-way  up  it.  This  is  also  the  case  in  Cirrhoteuthis,  where  the 
more  or  less  circular  fin-lobes  rise  from  the  body  on  stalk -like  bases. 

The  triangular  or  semicircular  fins  of  Cranchia,  Histioteuthis,  Onychoteuthis,  Loligo 
(Fig.  34,  p.  23),  Loligopsis,  Ommastrephes,  etc.,  are  found  at  the  dorsal  end  of  the 
visceral  dome,  on  its  anterior  side. 

In  many  Dibranchia,  there  is  a  concrescence  of  the  free  edge  of  the  mantle  fold 
with  the  integument  of  the  "  head"  (Kopffuss),  which  lies  below  it.  This  connec- 
tion is  effected  by  means  of  a  muscular  band,  which  passes  over  the  neck  (nuchal 
band).  In  most  Decapoda,  this  connection  is  wanting,  and  the  edge  of  the  mantle 
is  free  all  round  the  body  ;  the  exceptions  are  the  genera  Sepiola,  Cranchia,  and 
Loligopsis,  which  have  a  narrow  connection  of  this  sort.  All  Octopoda  have  this 
concrescence,  commencing  with  the  Argonauta ;  it  lengthens  in  Philoncxis  and 
Octopus,  till  in  Cirrhoteuthis  it  spreads  to  the  posterior  surface  (physiologically  the 
ventral  surface),  so  that  the  edge  of  the  mantle  remains  free  only  at  the  aperture 
through  which  the  funnel  or  siphon  is  protruded. 

Arrangements  for  fastening  the  mantle  fold  to  the  adjacent 
body  wall  are  very  common.  Such  attachment  is  either  temporary 
or  permanent.  In  the  former  case,  there  are  prominences  with  cor- 
responding depressions  for  locking  the  mantle  (appareil  de  resistance) ; 
in  the  latter  case,  dermal  or  muscular  fusions  take  place  between  the 
mantle  and  body  wall. 

1  Cf.  note,  p.  37. 


vir  MOLLUSCA— THE  SHELL  55 

1.  Apparatus  for  locking  the  mantle. — These  are  paired  or  unpaired.    The  former 
are  to  be  found  on  the  posterior  side  of  the  body,  in  the  mantle  cavity,  near  its 
lower  end  ;  they  lie   to  the  right  and  left  at  the  base  of  the  funnel,  and  on  the 
corresponding  points  of  the  inner  surface  of  the  mantle  fold.     The  unpaired,  on 
the  contrary,  are  found  on  the  anterior  surface  of  the  neck.     Since  all  the  arrange- 
ments serve  the  purpose  of  cutting  off  the  mantle  cavity  from  the  external  medium, 
it  is  easy  to  see  that  their  development  is  in  inverse  ratio  to  the  extent  of  the  con- 
crescence of  the  edge  of  the  mantle  round  the  neck  before  mentioned.     Where  no 
concrescence  is  found,  as  in  Sepia,  the  arrangements  for  locking  the  mantle  are 
highly  developed  ;  while,  where  the  line  of  concrescence  is  very  long,  as  in  Octopus, 
the  locking  apparatus  may  be  altogether  wanting.     The  locking  apparatus  consists, 
in  general,  of  cartilaginous  prominences  (often  accompanied  by  depressions)  on  the 
inner  surface  of  the  mantle  fold,  i.e.  the  surface  turned  towards  the  mantle  cavity, 
which  exactly  fit  corresponding  cartilaginous  depressions  accompanied,  as  the  case 
may  be,  by  prominences,  on  the  opposite  body  wall  (cf.  Fig.  80).     The  special  forms 
of  the  mantle  and  nuchal  locking  cartilages  are  of  importance  in  classification. 

The  cartilaginous  arrangements  for  locking,  which  are  almost  always  found  in  the 
Decapoda  (they  are  wanting  only  in  Oicenia,  and  Cranchia),  are  still  retained  in  a 
few  Octopoda  in  the  form  of  more  or  less  modified  fleshy  processes  (Argonauta, 
Tremoctopus}.  The  nuchal  locking  apparatus  is  the  first  to  disappear  on  the  rise  of 
the  pallio-nuchal  concrescence.  It  has  disappeared  among  the  Decapoda,  in  the 
genus  Sepiola,  where  the  mantle  is  firmly  attached  to  the  neck. 

2.  Permanent  connections  between  the  mantle  fold  and  the  adjacent  body  wall 
traversing  the  mantle  cavity  are  found  only  in  those  Cephalopods  which  have  no 
locking  apparatus.     Thus  in  Octopus  and  Elcdone  the  mantle  is  attached  to  the  body 
wall  by  means  of  a  median  muscle  above  the  funnel.     This  muscle  consists  of  two 
closely  -  applied  lamells,  having  the  anus  between  them.      In  Cranchia  the   free 
dorsal  edge  of  the  funnel  (at  its  so-called  base)  has  become  united  by  an  integu- 
mental  band  on  the  right  and  left  with  the  mantle  fold,  and  a  similar  arrangement 
is  found  in  Loligopsis. 

Water  pores. — Near  the  mouth,  or  at  the  bases  of  the  arms,  or  laterally  on  the 
head,  in  many  Cephalopods,  there  are  apertures  leading  to  integumental  pouches  of 
varying  size.  The  function  of  these  organs  is  unknown. 


IV.  The  Shell. 

General. 
The  Shape  of  the  Shell,  and  its  Relation  to  the  Soft  Body. 

All  the  various  forms  of  shell  found  in  the  Mollusca  are  deducible 
from  a  cup-  or  plate -like  shell  covering  the  dorsal  region.  Such  a 
shell  affords  sufficient  protection  for  animals  such  as  Fissurella,  Patella, 
etc.,  which  can  firmly  and  almost  immovably  attach  themselves  to  a 
hard  surface  by  the  sucker-like  foot.  The  soft  body  is  in  this  case 
protected  on  one  side  by  the  shell,  and  on  the  other  by  the  surface 
of  attachment.  Free  -  moving  Mollusca,  however,  show  a  tendency 
to  protect  the  whole  body  exclusively  by  means  of  their  shells,  and 
this  object  is  attained  in  various  ways. 

In  the  Chitonidce,  for  instance,  the  shell  is  made  up  of  consecutive 


56  COMPARATIVE  ANATOMY  CHAP. 

joints,  overlapping  in  such  a  way  as  to  be  movable  one  upon  the  other. 
This  segmented  shell  can  protect  the  whole  body,  since  it  allows  the 
Chiton  to  roll  up  like  an  Armadillo  or  a  Wood-louse. 

In  the  Lamellibranchia,  the  protection  of  the  whole  of  the  soft  body 
is  provided  for  by  the  development  of  a  bivalve  shell,  from  which  the 
foot  can  be  protruded,  and  which,  by  the  closing  of  its  two  valves, 
completely  envelops  the  soft  body  as  well  as  the  retracted  foot. 

In  the  Gastropoda,  Scaphopoda,  and  Cephalopoda,  the  most  complete 
protection  on  all  sides  of  the  body  by  means  of  the  shell  is  attained 
on  another  plan.  The  shell  becomes  much  elongated  and  turret- 
like,  and  is  thus  so  capacious  that  not  only  the  visceral  dome  but  the 
head  and  foot  also  can  find  place  in  it.  Even  the  only  remaining 
unprotected  aperture,  the  one  weak  point  of  this  fortification,  can  very 
often  be  completely  closed  by  a  hard  operculum. 

A  long,  turret-like  shell  is  an  inconvenient  burden  for  a  freely 
moving  animal,  being,  in  consequence  of  its  large  surface,  a  hindrance 
to. locomotion.  A  reduction  of  the  surface  is  brought  about  in  the 
Gastropoda  and  Cephalopoda  by  the  coiling  of  the  shell,  either  in  one 
plane  or  in  a  conical  spiral. 

In  the  latter  case  the  spiral  twist  is  almost  always  right-handed  or 
dextral. 

Iu  order  to  decide  the  direction  of  the  twist,  the  shell  should  be  held  in  such  a 
manner  that  the  point  of  the  spiral  is  uppermost,  while  the  aperture  is  directed 
downwards  and  towards  the  observer  (Fig.  60,  p.  60).  If,  in  this  position,  'the 
aperture  lies  on  the  right  of  the  axis,  the  shell  has  a  dextral  twist  if  to  the  left  its 
twist  is  left-handed  or  sinistral. 

We  have  a  striking  and  in  most  cases  unexplained  phenomenon  in 
the  reduction  and  even  complete  disappearance  of  the  shell,  which  takes 
place  not  only  in  nearly  all  the  classes,  but  even  within  some  of  the 
smaller  groups  of  Molluscs,  e.g.  the  Solenogastres  among  the  Ampliineura, 
a  few  Heteropoda  and  Titiscania  among  the  Prosobranchia,  many  Pul- 
monata,  very  many  Opisthobranchia,  and  most  extant  Cephalopoda. 

In  almost  all  cases  the  forms  in  which  the  shell  is  rudimentary  or 
wanting  can  be  shown  to  be  derived  from  forms  in  which  it  is  well 
developed.  All  shell-less  snails  (slugs)  have  shells  in  the  early  stages 
of  their  development. 

The  process  of  the  gradual  reduction  of  the  shell  to  a  rudiment, 
which  will  be  more  fully  described  later  on,  is  often  as  follows  :  (1) 
the  shell  becomes  internal ;  (2)  it  decreases  in  size,  so  that  it  no 
longer  can  cover  the  body  ;  (3)  the  visceral  dome  disappears ;  (4)  the 
shell  is  only  to  be  found  in  the  form  of  isolated  calcareous  particles 
in  the  dorsal  integument ;  (5)  even  these  vanish,  and  the  shell  is  only 
to  be  found  in  the  embryo. 

Only  in  a  few  cases  is  it  possible  distinctly  to  recognise  the  reason 
or  the  advantage  of  this  reduction  of  a  protective  covering  so  useful 
to  and  exercising  so  profound  an  influence  on  the  organisation  of  the 


viz  MOLLUSCA— THE  SHELL  57 

whole  race.  The  following  are  a  few  cases  in  which  the  utility  of 
the  reduction  of  the  shell  in  adaptation  to  special  conditions  is  to 
some  extent  evident:  (1)  In  free-swimming  marine  forms,  where  the 
shell  is  too  heavy  and  increases  friction ;  (2)  in  Testacella  and  allied 
forms,  which  prey  upon  earthworms,  where  a  large  shell  would  prevent 
them  from  following  their  prey  into  narrow  holes  and  passages ; 
(3)  in  Gastropods,  which  browse  among  thick  tangles  of  Corals, 
Bryozoa,  Hydroida,  or  Algae  (e.g.  many  Nudibranchia). 

The  loss  of  the  shell  is  generally  followed  by  compensatory 
adaptations  for  protection,  such  as  great  capacity  for  regeneration, 
especially  of  the  easily  detachable  appendages,  voluntary  amputation  of 
portions  of  the  body,  stinging  cells,  and  colouring  which  may  be 
protective  in  various  ways. 

The  carnivorous  Cephalopods  are  protected  (1)  by  their  extraordinary 
swimming  powers,  which  are  in  keeping  with  their  highly  developed 
organisation;  (2)  by  their  well-developed  sight ;  (3)  by  great  muscular 
strength ;  (4)  by  strong  jaws ;  (5)  by  the  discharge  of  the  secretion 
of  the  ink-bag;  (6)  by  their  partly  mimetic  changes  of  colour,  etc. 

Certain  peculiarities  of  organisation,  which  can  only  be  under- 
stood as  remains  of  a  shelled  condition  (e.g.  the  lateral  position  of  the 
genital  and  renal  apertures  and  also  to  some  extent  of  the  anus  in  the 
Nudibranchia),  always  persist  after  the  shell  has  disappeared. 

Chemical  Composition  of  the  Shell. 

The  shell  of  the  Mollusca  consists  principally  of  carbonate  of  lime,  with  traces 
of  phosphate  of  lime  and  of  an  organic  substance  related  to  chitin, — conchyolin. 
Besides  these,  various  colouring  matters  may  occur. 

Structure  of  the  Shell. 

The  shell  of  the  LamelMbrmichia  consists  of  three  layers,  the  innermost  layer 
being  applied  to  the  surface  of  the  mantle.  The  shell  is  to  be  regarded  as  a 
cuticular  structure. 

The  outer  layer  (shell -integument,  epidermis,  cuticle,  periostracum)  is,  so  far  as 
its  physical  constitution  is  concerned,  horny  and  wanting  in  lime.  It  generally 
disappears  off  the  older  portions  of  the  shell. 

The  middle  layer  (columnar,  prismatic,  or.  porcelanous  layer)  consists  of  slender 
prisms  of  carbonate  of  lime,  usually  perpendicular  to  the  surface  of  the  shell  and 
closely  crowded  together. 

The  inner  (nacreous)  layer  has  a  finely  lamellated  structure.  The  very  delicate 
transparent  lamiuge  of  which  it  is  composed  are  thrown  into  slight  waves  ;  these 
cause  the  wavy  lines  on  that  surface  of  the  shell  which  lies  on  the  mantle,  which, 
by  interference,  produce  the  characteristic  nacreous  lustre.  The  pearls  of  the  pearl 
oyster  are  formed  of  the  same  substance  as  this  layer. 

The  constitution  of  these  three  layers  varies  greatly  in  details  both  in  the  Lamelli- 
branchia  and  in  other  Mollusca.  The  outer  and  middle  layers  are  formed  at  the  free 
margin  of  the  mantle,  the  inner  layer  is  yielded  by  the  epithelium  of  its  whole  outer 
surface. 

The  shell  in  the  Gastropoda  and  Cephalopoda  consists  principally  of  the  middle 


58  COMPARATIVE  ANATOMY  CHAP. 

or  porcelain  layer,  which,  however,  has  a  structure  very  different  from  that  of  the 
same  layer  in  the  Lamellibranchia.  This  layer  is  generally,  if  not  always  (at  least 
in  the  young),  covered  by  a  periostracum.  The  inner  (nacreous)  layer  is  very  often 
wanting. 

Growth  of  the  Shell. 

In  the  Arthropoda,  the  chitinous  exoskeleton,  which  we  may 
compare  with  the  Molluscan  shell,  develops  at  the  surface  of  the 
whole  body  and  its  appendages.  This  skeleton,  when  once  formed 
and  hardened,  encases  the  body  on  all  sides  within  fixed  boundaries, 
and  is  incapable  of  growth.  Hence  the  moults  of  the  Arthropoda,  by 
which  alone  growth  becomes  possible. 

The  Molluscan  shell,  on  the  contrary,  is  open.  In  the  Gastropoda 
and  Cephalopoda,  it  assumes  the  shape  of  a  conical  mantle,  coiled  round 
a  single  axis  and  open  at  the  base  of  the  cone.  By  continual  additions 
at  the  edge  of  its  aperture,  it  grows  with  the  growth  of  the  animal, 
without  materially  altering  its  form.  The  lines  on  the  surface  of  the 
shell  of  the  adult  snail  register  its  phases  of  growth.  During  growth, 
the  oldest,  uppermost  coils  or  whorls  of  the  shell  either  continue  to  be 
filled  by  the  apex  of  the  visceral  dome  (in  many  Gastropods),  or  are 
deserted  by  the  animal  which,  as  the  shell  grows,  withdraws  farther 
and  farther  from  its  tip.  These  whorls  may  remain  empty,  or  may  be 
partially  or  completely  filled  with  shell  substance.  In  the  latter  case, 
they  may  be  successively  thrown  off.  The  Nautilus  and  allied  forms, 
during  growth,  periodically  form  transverse  septa,  so  that  the  forsaken 
parts  of  the  shell  become  chambered  and  filled  with  gas,1  the  animal 
occupying  the  largest  and  last-formed  chamber,  which  opens  externally. 
In  the  Lamellibranchia,  the  growth  of  the  shell  keeps  pace  with  the 
growth  of  the  body  in  exactly  the  same  manner,  the  free  edge  of  the 
shell  valve  continually  receiving  additions  of  shell  substance  from  the 
edge  of  the  mantle  to  form  the  periostracum  and  the  prismatic  layers, 
while  the  whole  external  surface  of  the  mantle  yields  an  additional 
nacreous  layer.  The  consecutive  phases  of  growth  are  here  also 
registered  by  the  concentric  markings  on  the  surface  of  the  shell. 

Special. 
A.  Amphineura.     (Of.  pp.  39-42.) 

B.  Gastropoda. 

A  few  details  concerning  the  shell  of  the  Gastropods  must  here  be  added.  As  a 
rule,  the  shell  is  coiled  spirally  round  an  axis.  This  spiral  is,  in  rare  instances,  so 
flattened  that  the  coils  come  to  lie  almost  in  one  plane,  giving  rise  to  a  nearly 
symmetrical  shell  (e.g.  Planorbis). 

There  are,  however,  among  the  Gastropoda,  uncoiled  shells  which  are  symmetrical, 
and  these  require  special  attention.  The  most  important  are  the  cup-shaped  or  more 
or  less  bluntly  conical  shells  of  the  Patellidce  and  Fissurcllidce.  Since  (1)  we  derive 

1  Of.  note,  p.  37. 


VII 


MOLL&8CA—THE  SHELL 


59 


the  Gastropoda  from  bilaterally-symmetrical  ancestors  with  symmetrical  shells  ;  and 
since  (2)  the  Fissurellidcc  undoubtedly  possess  the  most  primitive  organisation  of  all 
the  Gastropoda,  and  thus  stand  nearest  to  the  racial  form,  and  are  moreover  (3) 
strikingly  symmetrical  in  their  organisation,  it  seems,  at  first  sight,  natural  to 
consider  this  symmetry  a  primitive  feature.  Certain  peculiarities  of  the  nervous 
system,  however,  especially  the  crossing  of  the  pleuro-visceral  connectives,  taken  in 
connection  with  other  conditions  explained  more  fully  elsewhere,  make  it  certain 
that  the  cup-shaped  shell  of  Fissurella  is  only  secondarily  symmetrical,  i.e.  that 
Fissurella  is  descended  from  forms  which  possessed  a  spirally  coiled  shell.  The  same 
is  the  case  with  the  Patellidce. 

The  following  important  facts  are  in  harmony  with  this  conclusion  :  (1)  the 
young  shell  of  FissureUa  is  asymmetrical  and  coiled,  and  it  only  gradually  assumes 


FIG.  50.— Shells  of— A,  Pleurotomaria ;  B,  Polytremaria ;  C  and  E,  Emarginula  :  D,  Haliotis ; 
F,  Fissurella ;  G  and  H,  stages  in  the  development  of  the  shell  of  Fissurella ;  I,  shell  of  the 
twisted  racial  form  of  the  Gastropoda  with  marginal  cleft ;  K,  the  same  with  apical  aperture ; 
L,  shell  of  Lamellibranch  ;  M,  shell  of  Dentalium,  seen  from  the  apical  aperture.  The  holes  and 
clefts  of  the  shells  are  black  ;  o.  mouth ;  a,  anus  ;  ct,  ctenidium. 

the  symmetrical  form  (Fig.  59,  G,  H)  ;  (2)  the  apparently  symmetrical  shape  of  some 
forms  nearly  related  to  Fissurella  and  Patella  prove  on  closer  inspection  to  be 
somewhat  asymmetrical,  the  apex  especially  being  more  or  less  excentric  ;  (3) 
other  forms  nearly  allied  to  Fissurella,  such  as  Haliotis,  Scissurella,  and  Pleuro- 
tomaria, have  spirally  coiled  shells  (Fig.  59,  A,  B,  C,  D). 

In  the  Fissurellidcc.  many  Plcwrotomariidee,  and  the  Haliotidcc.  i.e.  in  the  most 
primitive  Gastropods,  peculiar  and  noteworthy  perforations  of  the  shell  occur,  such 
as  are  occasionally  found  in  other  divisions.  These  perforations  lie  above  the  slit 
in  the  mantle  which  is  characteristic  of  this  order  (cf.  p.  43),  and  they  everywhere 
establish  communication  between  the  mantle  cavity  and  the  exterior,  especially 
needed  when  the  mouth  or  edge  of  the  shell  is  closely  applied  to  the  object  on  which 
the  creature  crawls. 


60  COMPARATIVE  ANATOMY  CHAP. 

In  Scissurella,  Pleurotomaria,  and  Emarginula,  there  is  a  median  indentation  in 
the  anterior  edge  of  the  shell,  which  corresponds  with  an  incision  in  the  mantle 
edge.  This  is  the  case  in  the  young  Fissurella,  but,  during  further  development, 
the  edge  of  the  shell  grows  across  the  incision,  so  that  in  the  adult  animal  the 
aperture  lies  near  the  apex.  Beneath  it  is  the  anus,  placed  high  up  in  the  mantle 
cavity.  If  such  a  cleft  were  to  arise  at  both  the  anterior  and  posterior  edges,  and 
to  become  very  deep,  a  double  shell  would  result  comparable  with  the  bivalve  shell 
of  the  Lamellibranchia.  It  is  in  fact  probable  that  this  notching  of  the  shell  edge 
is  of  great  phylogenetic  significance. 

In  Haliotis  we  have  a  row  of  perforations  of  the  shell,  the  process  of  formation 
of  the  perforation  in  Fissurella  being  often  repeated  ;  the  older  apertures  are  always, 
however,  closed  by  shell  substance,  and  the  younger  only  remain  open  as  long  as 
they  lie  immediately  over  the  respiratory  cavity. 

In  very  many  Prosobraiichia  (the  Siphoniata  of  earlier  writers),  there  is,  at  the 
columnar  edge  or  lip  of  the  shell,  a  notch  which  gives  passage  to  a  channel -like  fold 
of  the  mantle  margin.  This  channel  keeps  up  communication  between  the  mantle 
cavity  and  exterior,  even  when  the  shell  is  closed  by  the  operculum.  Instead  of  a 

A  B 


FIG.  60.— A,  Dextrally  twisted ;  B,  sinistrally  twisted  shell  of  Helix  pomatia. 

notch,  a  more  or  less  long  process  or  beak  may  enclose  a  corresponding  process  of  the 
mantle,  the  siphon.  The  latter  may  become  a  tube  by  th'e  apposition  of  its  edges. 

It  has  already  been  mentioned  that  the  shells  of  most  Gastropods  are 
dextrally  twisted.  There  are,  however,  a  few  families,  genera,  or  species  in  which 
the  shell  has  a  sinistral  twist ;  and  in  some  species  where  the  twist  is  dextral,  a  few 
individuals  with  sinistral  twist  may  occur,  and  vice  versa.  It  is  a  curious  fact  that 
some  species,  in  which  the  shell  has  a  sinistral  twist,  show  the  asymmetry  of  the 
dextral  twist  in  the  soft  body,  whereas,  in  others,  the  asymmetry  of  the  soft  body 
corresponds  with  the  twist  of  the  shell.  We  shall  return  to  this  point. 

For  details  as  to  the  growth  of  the  shell,  and  the  capacity  of  the  animal  to 
dissolve  the  shell  already  formed,  both  of  which  are  points  full  of  interest,  we  must 
refer  to  special  works  on  Conchology,  as  also  for  detailed  descriptions  of  forms  of  the 
shell  and  opercula,  and  differences  due  to  age. 

Progressive  reduction  of  the  shell  occurs  in  each  of  the  three  divisions  of 
the  Gastropoda.  In  the  Prosobranchia,  this  has  only  been  observed  in  marine, 
free-swimming  Heteropods  and  in  Titiscania ;  in  the  Pulmonata,  it  is  much  more 
common  ;  and  in  the  Opisthobranchia,  so  frequent  that  nearly  all  the  members  of 
this  division  have  more  or  less  rudimentary  shells.  Many  adult  Opisthobranchia 
have  even  lost  every  trace  of  a  shell  (Pteropoda  gymnosomata,  Nudibranchia,  and 
most  Ascoglossa),  although,  in  their  earliest  stages  at  least,  they  possessed  a  coiled 


vii  MOLLUSCA—THE  SHELL  61 

shell,  for  the  closing  of  which  there  may  even  be  an  operculum,  secreted  by  the  foot, 
as  in  the  Prosobranchia. 

The  following  are  some  of  the  principal  stages  and  concomitant  phenomena  of 
the  reduction  of  the  shell :  (a)  The  well-developed  shell  ceases  to  be  large  enough  to 
shelter  the  whole  body,  (b)  The  shell,  which  becomes  thinner  and  smaller,  is  dorsally 
overgrown,  partially  or  altogether,  by  extensions  of  the  mantle,  (c)  As  the  shell 
(which  is  then  either  cup-  shield-  or  ear-shaped)  becomes  continually  smaller,  the 
visceral  dome  begins  to  be  levelled  down,  till  it  no  longer  rises  above  the  rest  of  the 
body,  its  contents  spreading  out  to  a  certain  extent  over  the  dorsal  surface  of  the  foot. 
(d]  The  external  asymmetry  of  the  body  passes  by  degrees  into  symmetry,  whereas 
the  internal  asymmetry  never  entirely  disappears,  (e)  The  shell  is  reduced  to  a 
number  of  isolated  calcareous  particles  in  the  integument  of  the  flattened  visceral 
dome.  (/)  There  is  at  last  no  trace  of  a  distinct  visceral  dome  ;  calcareous  particles 
are  to  be  found  in  the  dorsal  integument  of  the  long  and  now  naked  Gastropod. 
(g)  Even  these  particles  finally  disappear. 

In  connection  with  the  reduction  of  the  shell  in  OpisthobrancMa  and  Pulmonata, 
compare  the  section  on  the  mantle,  pp.  43-48. 

The  Heteropoda  present  the  following  interesting  series  : — 

Atlanta.  The  shell  is  very  light  and  thin,  but  large  and  spirally  coiled  (with 
an  incision  at  its  aperture)  ;  the  animal  can  entirely  withdraw  into  it,  and  close 
it  by  means  of  an  operculum  developed  on  the  distinct  metapodium. 

Carinaria.  The  shell  is  thin,  light,  and  delicate  ;  it  is  cup-shaped,  and  covers  the 
large  stalked  visceral  dome,  but  is  incapable  of  sheltering  the  long  and  thick 
cylindrical  body  and  foot.  There  is  no  operculum. 

Pterotrachea.  The  visceral  dome  is  small,  and  there  is  no  shell  and  no  operculum. 

C.  Lamellibranchia. 

The  two  lateral  valves  of  the  Lamellibranch  shell  are  connected,  at  their 
dorsal  edges,  by  means  of  a  hinge  and  a  ligament.  The  ligament  counteracts  the 
muscles  of  the  shell,  which  will  be  described  later  on,  and  which,  by  their  contraction, 
close  the  shell.  It  is  usually  composed  of  two  layers,  the  inner  layer  being  elastic, 
while  the  outer  is  not.  The  outer  non-elastic  layer  passes  into  the  epidermis  or 
periostracum  of  the  shell.  The  inner  layer  of  the  ligament  is  elastic  and  calcareous, 
and  is  often  called  cartilage,  but  this  is  histologically  incorrect. 

The  ligament  lies  either  externally,  distinctly  seen  dorsally  between  the 
prominences  of  the  hinge  edges  of  the  valves,  or  internally,  stretched  between  the 
apposed  edges  of  the  hinge,  which  are  furnished  with  depressions  for  its  reception. 
These  depressions  can  easily  be  distinguished  from  those  belonging  to  the  hinge 
itself,  since  the  former  are  alike  on  the  two  valves,  whereas  the  furrows  and  other 
depressions  belonging  to  the  one  face  of  the  hinge  correspond  with  teeth,  ridges, 
etc.,  on  the  opposite  face. 

When  the  elastic  "cartilage  "  of  the  ligament  is  at  rest,  as  in  a  dead  bivalve,  or 
when  the  adductor  muscles  of  the  living  animal  are  relaxed,  the  valves  open.  When 
the  adductors  contract,  the  "cartilage"  is — apparently  in  all  cases — compressed. 
On  the  other  hand,  when  the  adductors  are  relaxed,  the  elasticity  of  the  "cartilage  " 
forces  the  shell  open  again  (Fig.  61). 

The  continuity  established  between  the  two  valves,  by  means  of  this  dorsal 
ligament,  causes  the  Lamellibranch  shell  to  appear  to  consist,  strictly  speaking,  of 
one  dorsal  piece,  developed  to  the  right  and  left  ventrally  into  two  valves.  The 
constitution  of  the  ligament  and  hinge  are  of  importance  in  classification. 

We  must  refer  the  reader  to  systematic  zoological  works  for  the  special  forms 
taken  by  the  shell,  and  content  ourselves  with  the  following  remarks  : — 


COMPARATIVE  ANATOMY 


CHAP. 


The  Lamellibranch  shell  is  originally  symmetrical,  that  is  to  say,  the  two  valves, 
apart  from  the  almost  invariable  asymmetry  of  the  hinge,  are  exactly  alike 
(equivalve).  This  is  the  case  in  most  of  the  Lamellibranchia.  The  two  valves 
may,  however,  become  unlike,  i.e.  the  shell  (and  to  a  much  lesser  extent,  and  only 
in  unimportant  details,  the  soft  body  also)  may  become  asymmetrical.  As  far  as 
we  can  at  present  judge,  this  asymmetry  is  caused  by  adaptation  to  an  attached 
manner  of  life. 

The  left  valve  of  the  Oyster  is  firmly  cemented  to  the  surface  on  which  it 
rests.  This  valve  is  thicker,  more  convex  and  spacious,  and  forms  a  sort  of 
basin  in  which  the  soft  body  lies,  while  the  right  valve  acts  rather  as  a  lid,  and 
is  thinner  and  flatter.  We  have  thus  an  'upper"  (the  right)  and  a  "lower"  (the 
left)  valve,  but  it  is  hardly  necessary  to  point  out  that  this  use  of  the  terms  upper 
and  lower  has  as  little  morphological  significance  as  in  the  Pleuronectidse  among  the 
fishes.  The  attached  valve  is  sometimes  the  right,  sometimes  the  left,  and  this 


FIG.  til.— Diagram  in  illustration  of  the  mechanism  for  opening  and  closing  the  Lamelli- 
toranch  shell.  1,  2.  3,  The  three  layers  of  the  shell— 1,  prismatic  layer  ;  2,  cuticle  or  periostracum ; 
3,  nacreous  layer.  A,  Shell  closed  by  the  contraction  of  the  adductor  muscle  (6),  by  means  of 
which  the  elastic  inner  portion  of  the  ligament  (5)  is  compressed.  B,  Shell  opened  by  the  elastic 
pressure  of  the  inner  portion  of  the  ligament  during  the  relaxation  of  the  adductor  muscle.  4, 
Non-elastic  outer  portion  of  the  ligament,  which  passes  into  the  periostracum. 

variation  may  occur  within  one  and  the  same  genus  (Chamd),  or  even  species 
(Aetherid). 

Besides  the  above-named,  the  following  bivalves  are  also  attached,  and  have 
dissimilar  valves  :  Spondylus,  Gryphcea  p.  p. ,  Exogyra  p.  p. ,  and  especially  the 
fossil  Hippuritcs  (Rudistes),  in  which  the  right  valve  assumes  the  form  of  a  high  cone 
attached  by  its  point,  while  the  left  looks  like  a  lid.  The  conical  valve  has,  however, 
no  corresponding  internal  cavity,  but  is  almost  entirely  filled  up  with  shell  substance, 
so  that,  in  spite  of  the  form  of  the  shell,  the  space  occupied  by  the  animal  between 
the  two  valves  is  very  limited. 

This  same  condition  is  found  in  certain  fossil  Chamacea.  In  Jtcquienia,  the  left 
valve  is  produced  spirally  and  is  attached  by  its  point,  while  the  spirally-coiled 
flattened  right  valve  covers  it  like  a  lid,  so  that  the  whole  shell  closely  resembles 
a  Gastropod  shell  closed  by  its  operculum. 

There  are  also  free,  unattached  bivalves  with  unequal  valves,  e.g.  many 
Pcdinidce.  In  these  animals,  howrever,  many  peculiarities  of  organisation,  such  as 


VII 


MOLLUSCA—THE  SHELL 


63 


the  rudimentary  foot,  the  constitution  of  the  mantle  edge,  and  the  absence  of  siphons, 
indicate  descent  from  sedentary  forms.  In  the  case  of  other  forms  with  unequal 
valves,  however,  no  such  descent  can  be  established. 

In  Anornia  we  have  an  example  of  an  inequivalve  bivalve,  in  which  the  valve 
turned  to  the  surface  it  rests  on  is  flat  and  the 
upper  arched.  The  lower  valve  is  here  the  right 
one,  and  takes  the  exact  imprint  of  the  surface 
on  which  it  rests,  so  that,  for  example,  the  mark- 
ings of  the  shell  of  the  Peden  or  the  Oyster,  to 
which  Aiiomia  frequently  attaches  itself,  are 
exactly  reproduced.  In  this  right  attached  valve 
there  is  a  perforation  into  which  a  shelly  plug, 
the  calcined  byssus,  fits  ;  by  means  of  this,  the 
animal  fixes  itself  to  its  substratum.  The  ex- 
planation of  this  perforation  is  seen  in  the  course 
of  development.  It  commences  as  a  simple  notch 
at  the  edge  of  the  shell,  as  found  also  in  other 
bivalves,  for  the  passage  of  the  byssus.  By  the 
further  growth  of  the  shell,  this  notch  to  a 
certain  extent  is  grown  round,  and  thus  ap- 
parently travels  away  from  the  edge  of  the  shell, 
with  which,  however,  it  is  still  really  connected  (Fig.  62).  In  related  forms  (Carolia) 
this  aperture  becomes  quite  filled  up  by  a  homogeneous  calcareous  mass. 

Impressions  on  the  inner  surfaces  of  the  shell.  — Various  organs  of  the  Mollusc, 
attached  to  or  adjacent  to  the  inner  surface  of  the  shell,  leave  more  or  less  distinct 
impressions  on  this  surface,  which  are  visible  when  it  is  empty.  These  impressions 
are  of  great  importance,  especially  to  the  palaeontologist,  for  by  their  means  fairly 


FIG.  62.— Three  stages  in  the  develop- 
ment of  the  shell  valves  of  Anomia. 
A,  Very  young  shell ;  B,  older  shell  with 
notch  for  the  byssus  ;  C,  still  older  shell, 
the  byssus  notch  surrounded  by  .the  shell 
and  persisting  as  a  hole  (after  Morse). 


Fi<;.  < ',3.— Dimyaria,  inner  surface  of  the  left  shell  valve.  A,  Cytherea  chione  (SinupaUiata); 
B.  Lucina  Pennsylvanica  (iHtegripaUiata);  1,  impression  of  the  anterior;  2,  impression  of  the 
posterior,  adductor  ;  3,  sinus  of  the  pallial  line  (4) ;  5,  ligament. 

safe  conclusions  may  be  arrived  at  as  to  certain  points  in  the  organisation  of  the 
soft  body  which  has  disappeared. 

1.  The  most  distinct  impressions  are  those  caused  by  the  adductor  muscles. 
Where  there  are  two  powerful  adductor  muscles,  one  anterior  and  the  other  posterior 
(Dimyaria),  there  are  two  impressions  in  the  corresponding  parts  of  the  inner  surface 
of  the  shell  (Fig.  63).  In  cases  where  the  anterior  muscle  is  rudimentary,  while 
the  posterior  is  unusually  powerful,  and  has  moved  anteriorly  towards  the  middle 
of  the  shell  (Monomyaria),  there  is  only  one  large  impression  (Fig.  64).  The  anus 


64  COMPARATIVE  ANATOMY  CHAP. 

is  always  to  be  found  close  to  the  posterior  (which  in  the  Monomyaria  is  the  only) 
adductor. 

2.  Parallel  to  the  edge  of  the  shell,  and  more  or  less  removed  from  it,  we  find 
on  the  inner  surface  of  the  shell  the  so-called  pallial 
line,  caused  by  the  muscle  fibres  which  attach  the 
edge  of  the  mantle  to  the  valves. 

The  course  taken  by  this  line  undergoes  charac- 
teristic modification  in  such  Lamellibranchs  as  have 
siphons ;  at  the  posterior  part  of  the  shell  it  suddenly 
bends  forward  and  upward,  and  then  again  passes 
backward  and  upward  towards  the  lower  edge  of  the 
posterior  adductor.     The  pallial  line,  in  this  case, 
forms  an  indentation,  leaving  a  sinus  or  bay  opening 
posteriorly,  the  pallial  sinus,  which  has  been  utilised 
for  systematic   purposes    (Sinupalliata,  Integripal- 
liata,  Fig.  63).     The  sinus  marks  the  line  of  attach  - 
Fio.  64.—  Monomyarian,  internal    ment  of  the  sipho-retractor  muscles  ;   the  stronger 
surface  of  a  shell  valve  of  Perna    these  retractors  and  the  better  developed  the  siphons 
Ephippiuin      l.  Hinge  edge;  2,  im-    th    ^          &nd  dearer  ig  the  ^ 
pression  of  adductor.  E..  . 

3.  The  foregoing  impressions  are  the, -most  dis- 
tinct and  constant,  but  others  may  occur  as  well,  caused  by  the  protractors  and 
retractors  of  the  foot,  by  the  muscles  or  ligaments  which  attach  the  visceral  dome 
to  the  shell,  etc.  ;  but  these  cannot  be  further  described. 

In  most  Lamellibranchia,  when  the  shell  is  closed,  the  edges  of  the  two  valves 
meet  exactly,  so  that  the  soft  body  can  be  entirely  enclosed  and  cut  off  from  the 
exterior  (closed)  shell.  There  are,  however,  shells  in  which,  in  the  closed  condition, 
the  valves  gape  posteriorly,  or,  more  frequently,  both  posteriorly  and  anteriorly 
(e.g.,  Myidce,  Glycymeridce,  Solenidce).  This  is  accounted  for  by  the  great  develop- 
ment of  the  siphons  and  of  the  foot,  which  can  only  partially  (Myidce,  Solenocurtus] 
or  with  difficulty  be  withdrawn  into  the  shell.  Such  gaping  shells  are  found  in  most 
boring  bivalves,  whose  shell  formation  is  specially  interesting  owing  to  the  develop- 
ment of  accessory  valves  or  calcareous  tubes.  In  this  respect  Pholas,  Pholadidea, 
and  Jouannetia  represent  the  most  important  stages  in  a  remarkable  series. 

The  shell  of  Pholas  is  elongated  longitudinally,  and  gapes  anteriorly  and  ventrally 
for  the  passage  of  the  short  club-shaped  foot,  and  posteriorly  for  that  of  the  strongly 
developed  siphons.  As  many  as  three  accessory  valves  are  developed  dorsally 
(prosoplax,  mesoplax,  metaplax). 

The  shell  of  Pholadidea  somewhat  resembles  that  of  Pholas.  In  the  young 
animal  it  gapes  anteriorly,  as  in  Pholas,  for  the  passage  of  the  foot.  Posteriorly, 
each  valve  is  produced  into  a  horny  process,  which  is  succeeded  by  an  accessory 
piece  (siphonoplax),  hollowed  out  like  a  trough.  The  siphonoplax  of  the  one  valve 
often  fuses  with  that  of  the  other  to  form  a  single  tube  for  the  reception  of  the 
siphons.  There  are  two  pieces  of  prosoplax,  while  the  nieso-  and  metaplax  are 
rudimentary.  In  the  adult  the  boring  activity  is  suspended,  and  the  anterior 
opening  becomes  entirely  closed  by  the  secretion  of  an  accessory  piece,  the  callum 
(hypoplax).  The  functionless  foot  atrophies,  and  the  animal  can  move  no  farther 
in  the  substance  into  which  it  has  bored. 

The  shell  of  the  adult  Jouannetia  is  much  shortened  longitudinally,  and  is 
globular,  and  the  animal  cannot  move  in  the  round  hole  it  has  bored  for  itself  in  a 
block  of  coral.  Any  alteration  in  its  position  in  the  hole,  which  might  be  fatal  to 
the  animal,  is  avoided  by  means  of  a  posterior  tongue-like  process  of  the  shell, 
which,  however,  only  belongs  to  the  right  valve.  The  shell  is  completely  closed 
anteriorly,  and  a  foot  is  wanting  (cf.  also  Figs.  27,  28,  p.  19,  and  66,  p.  67). 


vii  MOLLUSCA—THE  SHELL  65 

The  adult  condition  of  Jouannetia  is  explained  by  its  developmental  history. 
The  shell  of  the  young  animal  is  like  the  segment  of  a  sphere,  whose  greatest  height 
is  hardly  half  of  the  radius.  It  covers  the  dorsal  upper  portion  of  the  body,  its  free 
edges  thus  bounding  a  very  wide  aperture,  which  corresponds  with  the  anterior  pedal 
gape  of  Pholas. 

In  this  Pholas-st&ge,  in  fact,  Jouannetia  really  possesses  a  foot.  Twisting  the 
body  about  and  rasping  the  stone  with  the  anterior  edge  of  the  shell,  the  animal 
excavates  a  hole,  which  is  spherical  in  consequence  of  the  shape  of  its  shell.  When 
this  hole  is  made,  new  accessory  shell  material  is  secreted  at  the  free  edge  of  the 
sliell  ;  this  forms  the  "callum,"  and  as  the  edge  of  the  mantle  follows  the  lines  of 
excavation,  the  form  of  the  accessory  shell  is  here  (as  in  Teredo}  determined  by  the 
form  of  the  hole,  and  the  sphere  of  which  the  original  shell  was  but  a  segment  is 
completed. 

Setting  aside  a  few  related  forms  (Martesia,  Teredina,  Xylophaga,  Gastrochaena, 
and  Fistulatia),  in  which  the  conditions  are  somewhat  similar,  we  come  to  the  ship- 
worm  Teredo  (Fig.  29,  p.  20).  This  animal  has  a  long  tubular  mantle  which  is 
produced  posteriorly  in  two  long  siphons.  The  body  lies  at  the  anterior  end  of  the 
mantle.  Teredo  bores  cylindrical  passages  in  wood.  The  valves  of  the  shell  are 
very  small  in  comparison  with  the  body  ;  they  take  the  form  of  tri-lobate  pieces, 
which  encircle  the  anterior  end  of  the  mantle.  This  rudimentary  shell  gapes 
anteriorly  for  the  passage  of  the  pestle-shaped  foot,  and  very  widely  posteriorly. 
The  mantle  further  secretes  over  its  whole  surface  a  calcareous  tube  which  lines  its 
burrow,  but  which  does  not  fuse  with  the  shell  valves.  Two  small  accessory  shell- 
pieces,  the  so-called  "palettes,"  lie  at  the  place  where  the  siphons  separate.  If  the 
anterior  portion  of  the  animal  reaches  (i.e.  if  it  bores  through  to)  the  water,  the 
calcareous  tube  is  rounded  off  and  closed. 

Aspergillum  (Brechites,  Fig.  30,  p.  20,  and  Fig.  65)  and  Clavagella  show  similar  con- 
ditions. In  the  club-shaped  shell,  which  inserts  its  anterior  thicker  end  into  rock, 
shell,  coral,  or  sand,  we  can  distinguish  a  true  and  a  false  shell.  The  false  shell 
forms  by  far  the  larger  portion  of  the  tube,  and  corresponds  with  the  secreted  tube 
of  Teredo,  and  with  a  callum  like  that  of  Pholas.  The  true  shell  is  very  small 
and  lies  anteriorly.  The  two  valves  of  this  true  but  rudimentary  shell  are,  in 
Aspergillum,  placed  saddle-like  over  the  anterior  end  of  the  tube,  with  which  they 
are  firmly  fused  (Fig.  30,  p.  20).  Were  they  isolated,  their  gape  would  be 
unusually  wide,  not  only  anteriorly  and  posteriorly,  but  ventrally.  The  shell- tube  is 
open  posteriorly,  over  the  apertures  of  the  siphons  ;  anteriorly,  however,  it  is  closed 
(in  the  adult)  by  means  of  a  disc  perforated  like  the  rose  of  a  watering-can,  which 
corresponds  in  position  with  the  callum  of  the  Pholadidce.  The  perforations  at  the 
edge  of  the  disc,  or  even  over  its  whole  surface,  are  sometimes  produced  into  cal- 
careous, and  at  times  dichotomously  branched  tubules.  In  the  middle  of  the  disc 
there  is  sometimes  found  a  narrow  slit-like  aperture  corresponding  with  the  pedal 
aperture  in  the  mantle  beneath,  but  this  is  often  wanting.  Less  frequently,  we  find 
another  aperture  in  the  ventral  middle  line,  corresponding  with  the  fourth  mantle 
aperture  above  described  (p.  51). 

Aspergillum  buries  its  anterior  end  in  mud  or  sand,  but  its  whole  organisation, 
and  especially  its  shell  arrangement,  point  to  a  former  boring  mode  of  life. 

Clavagella,  which  is  nearly  related  to  Aspergillum,  bores  into  rock  or  the  cal- 
careous shells  of  various  other  animals.  The  arrangement  of  its  shell  differs  from 
that  of  Aspergillum  chiefly  in  the  somewhat  greater  size  of  its  true  valves,  and  in 
the  fusion  of  only  the  left  valve  with  the  calcareous  tube,  the  right  lying  free 
within  that  tube. 

In  the  Pholadidce,  the  ligament,  which  is  still  found  at  the  hinge,  no  longer  acts 
for  opening  the  shell.  In  consequence  of  a  peculiar  arrangement  of  the  anterior 
VOL.  II  F 


UNIVJE 

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66 


COMPARATIVE  ANATOMY 


CHAP. 


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vii  MOLLUSCA—THE  SHELL  67 

adductor,  the  opening  of  the  shell,  such  as  it  is,  is  brought  about  by  the  muscles. 
The  anterior  and  upper  edges  of  the  valves,  are  bent  outward,  and  to  these  edges  the 
anterior  muscle  is  attached.  We  thus  have  external  instead  of  internal  points 


FIG.  66.— Pholas  dactylus,  right  valve,  internal  aspect  (after  Egger).  1-2,  Axis  round  which  the 
valves  move  upon  one  another  ;  3-4,  longitudinal  axis  of  the  shell ;  5-8,  line  connecting  the  shell 
muscles  ;  6,  anterior  muscle  ;  7,  posterior  muscle  ;  9,  rotating  point  of  the  valves  ;  10,  anterior  and 
upper  edge  of  the  shell,  which  is  bent  outwards,  and  to  which  the  muscle  6  is  attached  ;  6-9,  shorter 
anterior ;  9-7,  longer  posterior  arm  of  the  lever. 

of  attachment,  and  the  whole  shell  may  be  compared  to  a  double -armed  lever 
acting  along  the  longitudinal  axis  of  the  body,  its  fulcrum  being  at  the  point  where, 
in  other  bivalves,  the  hinge  is  found.  When  the  anterior  muscle  contracts,  the  shell 
opens  posteriorly  and  ventrally  ;  when  the  posterior  adductor  contracts,  the  shell 
closes  (Fig.  66). 

D.  Cephalopoda. 

The  Cephalopoda  are  all  to  be  derived  from  an  ancient  fossil  form  which  possessed 
a  chambered  shell,  in  the  last  and  largest  portion  of  which  the  animal  lived,  leaving 
the  rest  of  the  shell  empty,  or  rather  filled  with  gas  (or  water)  and  traversed  by 
the  siphon  or  siphuncle.  Such  a  shell  is  now  found  only  in  the  sole  living  repre- 
sentative of  the  Tetrabranchia,  the  Nautilus,  an  animal  of  great  importance  to  the 
comparative  anatomist.  Many  fossil  forms  allied  to  the  Nautilus,  and  grouped 
in  the  order  Nautiloidea,  possessed  such  a  shell,  as  did  also  the  Ammonoidea,  with 
their  enormous  wealth  of  forms  which,  rightly  or  not,  are  generally  considered  to 
be  nearly  related  to  the  Nautiloidea,  i.e.  to  belong  to  the  Tetrabranchia.  In  nearly 
all  these  animals  the  shell,  when  coiled  at  all,  is,  unlike  the  Gastropod  shell,  coiled 
anteriorly  or  exogastrically. 

One  group  of  the  Nautiloidea,  the  Endoceratidce,  which  includes  only  very  old 
forms  (Cambrian  and  Lower  Silurian),  is  distinguished  by  the  fact  that  the  chambers 
of  its  straight  shell,  which  were  filled  with  gas  (or  water),  lay  at  the  side  of  and  not 
behind  the  inhabited  chamber.  There  was  no  real  siphuncle,  but  the  upper  end  of 
the  visceral  dome,  much  narrowed  by  the  air  chambers,  stretched  as  far  as  to  the 
apex  of  the  shell. 

In  other  Nautiloidea,  fcthe  air  chambers  always  lie,  as  in  Nautilus,  above  the 
occupied  chamber,  and  are  traversed  by  a  thin  membraneous  siphuncle,  which,  how- 
ever, in  old  forms,  is  much  thicker,  and  represented  the  narrow  prolonged  portion  of 
the  visceral  dome  (Fig.  32,  p.  22). 

Some  forms  of  Nautiloidea  have  shells  coiled  endogastrically  ;  this  is  never  the 
case,  however,  when  the  shell  forms  a  complete  spiral.  The  sutures,  which  corre- 
spond with  the  lines  of  insertion  of  the  septa,  are  simple  in  the  Nautiloidea,  as 


68    -  COMPARATIVE  ANATOMY  CHAP. 

compared  with  those  in  the  Ammonoidea,  in  which  they  are  folded  in  a  complicated 
manner. 

Nautiloidea. — In  the  following  table  we  have  the  chief  forms  of  the  shell  among 
the  Nautiloidea  : ] — 

(a)  Orthoceras  group.— Shell  straight  or  slightly  bent.     Silurian— Trias. 

(b)  Cyrtoceras  group. — Shell  curved  like  a  horn,  but  not  regularly  spirally 

coiled.     Cambrian— Permian. 

(c)  Gyroceras  group. — Shell  regularly  spirally  coiled,  the  coils,  however,  not 

touching  each  other.     Silurian — Permian. 

(d)  Nautilus  group. — Shell  regularly  spirally  coiled,  the  coils  touching,  or  the 

outer  clasping  the  inner.     Silurian — recent. 

(e)  Lituites.  —  Shell    at  first    regularly  spirally  coiled,  straightening    later. 

Silurian. 

The  siphuncle  runs  either  through  the  centre  of  the  septa,  or  through  their 
anterior  or  posterior  sides. 

Ammonoidea. — The  shells  of  the  (fossil)  Ammonoidea  are  distinguished  by  very 
complicated  sutures,  their  zigzag  lines  are  like  the  outlines  of  sharply-indented  leaves 
or  richly -branched  mosses,  they  are  due  to  the  extraordinary  folding  of  the  edges  of 
the  septa,  which  are  attached  to  the  inner  surface  of  the  shell.  The  siphuncle  is 
always  very  thin  in  the  Ammonoidea,  and  almost  always  pierces  the  septa  on  the 
posterior  side. 

The  following  quotation  summarises  the  chief  peculiarities  in  the  form  of  the 
Ammonite  shell : — 2 

"The  shell,  as  a  rule,  forms  a  closed  symmetrical  spiral,  the  coils  touching  or 
clasping  one  another.  Some  of  the  oldest  forms  are  straight,  or  in  youth  incom- 
pletely coiled.  In  certain  groups  of  the  Ammonoidea  we  find  a  tendency  repeated  at 
different  times  (Trias,  Jurassic,  Chalk)  to  depart  from  the  close  symmetrical  spiral, 
and  to  adopt  what  are  called  accessory  forms.  The  first  step  in  this  process  of  change 
is  in  most  cases  the  detachment  of  the  occupied  chamber  from  the  next  inner  whorl  ; 
then,  little  by  little,  the  inner  whorls  also  separate,  though  they  still  remain  in  the 
same  plane — the  Crioceras  stage.  Sometimes  the  shell  grows  straight  for  a  time, 
then  becomes  hooked — the  Ancyloceras  and  Hamites  stages,  and,  if  only  the  occupied 
chamber  separates  from  the  coiled  part— the  Scaphites  stage.  Finally,  entirely  straight 
shells  arise  in  the  Baculites  stage.  Rarely,  the  coils  leave  the  symmetrical  plane  and 
assume  the  shape  of  a  snail's  shell ;  in  this  case,  the  shells  may  be  either  closely  or 
loosely  coiled, — the  Turrilites  stage." 

Dibranchia. — The  shells  of  all  known  Dibranchia,  extinct  or  recent,  are  more  or 
less  rudimentary,  since  they  are  never  capable  of  sheltering  more  than  a  small  portion 
of  the  animal.  Further,  they  are  always  internal,  on  the  anterior  side  of  the  visceral 
dome,  and  are  overgrown  by  a  fold  of  the  integument.  In  Spirula  (Fig.  33,  p.  23) 
alone,  the  shell  is  not  completely  overgrown,  a  portion  at  the  apex  of  the  visceral 
dome  remaining  uncovered. 

The  shell  of  the  (fossil)  Belemnites  (Fig.  67  C)  is  straight,  conical,  and  chambered; 
the  septa  are  near  one  another,  and  are  pierced  on  the  posterior  or  ventral  side  by 
the  thread-like  siphuncle,  which  is  enclosed  in  short,  calcareous  sheaths.  The  apex 
of  the  shell  (phragmocone)  is  protected  by  a  conical  calcareous  sheath  (rostrum  or 
guard),  the  only  part  usually  preserved.  The  anterior  wall  of  the  last  chamber 
is  produced  downwards  into  a  broad  thin  process,  the  pro-ostracum. 

In  Spirulirostra  (Fig.  67  D),  the  phragmocone  begins  to  bend  posteriorly  (endo- 
gastrically).  The  rostrum  is  triangular  and  pointed  at  the  top. 

1  Steinmann-Doderlein,  JSlemente  der  Paldontologie,  1890.  2  Ibid. 


VII 


MOLLUSCA—THE  SHELL 


69 


In  Spirula  (E),  the  shell  is  coiled  spirally  and  endogastrically.  The  siphuncle 
is  thick,  and  is  surrounded  along  its  whole  length  by  septal  envelopes.  The  rostrum 
is  rudimentary,  and  there  is  no  pro-ostracum. 

Starting  again  from  the  Belemnites,  the  modification  of  the  shell  may  take  another 
direction.  The  phragmocone  may  become  smaller  and  shorter  in  comparison  with 
the  continually  lengthening  pro-ostracum  (e.g.  Ostracoteuthis,  F).  The  rostrum 
also  may  become  thinner  and  smaller.  Finally,  the  shell  may  be  reduced  to  a  very 


FIG.  67.— A-H.,  Diagrammatic  median  sections  through  the  shells  of  eight  extant  or  fossli 
Dibranchia,  from  the  left  side.  The  point  of  the  visceral  dome  is  turned  down  wards,  the  posterior 
side  of  the  shell  is  to  the  left  and  the  anterior  to  the  right  (cf.  the  position  of  the  Cephalopod  body, 
p.  36).  A.  Sepia;  B,  Belosepia  (fossil);  C,  Belemnite  (fossil);  D,  Spirulirostra  (fossil);  E, 
Spirula;  F,  Ostracoteuthis  (fossil);  G,  Ommastrephes ;  H,  Loligopsis;  ph,  chambered  shell = 
phragmocone ;  pr,  pro-ostracum ;  r,  rostrum  (guard) ;  s,  siphuncular  canal,  or  space  which  con- 
tains the  siphuncle  ;  1,  2,  3,  last  three  septa  (the  most  recent) ;  a,  anterior  wall  of  the  siphuncle ;  p, 
posterior  ;  x,  anterior  edge  of  the  first  septal  or  siphuncular  en velope  =  anterior  or  posterior  edge  of 
the  siphuncular  canal. 

small  hollow  cone  at  the  end  of  a  long  narrow  horny  lamella  which  corresponds 
with  the  pro-ostracum,  and  is  called,  in  the  extant  Decapoda,  the  gladius  or  calamus 
(or  pen)  (Loligo,  Ommastrephes  (G),  Onychoteuthis).  In  Dosidicus,  this  terminal  cone 
is  almost  solid,  and  in  Loligopsis  (H)  it  is  nothing  more  than  a  thickening  at  the 
upper  end  of  the  gladius  :  in  other  Decapoda,  there  is  no  trace  of  it  on  the  gladius. 
In  the  Octopoda,  the  shell  has  completely  disappeared. 

Again  starting  from  the  Belemnite,  the  shell  may  develop  in  a  third  direction 
to  form  the  Sepia  shell.     The  transition  form  is  found  in  Belosepia  (B)  (Eocene), 


70 


COMPARATIVE  ANATOMY 


CHAP. 


that  is,  if  this  interpretation  is  correct.  This  shell  is  somewhat  bent,  the  septa 
are  crowded  together  and  slope  downwards  anteriorly.  They  are  penetrated 
posteriorly  by  an  extremely  thick  siphon,  which  is  enclosed  throughout  its  whole 

length  in  an  envelope  with  a  very  thick  anterior 
wall.  The  completely  enclosed  siphuncular 
space  is  thus  a  wide  funnel  running  through  the 
chambers  of  the  shell  on  its  posterior  side  (Fig. 
67  B).  The  phragmocone  is  enclosed  in  a  thick, 
strongly  -  developed  rostrum,  and  its  anterior 
and  lateral  walls  are  produced  downwards  into 
a  broad,  posteriorly  concave  shell  (pro- 
ostracum  ?). 

These  arrangements  seem  to  have  culminated 
in  the  extant  Sepia  (Figs.  67  A  and  68).  The 
siphuncular  space  fits  over  the  visceral  dome 
like  a  mould.  The  anterior  portions  of  the 
septa  slope  downward  much  more  obliquely 
from  behind  anteriorly,  so  that,  in  a  back  view 
of  the  shell,  the  whole  area  of  the  last  septum  is 
visible  at  the  surface  (Fig.  68,  1).  The  septa 
are  thin  calcareous  lamellae,  closely  superim- 
posed one  upon  the  other,  with  very  narrow  air 
chambers  between  them  ;  and  these  latter  are 
traversed  by  perpendicular  trabeculae.  The  shell 
is  thus  very  light,  its  specific  gravity  is  less  than 
that  of  water.  Behind  the  siphuncle,  on  the 
posterior  very  much  shortened  side  of  the  shell, 
the  short  septa  are  closely  contiguous,  without 
any  intervening  air  spaces. 

The  dorsal  end  of  the  shell  is  enclosed  in  a 
small  pointed  rostrum.  The  whole  anterior  sur- 
face is  covered  by  a  thin  lamella  of  conchy olin, 
which  projects  laterally  beyond  the  edge  of  the 
shell,  and  is  itself  covered  by  a  calcareous  layer 
which  is  an  anterior  and  ventral  extension  of 
the  rostrum. 

The  female  Argonaut  is  the  single  exception 
to  the  rule  stated  above,  that  in  the  Octopoda 
the  shell  has  entirely  disappeared.  This  animal 
has  a  light,  thin  external  shell  coiled  anteriorly 
or  exogastrically,  which  is  not  firmly  attached 
to  the  body  at  any  point,  and  serves  more  for 
receiving  the  eggs  (Figs.  35,  36,  pp.  24,  25)  than 
for  protecting  the  body.  This  shell  is  sur- 
rounded and  secured  by  lobate  processes  of  the 
anterior  pair  of  arms.  It  has  no  nacreous  layer,  but  is  porcelanous,  and  is 
apparently  produced  from  the  integument  of  the  visceral  dome  and  the  mantle. 
The  dorsal  pair  of  arms  is  said  only  to  deposit  the  so-called  black  layer  on  its 
surface. 

It  is  usually  considered  that  this  Argonaut  shell  is  not  the  homologue  of  the 
shell  of  other  Cephalopods,  but  is  a  formation  peculiar  to  the  Argonaut  female.  An 
opposite  view  has,  however,  recently  been  very  ably  advanced — that  the  Argonaut 
shell  is  an  Ammonite  shell  which  has  lost  its  septa  and  siphuncle  and  also  its 


-i4— s 


r 


FIG.  68.  — Shell  of  Sepia  aculeata. 
Posterior  (physiologically  ventral)  aspect. 
Lettering  as  in  Fig.  67.  The  last  septum 
1  is  seen  in  its  whole  extent ;  s,  the  mouth 
of  the  broad,  slipper-shaped  siphuncular 
cavity  ;  I,  lateral  wall  of  the  cavity  ;  a-/3, 
line  of  the  section  which  in  Fig.  67  A  is 
diagrammatised.  The  two  figures  should 
be  compared  (principal  details  after 
D'Orbigny). 


fi  UNI  V 

vn  MOLLUSCA*-THE  PALLIAL  COMPLEX  71 

nacreous  layer.1  Should  this  view  prove  correct,  the  Cephalopods  would  have  to 
be  differently  classified.  The  division  into  Tetrabranchia  and  Dibra/ichia  would 
have  to  disappear,  as  we  cannot  tell  whether  the  fossil  Ammon&idea  Avere  Tetra- 
branchia. and  are  also  ignorant  as  to  when  the  Dibrauchia  developed  from  the  Tetra- 
branchia. The  Cephalopods  would  then  have  to  be  divided  into  (1)  Nautiloidea 
with  the  extant  genus  X<n'tilus  ;  (2)  Ammonoidea  with  the  still  living  Octopoda  ;  and 
(3)  JSelemnoidea  with  the  extant  Dccapoda. 

Bivalve  shelly  plates  called  aptychi  have  been  found  sometimes  in  the  last 
chamber  of  the  Ammonoidea,  sometimes  isolated.  These  have  been  proved  to  belong 
to  the  bodies  of  certain  species  of  Ammonoidea,  and  have  been  considered  by  some  to 
be  protectives  for  the  nidameiital  gland,  by  others  as  opercula,  and  by  others  again 
as  the  analogues  or  homologues  of  the  infundibular  cartilage  of  the  Decapoda.  No 
one  of  these  three  views  has  as  yet  been  generally  accepted. 


V.   Arrangement  of  the  Organs  in  the  Mantle  Cavity  and  of 
the  Outlets  of  Inner  Organs  in  that  Cavity. 

A  discussion  of  this  subject  at  this  stage  will  help  to  explain  the  asymmetry  of 
the  Gastropoda  and  to  simplify  the  discussion  in  later  chapters. 

There  are,  in  the  mantle  cavity,  many  important  organs  crowded  together  in  a 
comparatively  small  space,  and  into  it  also  open  all  the  apertures  of  the  inner  organs 
except  the  oral  aperture  of  the  alimentary  canal.  The  term  "circum-anal  complex." 
though  especially  applicable  to  the  arrangement  in  the  Gastropoda,  is  not  so  suitable 
as  "pallial  complex,"  which  applies  to  nearly  all  Mollusca,  and  comprises  not  only 
the  pallial  organs  themselves,  but  the  apertures  of  inner  organs  that  lie  in  the 
mantle  cavity. 

The  most  important  constituents  of  the  pallial  complex  are  the  ctenidium,  the 
osphradium  (Spengel's  07-gau,  olfactory  organ,  or  accessory  gill),  the  hypobranchial 
gland,  the  anus,  and  frequently  the  rectum  as  well,  the  nephridial  apertures  and 
often  the  renal  organ  also,  the  genital  apertures,  and  frequently  the  pericardium, 
with  the  enclosed  heart. 

Starting  with  the  Chitonidce,  which,  as  has  already  been  described  (p.  42). 
must  be  considered  as  the  most  primitive  of  all  living  Molluscs,  we  have  : — 

The  median  anus,  lying  at  the  posterior  end  of  the  body  in  the  mantle  groove  ; 
on  each  side  of  it  anteriorly  the  nephridial  apertures,  and  again  on  each  side,  in 
front  of  these,  the  genital  apertures. 

Assuming  this  to  be  the  primitive  arrangement,  we  have  the  following  important 
variations. 

A.  Gastropoda. 
1.  Prosobranchia. 

>'.  Diotocardia. — In  Fissure? la,  the  pallial  complex  is  still  quite  symmetrical, 
but  instead  of  lying  posteriorly,  as  in  Chiton,  it,  together  with  the  mantle  and  the 
pallial  cavity,  lies  on  the  front  of  the  visceral  dome.  We  have  to  imagine  that  the 
whole  complex  has  shifted  forward  along  the  right  side  of  the  body,  so  that  the  gill 
originally  on  the  left  has  come  to  lie  on  the  right  anteriorly,  and  that  originally  on 
the  right  now  lies  anteriorly  on  the  left,  and  the  same  applies  to  the  other  organs 
belonging  to  the  complex. 

1  Steinmann,  Bericht  Freiburg  Gesellsch.,  iv.  pp.  113-129. 


72 


COMPARATIVE  ANATOMY 


CHAP. 


In  order  to  prevent  confusion,  the  hypothetical  original  position  of  each  organ 
will  be  denoted  by  ur  ( =  originally  right)  and  ul  ( =  originally  left)  in  brackets. 

In  the  upper  part  of  the  mantle  cavity  in  Fissu-rella,  beneath  the  median 
aperture  in  the  mantle  and  shell,  lies  the  anus,  and  immediately  to  its  right,  the 
right  (ul)  nephridial  aperture,  immediately  to  its  left  the  left  (ur)  nephridial 
aperture  ;  the  right  (ul)  and  left  (itr).  ctenidia,  again,  lie  symmetrically  to  the  right 
and  left.  There  are  no  distinct  osphradia,  and  the  genital  apertures  are  wanting  as 
the  genital  gland  opens  into  the  right  nephridium. 

Haliotis. — The  mantle  cavity  has  here  shifted  to  the  left,  and  the  rectum, 
attached  to  the  mantle  fold,  runs  forward  some  way  through  it,  so  that  the  anus  is 
at  a  considerable  distance  from  the  posterior  apex  of  the  cavity.  On  the  right  of  the 
rectum  lies  the  right  (ul),  and  to  its  left  the  left  (ur)  ctenidium,  both  fastened  to 
the  mantle,  and  stretching  far  forward.  The  right  and  left  nephridial  apertures 
lie  near  the  bases  of  the  ctenidia,  in  the  upper  and  posterior  part  of  the  mantle 


FIG.  70. — The  same  specimen  from  the  left 
side.     Lettering  as  before  ;  o,  mouth. 


FIG.  69.— Anterior  portion  of  Patella,  from 
above,  after  removal  of  the  mantle  fold  (after 
Ray  Lankester).  a,  Tentacle ;  b,  foot ;  c, 
pedal  muscles  (shell  muscles) ;  d,  osphradia  ; 
e,  mantle  fold  ;  /,  aperture  of  the  right  nephri- 
dium ;  g,  anal  papilla  and  anus ;  h,  papilla  and 
aperture  of  the  left  nephridium  ;  i,  left  nephri- 
dium ;  A-,  right  nephridium ;  I,  pericardium  ; 
n,  digestive  gland  (liver) ;  m,  cut  edge  of  the 
mantle ;  p,  snout. 

cavity.  Between  the  rectum  and  the  left  ctenidium,  also  on  the  mantle,  is  found 
the  long,  well-developed  hypobranchial  gland  (mucous  gland),  which  stretches  as  far 
forward  as  the  gill.  Only  a  small  portion  of  the  gland  lies  to  the  right  between 
the  rectum,  as  far  as  it  runs,  and  the  right  ctenidium.  There  are  two  osphradia 
which  run  as  bands  along  the  axes  of  the  ctenidia  facing  the  mantle  cavity. 

Turbinidae  and  Trochidse.  —  Only  the  left  (ur)  ctenidium  of  Haliotis  is  here 
retained  ;  it  lies  far  to  the  left  on  the  roof  of  the  mantle  cavity,  i.e.  on  the  mantle. 
The  rectum  runs  far  forward  along  this  roof.  Two  nephridial  apertures  lie  on 
papillae  in  the  base  of  the  cavity,  at  the  sides  of  the  rectum.  The  hypobranchial 
gland  is  found  in  various  stages  of  development,  the  highest  being  attained  in  the 
Turbinidce.  It  is  largest  between  the  rectum  and  ctenidium,  i.e.  between  the 
right  side  of  the  latter  and  the  left  side  of  the  former.  In  the  Turbinidce,  however, 
a  portion  of  it  lies  to  the  right  of  the  rectum.  There  is  a  diffuse  osphradium  on  the 
axis  of  the  gill. 

Neritina. — There  is  here  only  one  gill  (the  left  (ur)  in  Haliotis)  shifted  somewhat  far 
to  the  right.  The  rectum  lies  asymmetrically  to  the  right  in  the  respiratory  cavity, 


VII 


MOLLUSCA—THE  PALLIAL  COMPLEX 


73 


reaching  so  far  forward  that  the  anus  is  found  near  the  right  edge  of  the  mantle  cleft. 
There  is  only  one  nephridial  aperture  to  the  left  of  the  base  of  the  ctenidium,  far  up^in 
the  mantle  cavity.  The  inner  surface  of  the  mantle,  between  the  rectum  on  the  right 


_^ 


20 


Jui 


FIG.  71.— Pyrula  tuba,  male,  taken  out  of  the  shell  (after  Souleyet).  The  mantle  is  cut  open 
along  its  base  and  right  side,  and  laid  back  to  the  left ;  the  position  of  the  pallial  organs  is  thus 
reversed.  1,  Proboscis;  2,  snout ;  3,  foot ;  4,  penis ;  5,  seminal  duct,  which  is  continued  at  15; 
6,  floor  of  the  pallial  cavity  =  nuchal  integument;  7,  colmnellar  muscle;  8,  intestine;  9,  heart  in 
the  opened  pericardium  ;  10,  digestive  gland  (liver)  ;  11,  testes  ;  12  and  13,  renal  organs  ;  14,  renal 
aperture  ;  15,  seminal  duct ;  16,  rectum  ;  17,  hypobranchial  gland  ;  18,  anus  ;  19,  ctenidium  (gill)  ; 
20,  mantle  ;  21,  osphradium  ;  22,  respiratory  siphon. 

and  the  gill  on  the  left,  is  glandular,  and  represents  the  slightly  differentiated  hypo- 
branchial  gland.     The  genital  aperture  lies  close  to  the  anus. 

Docoglossa. — In   the   Patellidce  (Figs.  69,  70)  a   short   conical  portion  of  the 


74  COMPARATIVE  ANATOMY  CHAP. 

rectum  projects  into  the  small  mantle  cavity.  This  anal  cone  is  not  median,  but 
is  distinctly  shifted  to  the  right.  To  its  right  and  left  lie  the  nephridial  apertures, 
raised  on  short  conical  papilla?.  There  is  no  separate  genital  aperture.  In  some 
forms  (Tectura,  Scurria,  Acmcca)  one  ctenidium  is  found  attached  to  the  mantle,  on 
the  left  side  of  the  pallial  cavity.  Further  details  as  to  the  gills  in  the  Patellidce 
will  be  given  later  on.  We  further  find,  on  the  floor  of  the  cavity,  on  each  side, 
traces  of  an  osphradium  in  the  shape  of  a  small  patch  of  sensory  epithelium,  which 
may  be  raised  on  a  prominence.  It  is  doubtful  if  the  prominence  found  in  Patella 
close  to  each  osphradium,  containing  a  blood  sinus  divided  up  by  septa,  can  be 
considered  as  a  rudimentary  gill.  These  prominences  rise  from  the  floor  of  the 
mantle  cavity,  whereas  in  Tectura,  for  example,  in  which  a  true  gill  still  occurs  on 
the  left,  it  lies  far  removed  from  the  left  osphradium,  in  the  usual  position  on  the 
roof  of  the  cavity,  i.e.  on  the  inner  surface  of  the  mantle. 

b.  Monotocardia. — In  this  division,  the  numerous  forms  of  which  show  little 
variety  of  organisation,  the  arrangement  of  the  pallial  complex  is  very  uniform. 
The  single  genital  aperture  is  always  distinct  from  the  single  nephridial  aperture. 
The  position  of  the  organs  in  the  spacious  pallial  cavity  (Fig.  71),  from  right 
to  left,  is  as  follows  : — 

1.  To  the  extreme  right,  lies  the  afferent  duct  of  the  genital  organs  (ovary  or 
seminal  duct),  which  runs  more  or  less  far  forward,  in  the  mantle  cavity. 

2.  In  contact  with  this,  but  quite  on  the  roof  of  the  cavity,  is  the  rectum. 

3.  To  the  left  of  the  rectum,  far  back  in  the  base  of  the  mantle  cavity,  lies  the 
slit-like  nephridial  aperture,  which  pierces  the  wall  separating  the  cavity  from  the 
renal  organ  behind  and  above  it.     Exceptions  occur  in  Paludina  and  Valvata,  in 
which  this  aperture  is  shifted  forward  to  the  end  of  a  urinary  duct  which  runs  on 
the  mantle. 

4.  On  the  roof  of  the  mantle  cavity  are  found  the  hypobranchial  glands  (mucous 
and  purple  glands),  which  are  developed  in  varying  degrees. 

5.  Quite  to  the  left,  and  also  on  the  roof  of  the  cavity,  the  ctenidium,  feathered 
on  one  side  (the  left  (ur)  of  Haliotis  and  Fissurella),  at  whose  base,  deep  back  in  the 
cavity,  the  pericardium  is  visible  with  the  ventricle  and  auricle  seen  through  it. 

6.  Finally,    to   the   extreme   left,  lies  the   osphradium,  which   is   always   well 
developed  and  sharply  circumscribed,  and  is  either  filamentous  or  feathered  on  two 
sides,  and  attached  to  the  roof  of  the  pallial  cavity. 

The  position  of  the  organs  in  the  pallial  complex  of  the  Heteropoda,  certain  forms 
of  which,  such  as  Atlanta,  are  closely  related  to  the  other  Monotocardia,  requires  to 
be  re-investigated.  The  osphradium  lies  at  the  base  of  the  gill. 

2.  Pulmonata. 

In  the  Pulmonata,  the  single  or  double  ($  and  6  )  genital  aperture  (Fig.  72)  no 
longer  belongs  to  the  pallial  complex,  but  lies  outside  the  mantle  cavity  laterally  on 
the  head  or  neck.  In  Onddium  the  male  aperture  lies  anteriorly  under  the  right 
tentacle,  the  female  posteriorly,  near  the  anus. 

Bearing  in  mind  that  the  mantle  or  pulmonary  cavity  communicates  with  the 
exterior  only  by  means  of  the  respiratory  aperture  lying  on  the  right,  we  have  the 
following  arrangement  of  the  pallial  complex  as  typical  (excluding  such  aberrant 
forms  as  Daudebardia,  Testacella,  and  Onddium). 

1.  On  the  extreme  right  of  the  pulmonary  cavity  lies  the  rectum,   the  anus 
opening  in  the  respiratory  aperture. 

2.  On  the  roof  at  the  back  of  the  cavity  lies  the  nephridium  (kidney). 

3.  To  the  left,  near  the  kidney,  also  far  up  in  the  cavity,  and  on  its  roof,  lies 


vii  MOLLUSCA—THE  PALLIAL  COMPLEX  75 

the  pericardium,  containing  the  ventricle  and  auricle,  the  latter  lying  in  front  of 

S 

7 


r, 

fffr, 

FIG.  72. — Helix  aspersa,  fully  extended  from  the  right  (after  Howes),  a,  Anus  appearing  in  the 
respiratory  aperture,  pl-2  ;  s,  shell ;  p,  edge  of  shell  aperture  ;  ga,  genital  aperture  ;  ti,  optic  tentacle  ; 
t,  anterior  tentacle  ;  ?->,  upper  lip. 

the  former.     From  the  ventricle  the  aortic  trunk  runs  upward  and  backward,  and 
from  the  auricle  rises  the  pulmonary  vein, 
which  runs  forward  along  the  roof  of  the 
pulmonary  cavity. 

4.  The   respiratory  vascular   network 
spreads  over  the  whole  remaining  surface 
of  the  roof  of  the  pulmonary  cavity,  and 
is  thus  in  front  of  the  kidney  and  peri- 
cardium. 

5.  An  osphradium  has  till  now  only 
been  found  in  the  Basommatophora  (Plan- 
orbis,  Physa,  Limnaeus},  near  the  respira- 
tory aperture,  and  among  the  Stylommato- 
ph&ra  in   Testacella  on  the   floor  of  the 
pulmonary  cavity  at  its  extreme  posterior 
angle. 

The  floor  of  the  pulmonary  cavity  (the 
dorsal  nuchal  integument)  is  smooth  and 
devoid  of  organs. 

The  arrangement  of  the  efferent  ducts 
of  the  renal  organ  varies  and  deserves 
special  description  (Fig.  73). 

1.  The  anterior  side  of  the  renal  sac 
opens  on  a  simple  papilla  in  the  mantle 


cavity  (Bulimus  oblongus,  and  some  species 
of  Planorbis)  (Fig.  73  A). 

2.  The  papilla  lengthens  and  runs  for- 
ward as  a  straight  ureter  (primary  ureter). 
This  occurs  in  most  Basommatophora,  and 
some  species  of  Eulimus,  Cionella,  Pupa, 
Helix  (B). 

3.  The  ureter  runs  backward  along  the 

kidney,  and  opens  at  the  base  of  the  respiratory  cavity, 
of  Helix  (C). 

4.  A  secondary  urinary  duct  is  added,  becoming  constricted  from  the  wall  of 


FIG.  73. —  Six  diagrams  illustrating  the 
variations  in  the  renal  ducts  in  the  Pul- 
monata.  The  organs  are  supposed  to  be  seen 
through  the  mantle  above  them.  1,  Free  edge 
of  mantle  ;  2,  respiratory  aperture ;  3,  rectum ; 
4,  kidney ;  5,  pericardium ;  6,  auricle ;  7,  ven- 
tricle; 8,  primary  urinary  duct;  9,  secondary 
urinary  duct,  which,  in  D,  is  a  groove.  Further 
explanations  found  in  the  text. 


Testacella,  and  some  forms 


76  COMPARATIVE  ANATOMY  CHAP. 

the  pulmonary  cavity,  and  at  first  forming  a  more  or  less  closed  channel  along 
which  the  urinary  discharge  can  be  forwarded  from  the  base  of  the  cavity  to  the 
respiratory  aperture.  Some  species  of  Bulimus  and  Helix  (D). 

5.  The  secondary  urinary  duct  becomes  closed,  and  opens  either  alone  or  with 
the  anus  into  the  pulmonary  cavity.    Some  species  of  Bulimus,  Helix,  Daudebardia, 
Vitrina,  Hyalinia,  Zonites,  Arion,  etc.  (E). 

6.  The  end  of  the  secondary  urinary  duct  and  the  end  of  the  rectum  together 
form   a  cloaca  which   is   distinct  from  the  pulmonary  cavity,"  and  opens  close  to 
the  respiratory  aperture.     Limax,  Amalia,  and  some  species  of  Daudebardia  (F). 

When  the  primary  urinary  duct  runs  back  along  the  kidney  it  is  externally  in- 
distinguishable from  the  substance  of  the  latter,  and  it  thus  often  appears  as  if  the 
duct  rose  from  the  posterior  end  of  the  renal  organ. 

The  variations  which  occur  in  the  position  of  the  organs  of  the  pallial  complex 
in  the  carnivorous  Pulmonata  are  specially  interesting.  In  a  series  of  car- 
nivorous forms,  commencing  probably  with  Hyalinm  among  the  Stylommatophora, 
and  proceeding  through  Daudebardia  to  the  extraordinary  genus  Testacella,  we  find 
progressive  diminution  of  the  visceral  dome  and  its  displacement  to  the  posterior 
end  of  the  body,  simplification  and  diminution  of  the  shell,  and  further,  a  shifting 
back  of  the  liver  and  genital  organs  from  the  visceral  dome  into  the  nuchal  portion 
of  the  ccelom,  which  now  is  found  along  the  whole  length  of  the  dorsal  surface  of  the 
foot.  Finally,  in  Testacella  and  certain  Daudebardia,  the  visceral  dome  completely 
disappears,  and  the  pulmonary  cavity  covered  by  the  shell  is  alone  left,  the  cavity 
reaching  up  to  the  apex  of  the  shell.  The  floor  of  this  cavity,  and  indeed  the  whole 
cavity,  with  the  mantle  and  the  shell,  sink  down  into  the  body.  In  this  way 
Testacella,  which  follows  its  prey,  the  earthworm,  into  its  underground  passages,  is 
admirably  adapted  to  its  manner  of  life  ;  its  body  is  slender,  and  the  somewhat 
flat  shell  at  its  posterior  end,  which  does  not  stand  out  above  the  surrounding  sur- 
face of  the  body,  in  no  way  hinders  its  movements.  These  alterations,  however, 
especially  the  displacement  of  the  visceral  dome  to  the  posterior  end  of  the  body, 
are  accompanied  by  important  alterations  of  position  in  the  pallial  organs,  which 
finally  lead  to  the  condition  called  opisthopneumonic. 

It  is  important  to  note  that  concrescence  of  the  mantle  and  the  subjacent  dorsal 
integument  is  complete  except  at  the  respiratory  aperture  on  the  right,  and  that  the 
latter  shifts  farther  and  farther  back,  in  its  relation  to  the  pulmonary  cavity,  till,  in 
Testacella,  its  position  is  almost  terminal. 

The  first  important  step  in  the  displacement  of  the  pallial  organs  is  seen  in 
Daudebardia  rufa.  The  pericardium,  instead  of  lying  far  back  at  the  base  of  the 
pulmonary  cavity,  here  lies  far  forward  on  its  roof,  so  that  by  far  the  greater  portion 
of  the  vascularised  pulmonary  tissue  lies  on  the  roof  behind  the  pericardium  (Fig. 
74  A).  Daudebardia  rufa  is  thus  actually  opisthopneumonic.  But  in  this  case  the 
relative  position  of  the  ventricle  and  auricle  is  still  unaltered.  The  auricle  is,  as 
before,  placed  in  front  of  the  ventricle  ;  the  pulmonary  vein  from  the  auricle  is  thus 
obliged  to  bend  round  in  order  to  run  backward,  while  the  aorta,  which  becomes 
almost  exclusively  the  anterior  or  cephalic  artery,  ^supplying  that  portion  of  the  body 
which  lies  in  front  of  the  visceral  dome  (by  far  the  greatest  part),  must  bend  forward 
from  the  ventricle. 

In  another  Daudebardia,  D.  saulcyi,  the  case  is  somewhat  similar,  but  the 
kidney  and  pericardium  together  form  a  sort  of  sac  which  hangs  down  into  the  pul- 
monary cavity  from  its  roof.  In  this  sac,  the  ureter  lies  dorsally  and  the  peri- 
cardium ventrally  to  the  kidney.  The  floor  of  the  cavity  sinks  right  and  left  deep 
into  the  subjacent  region  of  the  body. 

If  we  imagine  that  the  pulmonary  vein  which  runs  back  from  the  anteriorly 


VII 


MOLLUSCA—THE  PALLIAL  COMPLEX 


77 


placed  auricle,  and  the  aorta  which  runs  forward  from  the  chamber  lying  behind  the 
auricle  have  pulled  these  chambers  round  in  such  a  way  that  the  flow  of  blood  can 
have  a  straight  course  (cf.  diagram,  Fig.  74),  the  ventricle  will  then  come  to  lie 
in  front  of  the  auricle.  Indeed,  the  pericardium  (with  the  ventricle  and  auricle) 
has  actually  twisted  round  180°.  In  this  twisting  it  has  been  followed  by  the 
kidney,  which  is  connected  with  it  by  the  reno-pericardial  ^aperture,  so  that  the 
latter  organ  no  longer  lies  to  the  right  but  to  the  left  of  the  pericardium,  the  aper- 
ture of  the  urinary  duct  remaining  at  its  former  place.  The  whole  reno-pericardial 
complex,  as  compared  with  its  typical  position  in  the  Pulmonata,  is  quite  reversed. 
This  reversal  is  characteristic  of  Testacella. 

It  is,  further,  noteworthy  that,  in  Testacella,  the  floor  of  the  pulmonary  cavity 
becomes  invaginated  anteriorly  into  the  body  below  it  to  form  a  large  air  sac.  The 
walls  of  this  sac  are  not  supplied  with  blood  vessels,  and  it  seems  to  serve  merely  as 
a  reservoir  of  air.  In  many  Testacellidce  the  reno-pericardial  complex  hangs  down  in 
the  shape  of  a  sac  into  this  air  sac  from  the  roof  of  the  pulmonary  cavity. 

In  the  Vaginulidce  and  the  Oncidia  the  arrangement  of  the  organs,  originally 
belonging  to  the  pallial  complex,  deviates  still  further  from  the  type.  A  shell  is 


FIG.  74.— Diagrams  to  illustrate  the  changes  of  position  in  the  pallial  organs  of  Daude- 
bardia  and  Testacella  (adapted  from  figures  by  Plate).  Mantle  organs  drawn  as  in  Fig.  73.  A, 
Daudebardia  rufa ;  B,  Hypothetical  stage,  the  pallial  complex  of  A  twisted  round  90° ;  C, 
Testacella.  1,  Respiratory  aperture  ;  2,  kidney ;  3,  ureter  or  urinary  duct ;  4,  reno-pericardial 
aperture  (renal  funnel) ;  5,  ventricle ;  6,  auricle ;  7,  aorta ;  8,  pulmonary  vein  ;  9,  pulmonary 
vascular  network. 

wanting  in  the  adult  and  a  mantle  also  ;  and  the  mantle-  or  pulmonary  cavity 
seems  in  consequence  to  have  atrophied.  The  pericardium  lies  posteriorly  to 
the  right,  sunk  into  the  integument,  the  ventricle  lying,  as  in  Testacella,  in  front 
of  the  auricle.  Respiration  takes  place  principally  through  the  skin  ;  in  the  amphib- 
ious Oncidia  it  is  assisted  by  dorsal  papillae.  In  Vaginulus,  the  urinary  duct  joins 
the  proctodaeum  to  form  a  cloaca  which  somewhat  widens  at  the  point  of  junction, 
and  opens  externally  at  the  posterior  part  of  the  body.  The  same  is  the  case  in 
most  Oncidia,  but  in  Oiicidium  cclticum,  the  urinary  duct  and  the  rectum  emerge 
separately,  but  one  close  to  the  other,  at  the  posterior  end  of  the  body.  Close  to 
these  apertures  lies,  in  all  cases,  the  female  genital  aperture  ;  the  male  aperture, 
however,  lies  anteriorly  to  the  right  below  the  tentacle. 

The  cloaca  just  mentioned,  which  is  filled  with  air,  has  given  rise  to  interesting 
discussions.  From  its  wall  there  rise  into  the  lumen  closely  packed  folds,  which 
may  also  be  continued  along  the  posterior  portion  of  the  urinary  duct.  The  cloaca 
has  therefore  been  considered  by  some  to  be  a  rudimentary  pulmonary  cavity,  into 
which  the  urinary  duct  and  the  rectum  open.  The  present  writer  holds  the  opinion, 


78  COMPARATIVE  ANATOMY  CHAP. 

provisionally,  that  this  cloaca  has  arisen  by  the  junction  of  the  terminal  portions  of 
the  secondary  ureter  with  the  rectum,  as  in  other  Pulmonata,  but  that  here  the  pul- 
monary cavity  having  atrophied,  it  opens  outward  direct,  i.e.  no  longer  through  a 
respiratory  aperture.  Others,  again,  have  thought  the  arrangement  in  Oncidium 
and  Vaginulus  to  be  primitive,  the  pulmonary  cavity  appearing  here  first  as  an 
insignificant  widening  of  the  terminal  portion  of  the  primary  ureter. 

If  this  were  the  case,  then  the  condition  described  above  (p.  75,  1)  for  Bulimus 
oblongus,  where  the  kidney  opens  on  a  papilla  direct  into  the  base  of  the  pulmonary 
cavity,  would  be  thus  explained  :  the  pulmonary  cavity  would  have  to  be  considered 
as  a  much  widened  primary  urinary  duct.  Then,  in  this  primary  ureter  (pulmonary 
cavity)  would  follow  the  successive  stages  of  the  development  of  the  secondaiy 
ureter,  at  first  an  open  and  later  a  partially  closed  channel,  and  finally  a  closed 
tube,  so  that  at  last,  as  in  Helix  pomatia,  the  primary  ureter  is  divided  into  two 
distinct  portions,  viz.  the  much  widened  pulmonary  cavity  and  the  secondary 
ureter.  But  in  the  Limnccidce,  for  example,  the  pulmonary  cavity  admittedly 
corresponds  with  the  mantle  cavity  of  other  Gastropods.  The  Pulmonata  would 
thus  fall  into  two  groups,  the  Nephropneusta  (Stylommatophora),  in  which  the 
pulmonary  cavity  =  the  widened  primary  ureter,  and  the  Branchiopneusta  (Basom- 
matophora,  p.  parte),  in  which  the  pulmonary  cavity  =  the  mantle  cavity  of  other 
Gastropods. 

We  consider  this  view  incorrect  because  of  the  uniformity  of  the  whole  organisa 
tion  in  the  Pulmonata,  and  especially  because  of  the  occurrence  of  an  osphradium  in 
the  pulmonary  cavity  of  a  Stylommatophore  (Nephropneusta],  viz.  in  the  genus 
Testacella.  For  the  osphradium  invariably  belongs  to  the  mantle  cavity,  being 
primitively  connected  with  the  ctenidium,  it  never  lies  in  the  urinary  duct. 

3.  Gastropoda  Opisthobranchiata. 

We  can  here  speak  of  a  pallial  complex  only  in  connection  with  the  .Tectibranchia, 
since  in  them  alone  is  a  distinct  mantle  fold  developed  on  the  right  side  of  the 


FIG.  75.— Aplysia,  right  aspect,  the  right  parapodium  (15)  turned  downwards;  the  pallial 
complex  is  seen  under  the  mantle  fold  7  (after  Lankester).  1,  Anterior  tentacle ;  2,  eyes  ;  3, 
posterior  tentacle  (rhinophore) ;  4,  left  parapodium  ;  5,  seminal  furrow ;  6,  genital  aperture ;  7, 
mantle  fold ;  8,  gland ;  0,  osphradium  ;  10,  outline  of  some  inner  organ  seen  through  the  integument ; 
11,  nephridial  aperture  ;  12,  ctenidium ;  13,  anus ;  15,  right  parapodium  ;  16,  anterior  portion 
of  the  foot.  (There  should  be  no  connecting  line  between  6  and  9.) 

body.    The  general  order  of  the  organs  in  the  pallial  cavity  (Fig.  75)  is  as  follows  :— 

1.   Far  back,  and  often  hardly  or  not  at  all  covered  by  the  mantle,  sometimes  at  the 

summit  of  a  conical  prominence,  lies  the  anus,  and  near  it  occasionally  an  anal  gland. 


VII 


MOLLUSGA—THE  PALLIAL  COMPLEX 


79 


2.  In  front  of  the  anus,  between  it  and  the  ctenidium,  is  the  nephridial  aperture. 
Following  these  there  may  be—  ^ 

3.  A  hypobranchial  gland. 

4.  The  ctenidium. 

5.  At  the  base  of  the  ctenidium  or  on  its  axis, 
the  osphradium. 

Were  this  complex  of  organs  to  be  shifted  along 
the  edge  of  the  body,  we  should  have  the  arrange- 
ment found  in  the  Monotocardia  among  the  Proso- 
brn.n<:hia.  The  correspondence  is,  however,  appar- 
ently marred  by  the  position  of — 

6.  The  genital  aperture,  which  in  the  Opistho- 
branchia  lies  farthest    forward  of  all   the  pallial 
organs. 

In  all  other  Opisthobranchia  (after  excluding  the 
Tectibranctiia}  the  pallial  complex  is  broken  up 
when  the  mantle  and  the  true  ctenidium  disappear. 
The  only  exception  to  this  is  found  in  the  Phyl- 
lidiidcc,  where,  apart  from  the  gills,  a  similar 
arrangement  to  that  in  the  Tcctibranchia  occurs. 
The  single  or  paired  genital  aperture  always  lies 
asymmetrically  on  the  right  side  in  front  of  the 
anus,  which  is  sometimes  found  asymmetrically  on 
the  right  side,  and  sometimes  has  a  median  dorsal 
position  between  the  middle  and  the  posterior  end 
of  the  body.  The  renal  aperture  lies  between  the 
anus  and  the  genital  aperture,  sometimes  close  to 
the  latter. 

In  the  Ptcropoda  gymnosomata  (Fig.  76)  the 
shell  and  mantle  are  wanting.  The  ctenidium, 
when  retained,  as  in  the  Dcxiobranchia  and  Pni'iiii<.>- 
il.-t'iiM,  lies  somewhat  far  back  on  the  right  side  of 
the  body,  far  behind  the  anus.  On  the  disappear-  ^ 

ance  of  the  mantle,  it  evidently  shifted  back  from  Flo/ 76._pneiimoderma,  from  the 
its  original  position  between  the  anus  and  the  genital  rjght  side,  diagrammatic  (after  Pel- 
aperture,  while  the  osphradium,  which  is  generally  seneer).  1,  Right  process  bearing 
found  close  to  the  ctenidium,  has,  as  far  as  has  yet 
been  observed,  retained  its  original  position. 

The  anus  lies  anteriorly  behind  the  right  fin  ; 
the  nephridial  aperture  lies  close  by,  either  distinct 
or  united  with  the  anus  at  the  base  of  a  common 
cloacal  depression.  Immediately  in  front  of1  this 
lies  the  osphradium,  then  follows,  considerably 
farther  forward  on  the  neck,  to  the  right  behind 
the  base  of  the  right  fin,  the  genital  aperture,  from 
which,  as  in  many  Tectibranchia,  a  ciliated  furrow 
runs  forward  along  the  surface  of  the  body  to  the 
aperture  of  the  penis,  which  lies  to  the  right  in  front  of  the  foot. 

All  Thccosoniata  have  a  mantle  and  a  mantle  cavity,  and  often  a  shell  as  well  ;, 
in  the  Cyinbuliidce,  the  latter  is  replaced  by  a  cartilaginous  pseudoconch,  a  sub- 
cutaneous formation  of  the  mantle. 

Among  the  Thccosomata,  the  Limacinidce  indicate  the  primitive  arrangement ; 
they  possess  a  dorsal  or  anterior  mantle  cavity,  a  coiled  shell,  and  an  operculum. 


hooks    (Hakensack)    evaginated ;    2> 
proboscis  ;  3,  right  buccal  tentacle  ; 

4,  position  of  the  right  nuchal  ten- 
tacle ;   5,  right  fin  (parapodium) ;  6, 
seminal  furrow  ;  7,  genital  aperture  ; 

5,  position  of  the  jaw  ;  9,  ventral  pro- 
boscidal  papilla  ;  10,  right  buccal  ap- 
pendage bearing  suckers  ;  11,  head  ; 
12,  aperture  of  penis ;  13,  right  anterior 
pedal  lobe ;   14,  anus ;   15,  posterior 
pedal  lobe  ;  16,  ctenidium ;  17,  pos- 
terior adaptive  gill ;  d,  v,  a,  p,  dorsal, 
ventral,  anterior,  posterior. 


80 


COMPARATIVE  ANATOMY 


CHAP. 


The  ctenidium,  however,  is  wanting.  In  the  base  of  the  pallial  cavity,  to  the  left, 
lies  the  pericardium,  and  immediately  in  front  of  it  the  kidney,  with  a  narrow 
aperture  into  the  cavity  ;  then  follow  the  osphradium  (where  this  has  been  found), 
and,  at  the  extreme  right  of  the  cavity,  the  anus  with  the  anal  gland.  The  mantle 
gland  (hypobranchial  gland,  shield)  is  found  on  the  roof  of  the  pallial  cavity.  The 
genital  aperture  lies  to  the  right  anteriorly  in  the  cephalic  region  ;  from  it  a 
ciliated  channel  or  furrow  runs  dorsally  to  the  aperture  of  the  penis,  which  lies 
anteriorly  between  the  fins. 

As  compared  with  the  Limacinidce,  i.e.  the  Thecosomata  with  coiled  shell,  the 


-7 


FIG.  77.— A,  B,  C,  Three  diagrams  to  illustrate  the  relation  of  the  Limacinidae  to  the 
Cavoliniidae  (after  Boas).  A,  Limacinidae ;  B,  hypothetical  intermediate  stage  between 
the  Limacinidae  and  the  CavoliniidaB.  The  visceral  dome  twisted  90°.  C,  Cavoliniidae.  All  the 
diagrams  from  the  ventral  or  posterior  side.  In  A  the  visceral  dome  is  drawn  straight,  whereas  it 
is  in  reality  coiled.  1,  Right  tin  (parapodium)  ;  2,  foot  bent  forward  ;  3,  genital  aperture ;  4,  ten- 
tacular appendage  of  the  mantle  edge  ;  5,  anus  ;  6,  masticatory  stomach  ;  7,  gonad. 

Cavoliniidce  and  Cymbuliidce,  or  Thccos&mata  with  straight  shell,  show  a  very 
different  arrangement  of  the  pallial  complex,  which  can  only  be  explained  by  the 
supposition  that  the  larger  posterior  portion  of  the  body  (the  visceral  dome)  of 
the  Limaciwidas,  with  all  the  pallial  organs  belonging  to  it,  has  twisted  round  the 
longitudinal  axis  of  the  body  180°,  in  relation  to  the  cephalic  region  with  the 
genital  apertures  belonging  to  it.  Sifch  a  twist  gives  the  organs  the  position  they 
actually  occupy  in  the  Cavoliniidce  and  Cymbuliidce  ;  the  posterior  (ventral)  pallial 
cavity  containing,  on  the  left  the  anus,  on  the  right  the  pericardium  and  kidney  and 
the  osphradium,  the  genital  aperture  occupying  its  original  position  to  the  right. 
The  cause  and  significance  of  this  twist  are  at  present  unknown. 


B.  Scaphopoda. 

There  is  no  gill  in  the  posteriorly  placed  mantle  cavity.  The  anus  lies  in  the 
middle  line  above  the  foot,  having  a  nephridial  aperture  on  each  side  of  it.  There 
are  no  distinct  genital  apertures. 


vii  MOLLUSCA—THE  PALLIAL  COMPLEX  81 


C.  Lamellibranchia. 

The  general  arrangement  of  the  organs  in  the  mantle  cavity  of  the  Lamelli- 
branchia has  already  been  described.  The  strict  symmetry  of  the  body  in  this 
class  must  again  be  pointed  out.  All  originally  paired  organs  remain  paired  and 
symmetrical. 

The  two  nephridial  apertures  lie  on  the  body  above  the  base  of  the  foot,  or 
farther  back  near  the  posterior  adductor  muscle  ;  they  usually  lie  beneath  the  point 
of  attachment  of  the  gill-axis,  between  it  and  the  line  of  concrescence  of  the  (inner) 
ascending  lamella  of  the  branchial  leaf  with  the  foot,  where  such  concrescence  takes 
place.  In  the  Septibranchia,  on  the  contrary,  the  apertures  open  into  the  upper 
pallial  chamber. 

The  outer  genital  apertures  may  be  wanting,  and  in  this  case  the  genital 
products  are  ejected  through  the  nephridial  apertures,  which  is  the  primitive 
arrangement.  When  present,  in  diceceous  bivalves,  they  are  always  found  in  one 
pair,  and  lie  on  each  side  just  in  front  of  the  nephridial  apertures,  sometimes  in  the 
base  of  a  common  pit  or  furrow,  less  frequently  at  some  distance  from  these  aper- 
tures. There  are  no  special  copulatory  organs. 

In  hermaphrodite  Lamellibranchia  the  arrangements  may  vary  as  follows : — 

1.  Both   kinds   of  sexual   products  may   be   ejected   on   each   side   through  a 
common  aperture  (Ostrcea,  Pecten,  Cyclas,  Pisidium,  etc.). 

2.  There  may  be,  on  each  side,  two  distinct  apertures,  one  male  and  the  other 
female  (Anatiiiacea). 

3.  The  seminal  ducts  and  the  oviducts  may  unite  before  opening  to  form  a 
short,  common,  terminal  piece  (Septibranchia). 

The  osphradium  is  paired  in  the  Lamellibranchia,  and  always  lies  near  the 
posterior  adductor  muscle  over  the  visceral  ganglion,  at  the  point  of  insertion  of  the 
branchial  axis  on  the  body.  A  pair  of  sensory  organs  is  found  in  many  Lamelli- 
branchia, one  on  each  side  of  the  anus  (abdominal  sensory  organs),  or  to  the  right 
and  left  on  the  mantle  at  the  inner  aperture  of  the  siphons  of  the  Siphoniata  (pallial 
sensory  organs). 

Hypobranchial  glands  have  been  found  in  the  Protobranchia  (Nuculidcc  and 
Solcnomyidce}.  They  are  large  and  well  developed,  and  belong  to  the  mantle,  lying 
in  the  posterior  part  of  the  body  above  the  base  of  the  gill  on  each  side,  to  the  right 
and  left  of  the  pericardium,  and  in  front  of  the  posterior  adductor. 

The  leaf-like  oral  lobes  (labial  palps),  one  occiirring  on  each  side  of  the  mouth, 
between  it  and  the  anterior  end  of  the  base  of  the  gill,  will  be  described  more  in 
detail  in  another  place. 

D.  Cephalopoda. 

In  the  Cephalopoda  the  primitive  symmetry  of  the  pallial  complex  is  on  the 
whole  retained. 

If  we  cut  open  the  mantle  of  the  Nautilus  (Figs.  78  and  79),  which  covers  the 
posteriorly  placed  pallial  cavity,  and  lay  it  back  on  all  sides,  the  following  organs 
are  revealed  : — 

1.  On  each  side  there  are  two  gills,  an  upper  and  a  lower. 

2.  The  anus  lies  on  the  visceral  dome,  between  the  bases  of  the  four  gills. 

3.  Below  the  base  of  each  gill  is  found  a  nephridial  aperture — making  four  in  all. 

4.  Close  to  the  two  upper  neplmdial  apertures  lie  the  two  so-called  viscero- 
pericardial  apertures. 

5.  Between  the  bases  of  the  lower  gills  there  are    in  each  sex,    two  genital 
VOL.  II  G 


82 


COMPARATIVE  ANATOMY 


CHAP. 


CMX 


FIG.  78.— Pallial  complex  and  siphon  of  Nautilus  pompilius  ?  (after  Bourne  and  Lankester). 
v,  Valve  of  the  siphon  ;  TO,  right  genital  aperture  ;  m,  the  mantle  fold,  with  the  nidameutal  gland, 
folded  back ;  an,  anus  ;  cp,  left  aperture  of  the  secondary  coelom  ;  Ihn,  left  upper  nephriclial  aper- 
ture ;  lo,  aperture  of  the  left  rudimentary  oviduct ;  Ivn,  left  lower  nephritlial  aperture.  The  four 
ctenidia  are  not  lettered. 


FIG.  79.— Pallial  complex  of  Nautilus  pompilius  £  (after  Bourne  and  Lankester).  pe,  Penis ; 
a,  muscle  band  of  the  siphon ;  hp,  aperture  of  the  left  rudimentary  seminal  duct ;  nepJia,  nephp, 
lower  and  upper  nephridial  aperture  of  the  left  side  ;  olf,  left  ospliradium  ;  viscper,  left  aperture  of 
the  secondary  coelom  ;  an,  anus  ;  x,  supra-anal  papilla  of  unknown  significance  ;  c,  mantle  cut  off. 


VII 


MOLLUSCA—THE  PALLIAL  COMPLEX 


83 


FIG.  80.— Sepia  Savignyana,  from  behind  (after  Savigny).  The  greater  part  of  the  mantle  cut 
open  and  laid  back  on  the  right  side  (left  in  the  figure),  a,  Prehensile  tentacle ;  b,  oral  arm ;  c, 
mouth  with  jaws  ;  (?,  lower  aperture  of  siphon ;  e,  eye  ;  /,  locking  apparatus  of  theinantle  g ;  h,  right 
ctenidium  ;  i,  siphon  ;  k,  locking  apparatus  of  the  mantle  on  the  visceral  dome ;  I,  upper  aperture 
of  siphon ;  7/1,  anus  ;  n,  depressor  infundibuli ;  o,  penis  ;  p,  right  nephridial  aperture  ;  q,  posterior 
integument  of  the  visceral  dome  ;  r,  fin. 


84  COMPARATIVE  ANATOMY  CHAP. 

apertures,  but  only  that  on  the  right  side  is  functional.     In  the  male,  the  aperture 
is  produced  into  a  tubular  penis. 

6.  Above  the  bases  of  the  lower  gills  there  is  an  osphradium  on  each  side  placed 
on  a  papilla. 

7.  Above  the  anus  there  is  a  large  median  papilla  of  unknown  significance. 

8.  The  nidamental  gland  lies  dorsally  in  the  mantle. 

If  we  compare  with  the  above  the  pallial  complex  of  a  dibranchiate  Cephalopod, 
such  as  Sepia  (Fig.  80),  we  find  the  following  arrangements  : — 

1.  There  is  one  gill  on  each  side. 

2.  Along  the  median  line  of  the  visceral  dome,  the  rectum  and  the  duct  of  the 
ink-bag  descend  together,  to  open  through  a  common  aperture  at  the  tip  of  a  papilla 
at  the  base  of  the  siphon. 

3.  On  each  side  near  the  rectum,  above  the  anus,  a  nephridiai  aperture  occurs 
on  the  point  of  a  papilla. 

4.  Of  the  two  paired  genital  apertures  only  the  left  has  been  retained  in  Sepia 
and  many  other  Cephalopods  ;   this  lies  near  the  left  nephridiai  aperture  at  the 
summit  of  a  large  papilla  (penis).     In  the  female  Octopus,  the  genital  apertures  are 
paired  and  symmetrical,  and  lie  to  the  right  and  left  of  the  rectum. 

5.  The  two  nidamental   glands   (in   Decapoda)  lie  in    the  visceral  dome,   sym- 
metrically with  regard  to  the  median  line  ;  they  open  above  the  nephridiai  apertures 
into  the  mantle  cavity. 


VI.  The  Respiratory  Organs. 
The  True  Gills  or  Ctenidia. 

The  most  important  of  the  pallial  organs  in  the  Mollusca  is  the  gill, 
for  it  is  in  order  to  protect  it  that  the  mantle,  and  with  it  the  pallial 
cavity,  develop.  The  gill  found  in  the  mantle  cavity  is  throughout 
all  the  divisions  of  the  Mollusca  a  homologous  organ,  to  be  derived 
from  the  gill  of  a  common  racial  form.  But  since  this  gill  is  wanting 
in  certain  Mollusca  (e.g.  many  Opisthobranchia),  and  is  functionally 
replaced  by  new  organs  which  are  morphologically  altogether  uncon- 
nected with  it,  it  has  been  found  useful  to  distinguish  the  primitive 
Molluscan  gill  by  the  name  of  etenidium.  This  word,  therefore,  has 
a  special  morphological  significance. 

The  etenidia  of  the  Mollusca  are  originally  paired  and  symmetri- 
cally arranged  ciliated  processes  of  the  body  wall,  carrying  two 
rows  of  branchial  leaflets,  and  projecting  into  the  mantle  cavity. 

Venous  blood  flows  into  the  gills  through  afferent  vessels 
(branchial  arteries),  and  after  becoming  arterial  by  means  of  the 
respiration,  flows  through  efferent  vessels  (branchial  veins)  back  to 
the  body,  passing  first  through  the  heart.  At  or  near  the  base  of 
each  etenidium  there  always  lies  a  sensory  organ,  which  is  considered 
as  olfactory,  the  so-called  osphradium  or  Spengel's  organ. 

Such  primitive  etenidia  are  met  with  first  in  that  group  of  the 
Mollusca  which  has  undoubtedly  retained  more  primitive  characteristics 
than  any  other,  viz.  the  Chitonidce  among  the  Amphineura.  They  are, 
further,  found  in  all  other  Mollusca  which  have  retained  the  original 


VII 


MOLL USC A—  RESPIRATORY  ORGANS 


85 


bilateral    symmetry  of    the    body,   such    as   the  LameUibmnchia,   the 
Cephalopoda,  and — a  point  of  great  importance — also  in  the  primitive 


FIG.  81.— Ctenidia  of  various  Molluscs  (after  Ray  Lankester).  A,  CMton ;  B,  Sepia ;  C, 
Fissurella ;  D,  Nucula  ;  E,  Paludina.  ft,  Longitudinal  branchial  muscle  ;  abv,  afferent  branchial 
vessel ;  ebi;  efferent  branchial  vessel  (branchial  vein)  ;  gl,  paired  lamellae  (leaflets)  of  the  feathered 
gill ;  in  D :  d,  position  of  the  axis  ;  a,  inner  ;  6  and  c,  outer  rows  of  branchial  lamellae  ;  in  E :  i, 
rectum  ;  br,  branchial  filaments  ;  a,  anus. 

Gastropoda,  the  Zeugobranchia.  In  the  latter, Jiowever,  the  left  ctenidium 
was  originally  the  right  and  vice  versd,  but  this  will  be  dealt  with  more 
in  detail  later. 


86  COMPARATIVE  ANATOMY  CHAP. 

With  regard  to  the  number  of  gills  originally  present  on  each  side 
of  the  body,  opinions  are  divided.  Those  who  hold  that  there  were 
several  seem  justified  by  the  arrangement  in  Chiton,  where  numerous 
consecutive  ctenidia  lie  in  a  longitudinal  row  in  the  branchial  furrow 
(mantle  cavity)  on  each  side,  and  also  by  that  in  the  Nautilus,  which 
is  rightly  considered  the  most  primitive  of  extant  Cephalopods,  where 
four  gills  are  found  (Tetrabranchia).  We  shall,  however,  see  later  that 
the  other  view,  viz.  that  the  Mollusca  originally  possessed  only  one 
pair  of  ctenidia,  has,  to  say  the  least,  equal  claim  to  be  accepted. 

In  all  other  Mollusca  with  paired  ctenidia,  including  the  Lamelli- 
branchia,  there  is  only  one  pair  at  the  posterior  part  of  the  body. 
Further,  in  the  racial  form  of  the  Prosobranchia,  a  single  pair  of  gills 
must  be  assumed  to  have  occupied  a  posterior  position  in  a  mantle 
cavity  which,  with  them,  shifted  forward  later  to  the  anterior  position. 
The  Zeugobranchia  still  retain  this  single  pair  of  gills. 

In  most  Prosobranchia,  the  asymmetry  of  the  body  is,  also  seen  in 
the  gills,  only  the  left  gill  of  the  two  in  the  Fissurellidce  and  Haliolida 
being  retained,  the  right  completely  disappearing.  In  the  forms  which 
most  resemble  the  Fissurellidce  and  Haliotidce,  the  single-gilled  Dioto- 
cardia  (Turbinidce,  Trochidce,  etc.),  the  gill  is  still  feathered  on  both 
sides,  but  in  all  Monotocardia  it  has  only  a  single  row  of  leaflets. 

In  one  division  of  the  Opisthobranchia,  the  TectibrancMa,  one 
ctenidium  is  still  retained,  that  on  the  right  side.  Other  Opistho- 
branchia have  lost  the  true  ctenidium  together  with  the  mantle  cavity  ; 
it  may  be  replaced  by  analogous  (but  not  homologous)  respiratory 
organs,  such  as  adaptive  gills. 

The  Pulmonata,  in  consequence  of  their  adaptation  to  aerial 
respiration,  have  lost  the  ctenidia. 

The  blood,  which  has  become  arterial  in  the  ctenidia,  reaches  the 
heart  through  the  auricle,  and  passes  into  the  body  through  the 
arteries.  It  is  therefore  evident  that  a  close  relation  must  exist 
between  the  gills  and  auricles.  This  relation  is  briefly  as  follows  : 
where  the  gills  are  paired,  the  auricles  are  paired,  and  unpaired  gills 
are  accompanied  by  a  single  auricle  on  that  side  of  the  body  on  which 
the  gill  is  retained.  Where  gills  are  paired,  there  is  almost  always 
only  one  pair,  and  then  there  is  one  right  and  one  left  auricle. 

The  Nautilus  has  four  gills,  and,  to  correspond,  two  right  and  two 
left  auricles.  The  Ghitonidce,  on  the  other  hand,  in  spite  of  their 
numerous  pairs  of  gills,  have  only  one  right  and  one  left  auricle. 

The  Scaphopoda  possess  neither  true  ctenidia  nor  any  other  localised 
gills.  Kespiration  may  take  place  at  the  various  soft-skinned  surfaces 
which  come  in  contact  with  the  water,  such  as  the  inner  surface  of  the 
mantle,  the  tentacles,  etc. 

A.  Amphineura. 

Chitonidae.— -A  single  ctenidium  of  a  Chiton  (Fig.  82)  may  serve  as  a  type  of  the 
Molluscan  gill  with  its  two  rows  of  leaflets.     The  plumose  ctenidium  rises  freely  from 


MOLLUSCA— RESPIRATORY  ORGANS 


87 


the  base  of  the  branchial  groove  (mantle  cavity).     The  axis  here  takes  the  shape  of 
a  thin  septum.     At  each  side,  on  the  broader  surface  of  the  septum,  extending  from 


FIG.  82.— Structure  of  the  ctenidium  of  a  Chiton  (after  B.  Haller).  A,  Single  ctenidium  with 
its  double  row  of  branchial  leaflets.  B,  Transverse  section  of  the  gill  along  the  line  a-b  in  Fig.  A. 
1,  Narrow  blood  sinus  in  the  branchial  leaflet;  2,  septum  in  its  axis;  3,  longitudinal  muscle;  4, 
afferent  branchial  vessel ;  5,  efferent  branchial  vessel ;  6,  nerves ;  7,  long  cilia  on  the  branchial 
axis.  C,  2  pairs  of  branchial  leaflets  cut  through  at  right  angles  to  their  surfaces,  along  the  line  e-f 
in  Fig.  B.  1,  Same  as  in  Fig.  B ;  8,  space  between  the  consecutive  branchial  leaflets.  D,  Longi- 
tudinal section  of  the  ctenidium  somewhat  laterally  to  the  axis,  and  parallel  to  its  septum,  along 
the  line  c-d  in  Fig.  A.  This  section  is  part  of  a  transverse  section  of  the  body.  Lettering  as  in 
Figs.  B  and  C.  In  addition  :  9,  olfactory  ridge  of  the  branchial  epithelium  ;  10,  general  afferent 
branchial  vessel ;  11,  general  efferent  branchial  vessel ;  12,  pleuro-visceral  strand  of  the  nervous 
system.  Tlie  branchial  epithelium  is  everywhere  indicated  by  a  thick  black  line. 

base  to  tip,  there  is  one  row  of  smooth,  delicate  branchial  leaflets.  In  outline  they 
are  more  or  less  semicircular,  and  stand  crowded  together  in  great  numbers  almost 
like  the  leaves  of  a  book.  The  entire  surface  of  the  branchial  epithelium  is 
ciliated  ;  on  the  axial  epithe- 
Hum,  the  cilia  are  remarkably  A 

long.  On  that  side  of  the  axis 
which  is  turned  towards  the 
foot,  a  blood-vessel  runs  from 
base  to  tip,  conducting  venous 
blood  to  the  gill  (afferent 
branchial  vessel).  On  the  op- 
posite side,  which  faces  the 
mantle,  another  vessel,  the 
branchial  vein,  runs  from  the 
tip  to  the  base  of  the  gill,  and 
carries  the  blood,  which  has 
become  arterial  by  respiration, 
to  the  general  branchial  vein, 
and  through  it  to  the  auricle. 
These  vessels  have  no  special 
endothelial  walls,  but  are  surrounded  by  circular  muscle  fibres.  The  branchial  vein 
is  accompanied  by  a  powerful  longitudinal  muscle.  At  the  base  of  each  branchial 
leaflet,  the  blood  flows  out  of  the  branchial  artery  through  an  aperture  into  the 
narrow  cavity  of  the  leaflet,  and  passes  through  a  similar  aperture  on  the  opposite 
side  of  the  axis  to  enter  the  branchial  vein.  Xerves  are  supplied  to  the  ctenidium 
from  the  pleuro-visceral  nerve  which  runs  close  to  its  base. 


FIG.  S3.— Diagrams  illustrating  the  arrangement  of  the 
gills  in  the  Chitonidae.  m,  Mantle ;  o,  mouth  ;  A:,  snout ;  /, 
foot ;  ct,  ctenidia  ;  a,  anus. 


88 


COMPARATIVE  ANATOMY 


CHAP. 


The  number  of  ctenidia  in  each  row  varies  very  much  in  the  different  species  of 
Chitonidce  ;  it  ranges  from  14  to  75.  The  row  extends  along  the 
whole  length  of  the  branchial  furrow  (Fig.  83  A),  or  else  (in 
Chiton  Icevis,  0.  Pallasii,  and  Chitonellus]  is  confined  to  its 
posterior  half  (B,  C). 

Solenogastres.  —  (Proneomenia,  Neomenia,  Chcetoderma}. 
The  mantle  cavity,  in  these  forms,  is  much  reduced,  consisting 
only  of  the  groove  on  each  side  of  the  rudimentary  foot ;  it 
opens  into  the  cloacal  cavity,  or  rather  widens  to  form  that 
cavity.  The  cloaca  is  thus  the  posterior  portion  of  the  mantle 
cavity.  In  Chcetoderma  (Fig.  84)  the  foot  has  disappeared,  and 
the  mantle  cavity  is  reduced  to  the  cloaca,  in  which  one  typical 
gill  lies  on  reach  side  of  the  anus.  These  gills  are  regarded  as 
the  last  ctenidia  of  the  rows  found  in  the  Chitonidce,  which  in 

Tf  Chitonellus  and  some  species  of  Chiton  are  already  confined  to 
end  of  tne  body  of  r  *        .         •. 

Chaetoderma       (dia-  the  posterior  half  of  the  body.     In  Neomenia,  there  is  no  longer 

gram  after  Hubrecht).  a  pair  of  ctenidia,  but  a  mere  tuft  of  filaments  rising  from  the 

1,  Gonad ;  2,  pericar-  wan  of  the  cloacal  cavity,  and  in  Proneomenia,  there  are  only 

dium  ;  3,  rectum  ;  4,  irregular  folds  of  the  cloacal  wall. 

nepnridium ;  5,  anus ;  °  ,     .  .,     ,          .„  ,        „,          . ,  . 

6       ctenidium-       7  ^n  'tne  relation  of  the   gills  in  the    Chitonidce  to  certain 

cloaca.  '  patches  of  epithelium,  which  may  perhaps   be   considered   as 

osphradia,  see  the  section  on  Olfactory  Organs,  p.  165. 


B.  Gastropoda. 

The  Fissurellidce  (Fig.  85,  A  and  B)  among  the  Prosobranchia  stand  nearest  to 
the  racial  form  of  the  Gastropoda.  The  mantle  cavity  is  anteriorly  placed  ;  into  it 
from  behind  and  above  project  two  long  gills  feathered  on  each  side  ;  these  lie 
symmetrically  to  the  middle  line,  and  to  the  right  and  left  of  the  anus.  The 
posterior  portion  of  their  axes  is  connected  by  a  band  with  the  floor  of  the  respiratory 
cavity,  while  the  anterior  pointed  portion  projects  freely. 

The  fact  that  in  the  Fissurellidce  (and  related  forms)  the  gills  are  paired  and 
symmetrical  is  very  significant.  It  points  to  the  primitive  character  of  these  forms, 
and  enables  us  to  compare  their  gills  with  those  of  the  lower  Lamellibranchia,  i.e. 
the  Protobranchia,  and  of  the  Cephalopoda.  We  must,  however,  again  emphasise 
the  generally-assumed  fact  that  the  left  gill  of  Fissurella  answers  to  the  right  gill  of 
the  Lamellibranchia  and  Cephalopoda,  and  the  right  gill  of  the  former  to  the  left  of 
the  latter,  these  latter  having  retained  their  primitive  symmetry  in  this  respect. 
This  assumption  becomes  the  more  plausible  when  we  consider  that  the  mantle 
cavity  with  its  organs  originally  lay  posteriorly  on  the  body,  and  shifted  forward 
secondarily  along  its  right  side. 

The  Haliotidce  are  closely  connected  with  the  Fissurellidce.  Their  spacious  mantle 
cavity  is,  however,  forced  to  the  left  side  by  the  great  development  of  the  columellar 
muscle.  There  are  two  gills,  feathered  on  both  sides,  of  which  the  right  is  the 
smaller.  The  axis  of  each  gill  has  united,  for  nearly  its  whole  length,  with  the 
inner  wall  of  the  mantle,  and  only  its  anterior  end  is  free  ;  its  tip  even  projects  a 
short  distance  beyond  the  respiratory  cavity. 

Although  the  Fissurellidce  and  Haliotidce  still  possess  two  gills,  other  Diotocardia 
have  retained  only  the  left  (ur)  and  larger  gill  of  Haliotis.  This  gill  is,  however, 
still  feathered  on  both  sides,  although  this  characteristic  is  obscured  in  a  peculiar 
manner.  The  septum  or  axis  of  the  gill,  to  the  broader  surfaces  of  which  the  branchial 
leaflets  are  attached,  and  one  edge  of  which  had,  in  Haliotis,  already  fused  with  the 


VII 


MOLLUSC  A— RESPIRATORY  ORGANS 


89 


inner  wall  of  the  mantle,  becomes  attached  to  the  mantle  by  its  other  edge 
also  (viz.  that  along  which  the  branchial  artery  runs),  somewhat  to  the  right 
of  the  first  line  of  concrescence.  In  this  manner,  which  is  illustrated  by  the 
accompanying  diagrammatic  sections  (Fig.  86),  the  mantle  cavity  is  divided 
by  the  branchial  septum  into  two  unequal  parts,  which  open  into  one  another 
anteriorly. 

Into  the  much  smaller  upper  division  the  one  row  of  smaller  branchial  leaflets 
projects,  while  the  opposite  row  of  larger  leaflets  hangs  down  into  the  lower  and 
larger  chamber.     The  anterior  end  of  the 
gill,  however,  is  still  free,  its  point  pro- 
jecting anteriorly  (Trochidce,   Turbinidce, 
Nentidas). 

In  the  Docoglossa  (Patellidce}  the  ar- 
rangement of  the  gills  is  very  varied. 
While  the  Lepctidce  have  no  gills  whatever, 
we  find  in  Patella  a  single  row  of  numerous 
small  branchial  leaflets  right  round  the 
body,  on  the  inner  or  under  side  of  the 
short  encircling  mantle  fold,  between  it  and 
the  foot.  This  row  is  broken  only  in  one 
place  anteriorly  on  the  left.  It  is,  how- 
ever, evident  that  these  gills,  which  some- 
what resemble  those  of  the  Chitonidce,  are 
no  true  ctenidia,  from  the  fact  that  there 
are  Docoglossa  (e.g.  some  forms  of  Tectura 
and  Scurria)  which  possess,  in  addition 
to  this  marginal  row  of  leaflets,  a  typical 
ctenidium  corresponding  in  every  way  with 
that  of  the  Turbinidce,  Trochidce,  etc. 
Other  forms,  such  as  Acmcea,  have  only  the 
true  ctenidium  and  no  marginal  branchial 
leaflets. 

In  the  large  second  division  of  the 
Prosobranchia  —  the;  Monotocardia  —  the 
arrangement  of  the  gills  is,  on  the  whole, 
remarkably  uniform.  There  is  only  a 
single  gill  feathered  on  one  side  (Fig.  71, 
p.  73),  united  to  the  mantle  along  almost 
its  whole  length  ;  this  gill  corresponds 
with  the  left  gill  in  Fissurella  and  Haliotis,  and  the  single  gill  in  Turbo  and 
Trochus.  It  generally  lies  quite  to  the  left  in  the  mantle  cavity. 

The  rise  of  this  gill  can  best  be  explained  by  recalling  the  arrangements  already 
described  in  Turbo  and  Trochus.  We  have  only  to  assume  that  the  row  of  small 
leaflets  turned  towards  the  mantle  in  Turbo  disappears,  and  that  the  branchial 
septum  unites  with  the  mantle  across  its  whole  width  (Fig.  86,  C,  D). 

A  few  anomalous  forms  alone  require  special  mention. 

1.  In  a  series  of  terrestrial  Monotocardia,  aerial  respiration  has  taken  the  place  of 
aquatic  respiration,  and  the  ctenidium  has  disappeared  (Acicula,  Cyclostoma,  Cyclo- 
phorus,  etc.). 

2.  The  Ampullaria  are  amphibian  Prosobranchia.     A  doubling  of  the  mantle 
'gives  rise  to  a  very  spacious  pulmonary  sac,  on  the  inner  surface  of  which  the  respira- 
tory vascular  network  spreads  out.     The  lower  wall  of  this  pulmonary  sac,  which 
forms  at  the  same  time  the  roof  of  the  mantle  cavity,  is  perforated  by  an  aperture 


FIG.  85.— Subemarginula  after  removal  of  the 
shell  (after  Fischer).  A,  from  above  ;  B,  from 
right  side.  The  mantle  cavity  is  exposed  by 
bending  back  the  mantle  fold  4.  1,  Snout ;  2, 
tentacle,  with  the  eye  on  its  short  stalk  behind 
it ;  3,  right  ctenidium  ;  4,  mantle  fold  ;  5,  shell 
muscle ;  6,  edge  of  the  mantle  encircling  the 
body  ;  7,  epipodium ;  8,  foot. 


90 


COMPARATIVE  ANATOMY 


CHAP. 


for  the  inhalation  and  exhalation  of  air.1  The  ctenidium  is  placed  to  the  extreme 
right  of  the  mantle  cavity,  a  position  which  is  in  some  way  connected  with  the  great 
development  of  the  pulmonary  sac.  It  nevertheless  answers  to  the  left  gill  in  other 
Monotocardia,  as  can  be  seen  from  its  innervation. 

3.  The  genus    Valvata  is   unlike   all   other  Monotocardia,   in   that  its   gill  is 
feathered  on  both  sides  and  projects  freely.     It  can,  further,  be  protruded  from 
the  pallial  cavity. 

4.  In  Atlanta,  among  the  Heteropoda,  the  gill  is  well  hidden  in  the  spacious 
mantle  cavity.     In  Carinaria,  it  is  only  slightly  protected  in  consequence  of  the 
small  development  of  the  mantle  fold.     In  Pterotrachea  there  is  no  mantle  fold,  and 
the  filamentous  branchial  leaflets  project  free  and  uncovered.     Firoloides  has  no 
gills. 

Opisthobranchia. — A  true  ctenidium  is  here  found  only  in  the  Tectibranchia  and 


FIG.  86.— General  Morphology  of  the  gills  of  the  Prosobranchia.  Diagrammatic  sections 
in  the  region  of  the  mantle  cavity,  from  behind.  '  A,  Haliotis  ;  B,  Trochus,  anterior  portion  of  the 
jJallial  cavity.  C,  Trochus,  middle  or  posterior  portion  of  the  cavity.  D,  Monotocardia.  1, 
Mantle  cavity ;  2,  rectum  or  anus,  r  right,  I  left  gill  of  Haliotis  (A),  which  latter  is  the  only  gill 
present  in  the  Azygobranchia  (B,  C)and  Monotocardia  (D).  i,  Branchial  leaflet  of  the  inner  row  ; 
e,  ditto  of  the  outer  row,  between  them  the  branchial  axis  or  septum  with  the  afferent  and  efferent 
branchial  vessels  (3  and  4) ;  5,  position  of  the  mantle  slit  in  Haliotis  (cf.  p.  43).  Further  explana- 
tions in  the  text. 

in  the  Steganobranchia  among  the  Ascoglossa.  It  lies,  often  incompletely  covered, 
in  the  mantle  cavity  which  is  developed  on  the  right,  and  is,  in  some  cases  at  least 
(e.g.  Pleurobranchus),  distinctly  feathered  on  both  sides. 

In  the  Pteropoda,  which  must  be  derived  from  the  tectibranchiate  Opistho- 
branchia, the  ctenidium,  when  present,  is  little  developed,  and  lies  on  the  right  side 
of  the  body.  It  answers  to  the  tectibranchiate  ctenidium. 

In  the  Gymnosomata,  this  true  gill  is  retained  only  in  the  Pneumodermidaz  as  a 
simple,  or  less  frequently  (Pneumoderma)  fringed,  process  on  the  right  side  of  the 
body  (Fig.  76,  p.  79).  New  gills,  on  the  other  hand,  may  develop  at  the  posterior 
end  of  the  body,  occurring  either  together  with  the  true  ctenidium  (Spongiobranchaa, 
Pneumoderma),  or  alone  (Clionopsis,  Notobranchcea),  until  they  in  their  turn  dis- 
appear (Clione,  Halopsyche). 

Among  the  Thecosomata,  the  CavoliniidcR  alone  (Fig.  87)  possess  a  gill  which 


1  In  this  and  in  the  closely-allied  Lanistes  there  is  in  addition  a  protrusible  siphon 
on  the  left  side  (t;.  Fischer  and  Bouvier,  C.  R.  cxi.  p.  200). 


VII 


MOLL USC A— RESPIRATORY  ORGANS 


91 


rises  in  the  form  of  a  series  of  fold-like  elevations  of  the  body  wall  in  the  pallial 


FIG.  ST.— Anatomy  of  Cavolinia 
tridentata  (after  Souleyet).  Shell 
and  mantle  removed,  and  visceral 
dome  partly  opened,  seen  from  behind 
and  below,  d,  Right ;  s,  left ;  1,  aper- 
ture of  the  penis  ;  2,  mouth  ;  3,  left  tin 
(para podium)  ;  4,  foot ;  5,  oesophagus  ; 
ti,  part  of  the  efferent  genital  appara- 
tus ;  7,  ventricle ;  8,  auricle  ;  9,  herma- 
phrodite gland  ;  10,  lateral  processes 
of  the  mantle  ;  11,  columcllar  muscle  ; 
12,  intestine ;  13,  digestive  gland 
(liver);  14,  stomach;  15,  ctenidium  ; 
16,  genital  aperture  ;  17,  anus. 


cavity,  and  which,  running  in  a  wavy  line,  forms  a  semicircle,  open  anteriorly,  the 
greater  portion  of  it,  however,  lying  on  the  right  side. 


C.  Lamellibranchia. 

The  Lamellibranchia  also  possess  typically  two  symmetrically  placed  gills,  each 
provided  with  two  rows  of  branchial  leaflets.  The  opinion  which  until  lately  was 
common,  that  the  Lamellibranchia  possessed  two  gills  on  each  side  of  the  mantle 
cavity,  has  been  shown  to  be  incorrect — these  two  gills  in  reality  answering  to  the 
two  rows  of  branchial  leaflets  of  one  typical  gill. 

It  is  worth  while  to  follow,  step  by  step,  the  interesting  series  of  modifications 
undergone  by  the  original  gill  in  the  Lamellibranchia. 

(a)  The  primitive  arrangement  is  found  in  the  Protobranchia.  Taking  Nucula 
(Fig.  21,  p.  14)  as  an  example,  we  find  a  gill  like  that  of  Fissurella,  consisting  of 
an  axis  along  which  the  branchial  artery  and  the  branchial  vein  run,  and  which  is 
attached  by  a  short  membraneous  band  to  the  posterior  and  upper  portion  of  the 
body  or  visceral  dome,  and  to  the  posterior  adductor  muscle.  On  this  axis  are 
attached  two  rows  of  short  flat  branchial  leaflets.  These  two  plumose  gills  converge 
posteriorly,  and  project  with  their  free  tips  into  the  mantle  cavity.  The  leaflets  of 
both  rows  are  directed  somewhat  downwards,  so  that  they  are  at  right  angles  to  one 
another.  In  Malletia  and  Solenomya,  on  the  contrary,  they  lie  in  the  same  plane, 
the  two  rows  standing  out  on  opposite  sides  of  the  axis.  In  Malletia,  this  plane  is 
horizontal,  but  in  Solenomya  it  trends  downwards  and  inwards.  The  number  of  leaflets 
on  the  very  slender  gill  of  Malletia  is  much  smaller  than  on  that  of  Xucula  ;  they 
are  consequently  neither  so  crowded  nor  so  flattened.  Each  leaflet  contains  a  blood 


92 


COMPARATIVE  ANATOMY 


CHAP. 


sinus,  which  is  a  continuation  of  the  branchial  artery.  Two  rods  of  connective 
tissue  run  along  the  lower  edge  of  each  leaflet  from  the  axis  to  its  tip,  and  serve  for 
its  support.  Similar  supports  are  found  in  almost  all  Lamellibranchia  and  in 
many  Gastropods. 

The  epithelium  of  the  branchial  leaflets  is  beset  with  long  cilia — (1)  at  the 
ventral  edge  ;  (2)  on  both  (anterior  and  posterior)  surfaces,  near  the  ventral  edge. 

The  first-named  cilia  form,  with  regard  to  the  whole  gill,  a  longitudinal  row  along 
the  free  ventral  edge  of  each  row  of  leaflets,  and  bring  about  a  current  in  the  water 
along  this  edge  from  behind  forward.  The  other  cilia  mentioned  above,  mingling 
together  like  the  bristles  of  two  brushes  which  are  pressed  together,  form  a  loose 
connection  between  the  successive  leaflets  of  the  row. 

(6)  In  the  Filibranchia  (Fig.  88  B)  the  leaflets  in  each  of  the  two  rows  are  very 
long  and  filamentous,  and  hang  down  far  into  the  mantle  cavity.  The  branchial 
filaments  of  the  two  rows  are  recurved  and  bent  back  upon  themselves,  so  that  in 
each  filament  a  descending  and  an  ascending  portion  can  be  distinguished.  The 
prolongation  of  the  filaments  corresponds  with  a  necessary  increase  of  the  respiratory 


FIG.  88.— Morphology  of  the  gills  of  the  Lamellibranchia,  diagrammatic  transverse  sections. 
A,  Protobranchia.  B,  Filibranchia.  C,  Eulamellibranchia.  D,  Septibranchia.  1,  Mantle ; 
2,  body  (visceral  dome) ;  3,  foot ;  e,  in  A,  branchial  leaflets  of  the  outer  row  in  the  feathered  gill, 
in  B,  branchial  filaments  of  the  outer  row,  in  C,  outer  branchial  leaf;  i,  branchial  leaflets  or 
filaments  of  the  inner  row  or  inner  branchial  leaf;  ej,  ascending  branch  of  the  outer  filament, 
or  lamella  of  the  outer  leaf;  ij,  ascending  branch  of  the  inner  filament,  or  lamella  of  the  inner  leaf; 
in  D,  s,  signifies  the  gill  which  has  become  transformed  into  a  muscular  septum  which  divides  the 
mantle  cavity  into  an  upper  (4)  and  a  lower  (5)  chamber,  the  -t\vo  communicating  by  means  of  slits 
(o)  in  the  septum.  Further  explanations  in  the  text. 

surface.  By  this  bending  back  of  the  filaments,  the  gills  make  the  most  of  the 
limited  space  afforded  by  the  mantle  cavity.  Each  filament  of  the  outer  row  is  bent 
outwards,  and  of  the  inner  row  inwards. 

The  filaments  of  each  row  may  be  so  crowded  together  that  the  whole  row  looks 
like  a  leaf  or  fringe.  This  branchial  leaf  consists  of  two  closely  contiguous  lamellse, 
one  the  descending  and  the  other  the  ascending,  the  two  passing  into  one  another 
at  the  lower  edge  of  the  leaf.  The  descending  lamella  is  formed  by  the  descending 
portions  of  the  filaments,  and  the  ascending  by  the  ascending  portions.  On  the 
outer  leaf,  the  ascending  lamella  is  the  outer  one,  on  the  inner  leaf  the  inner. 

In  the  Filibranchia,  the  separate  branchial  filaments  retain  their  independence — 
they  are  free,  i.e.  the  separate  filaments  of  a  series  are  unconnected  with  one 
another,  and  the  descending  and  ascending  portions  of  one  and  the  same  filament 
are  in  no  way  united.  There  are,  however,  on  both  the  anterior  and  posterior  sides 
of  the  filaments  places  covered  with  long  cilia  closely  crowded  together.  These 
ciliated  tufts  on  adjoining  filaments  mingle,  and  so  give  rise  to  a  sort  of  connection 
between  the  filaments  of  each  leaf. 

In  the  Mytilidce,  so-called  interfoliar  junctions  or  trabeculse  occur  at  certain 


vii  MOLLUSGA—  RESPIRATORY  ORGANS  93 

points  between  the  ascending  and  descending  portions  of  the  branchial  filaments, 
but  no  blood-vessels  run  into  them. 

In  Anomia,  the  dorsal  ends  of  the  ascending  portions  of  the  outer  lamella  are 
free,  but  in  the  Arcidce  united,  although  their  internal  cavities  are  not  in  communi- 
cation. In  such  cases,  the  interior  of  each  filament  is  divided  by  a  longitudinal 
septum  into  two  canals.  In  one  of  these  the  blood  flows  from  base  to  tip,  and  in  the 
other  back  from  tip  to  base,  i.e.  to  the  axis.  In  the  Mytilidce,  the  dorsal  ends  of  the 
recurved  portions  of  the  filaments  of  each  branch  have  grown  together,  and  their 
blood-vessels  communicate  at  the  points  of  junction,  i.e.  along  the  upper  edge  of  the 
ascending  lamella. 

(c)  Pseudolamellibranchia. — Each  leaf  of  the  gill  is  here  folded,  to  secure  increase 
of  surface.     The  plications  run  longitudinally  with  regard  to  the  filaments,  and  are 
thus  almost  dorso-ventral.    There  are,  therefore,  distinct  alternate  ridges  and  furrows 
on  each  leaf,  the  ridges  on  the  one  surface  corresponding  with  those  on  the  other,  and 
the  furrows  corresponding  with  furrows.     Each  ridge  or  furrow  is  formed  by  one 
filament ;  the  filament  forming  the  furrow  is  in  some  way,  such  as  greater  breadth, 
distinguished  from  the  others.     The  two  lamellae  of  each  leaf  of  the  gill  are  united 
here  and  there  by  trabeculae,  which  may  or  may  not  contain  blood-vessels.    They  occur 
either  between  the  opposite  furrows  or  between  the  opposite  ridges,  i.e.  between  the 
ascending  and  descending  portions  of  the  filaments  which  lie  either  in  the  furrows 
or  ridges.     The  upper  edge  of  the  ascending  lamella  of  the  outer  leaf  may  unite  with 
the  mantle.     The  consecutive  filaments  of  the  same  leaf  are  only  connected  by  means 
of  tufts  of  cilia. 

(d)  Eulamellibranchia  (Figs.  89-91). — The  branchial  leaves  are  either  smooth  or 
folded,  but  there  is  always  organic  connection,  by  means  of  numerous  vascularised 
junctions,  not  only  between  the  ascending  and  descending  lamellae,  but  between  the 
successive  filaments.    The  junctions  are  therefore  both  interfoliar  and  interfilamentar. 
This  leads  to  the  entire  disappearance  of  the  original  filamentous  structure  of  each 
leaf,  which  now  becomes  an  actual  leaf  or  lamella  with  perforations  or  slits,  the 
remains  of  the  spaces  between  the  original  filaments,  leading  into  an  internal  system 
of  sinuses  or  canals,  which  in  their  turn  are  the  remains  of  the  spaces  between  the 
ascending  and  descending  lamellae.     This  peculiar  arrangement  was  formerly  con- 
sidered typical  of  the  Lamellibranchia,  and  was  the  origin  of  their  name.     It  was 
supposed  that  the  animals  of  this  class  had  two  leaf-like  gills  on  each  side  of  the 
mantle  cavity,  i.e.  four  altogether,  but  we  now  know  how  the  two  branchial  leaves  on 
each  side  arose,  that  they  are  in  fact  the  two,  modified,  rows  of  leaflets  of  the  original 
plumose  gill  of  the  Protobranchia.     The  Lamellibranchia  in  reality  possess  only  one 
gill  on  each  side  in  the  mantle  cavity. 

The  blood  now  no  longer  flows  through  the  primitive  filaments  of  the  lamellae 
of  the  gills  and  back  again,  but  the  afferent  and  efferent  channels  lie  in  the  trabecular 
network  between  the  two  lamellfe  of  a  branchial  leaf. 

Instead  of  the  two  leaves  of  a  gill  hanging  down  into  the  mantle  cavity  parallel 
to  one  another,  the  outer  leaf  may  stand  up  dorsally  in  the  cavity,  so  that  the  two 
come  to  lie  in  the  same  plane  (Tellinidce  and  Anatinacea). 

The  ascending  lamella  of  the  outer  leaf  may  be  wanting  (Anatinacea,  Lascea], 
and  in  fact  the  entire  outer  leaf  may  be  absent  (Lucina,  Corbis,  Montacuta, 
Cryptodon}. 

In  all  Lamellibranchia,  with  the  exception  of  the  Protobranchia,  and  further,  of 
the  Arcidce,  Trigonidce,  and  Pectinidce,  the  gill  and  mantle  unite,  the  dorsal  edge  of 
the  ascending  (outer)  lamella  or,  where  this  is  wanting,  the  free  edge  of  the  single 
lamella  of  the  outer  leaf  becoming  fused  with  the  mantle.  In  the  same  way,  the 
dorsal  edge  of  the  ascending  (inner)  lamella  of  the  inner  leaf  may  become  fused  with 
the  upper  part  of  the  foot  (Fig.  88  C).  If  the  two  gills,  which  have  fused  with  the  foot, 


94 


COMPARATIVE  ANATOMY 


CHAP. 


fuse  with  each  other  behind  the  foot  in  the  middle  line  of  the  mantle  cavity,  they  form 
a  septum  which,  uniting  with  the  septum  formed  by  the  mantle  between  the  inhalent 
and  exhalent  siphons,  divides  the  cavity  into  an  upper  and  a  lower  chamber.  The 
water  flows  through  the  lower  (inhalent)  siphon  into  the  large  lower  chamber,  bathes 
the  gills,  and,  streaming  forward,  conveys  the  particles  of  food  it  contains  to  the 
mouth.  It  then  flows  back  along  each  side  of  the  foot  in  the  upper  chamber  of  the 
mantle  cavity  (which  is  itself  divided  into  two  canals  by  the  line  of  insertion  of  the 


FIG.  89.— Part  of  a  transverse  section  of  the  outer  branchial  leaf  of  Dreissensia  polymorpha 
(after  Peck).  /,  The  separate  filaments  ;  /,  sub-epithelial  fibres  ;  ch,  supporting  substance  of  the 
filaments  ;  lac,  lacunar  or  alveolar  tissue  ;  pig,  pigment  cells  ;  be,  blood  corpuscles ;  fe,  epithelium 
of  the  free  edge  of  the  branchial  filaments ;  lfe\,  lfe->,  two  rows  of  lateral  epithelial  cells  of  the 
branchial  filaments,  carrying  long  cilia  (ciliated  tufts) ;  Irf,  tissue  of  the  interlilamentar  junctions. 
Two  interfoliar  junctions  are  shown  in  the  figure. 

gill)  into  the  single  posterior  and  upper  chamber  behind  the  foot,  and  escapes  through 
the  upper  (exhalent)  siphon  (Fig.  26,  p.  18). 

(e)  Septibranchia  (Fig.  31  A  and  B,  p.  21 ;  and  Fig.  88  D,  p.  92).— These  Mussels 
were  formerly  erroneously  considered  to  be  gill-less.  As  a  matter  of  fact,  the 
branchial  septum  just  described  has  in  them  been  much  modified  in  structure,  and 
has  become  a  muscular  septum,  running  across  the  mantle  cavity  in  a  horizontal 
direction  and  joining  the  siphonal  septum  posteriorly,  while  anteriorly  it  passes 
round  the  foot.  This  septum  is  broken  through  by  various  perforations  and  slits, 
which  allow  of  communication  between  the  upper  and  lower  chambers  of  the  mantle 
cavity,  and  vary  in  the  different  genera. 


VII 


MOLLUSC  A— RESPIRATORY  ORGANS 

ot  A 


95 


FIG.  90.— Portions  of  transverse  sections  of  the  branchial  lamellae  of  Anodonta  (after  Peck). 
A,  Outer ;  B,  inner  lamella.  In  each  leaf  the  cross  sections  of  both  lamellae  are  seen,  and  also  the 
interfoliar  as  well  as  the  interfilamentar  junctions.  C,  A  part  of  B  much  magnified,  ol,  Outer ;  if, 
inner  lamella  of  the  same  leaf;  r,  blood-vessels;/,  the  separate  filaments  of  which  the  lamellae 
consist ;  lac,  lacunar  tissue ;  ch,  supporting  tissue  of  the  filaments,  with  firmer  supporting 
rods,  cltr. 

.f 


trf 


FIG.  91.— Portion  of  the  ascending  lamella  of  the  outer  branchial  leaf  of  Anodonta. 
diagrammatic  (after  Peck).  /,  The  separate  filaments,  connected  by  internlamentar  junctions  ; 
trf,  connective  tissue  of  the  latter  ;  r,  bloodvessels  ;  ilj,  interlamellar  junctions  ;  the  perforations 
in  the  lamella  (of  a  darker  shade)  are  the  spaces  remaining  between  the  filaments  and  their 
junctions,  through  which  the  water  needed  for  respiration  can  flow. 


96 


COMPARATIVE  ANATOMY 


CHAP. 


D.  Cephalopoda. 

The  gills  of  the  Cephalopoda  are  always  feathered  on  both  sides.  Those  of  the 
Dibranchia  have  been  the  most  thoroughly  investigated.  In  Sepia,  each  gill  has 
the  shape  of  a  slender  cone,  its  whole  length  being  applied  to  the  visceral  dome  in 
the  mantle  cavity,  in  such  a  way  that  the  base  is  directed  dorsally  towards  the  apex 

of  the  visceral  dome,  and  the  point  ventrally 
towards  the  free  edge  of  the  mantle  fold  or 
the  mantle  cleft  (Fig.  80,  p.  83).  The 
points  of  the  two  gills  diverge. 

The  two  rows  of  flat  triangular  branchial 
leaflets  (Fig.  92)  are  carried  by  the  two 
branchial  vessels,  each  leaflet  being  attached 
by  one  end  of  its  base  to  the  branchial  artery 
arid  by  the  other  to  the  branchial  vein.  In 
the  axis  of  the  gill  between  the  two  vessels, 
and  also  between  the  bases  of  the  two  rows 
of  leaflets,  a  channel  is  formed  which  com- 
municates by  a  slit  between  each  successive 
pair  of  leaflets  with  the  mantle  cavity  ; 
through  this  canal  the  respiratory  water 
freely  flows.  The  slits  in  this  axial  channel 
are  arranged  alternately  on  each  side,  like 
the  leaflets  between  whose  bases  they  lie. 
The  branchial  vein  forms  the  posterior  sup- 
port of  the  gill  turned  towards  the  mantle, 
and  the  branchial  artery  the  anterior  support 
turned  towards  the  visceral  dome.  The 
artery  is  united  along  its  entire  length  with 
the  integument  of  the  visceral  dome  by  a 
membrane  of  connective  tissue.  The  an- 


" blood-making  gland"  (9),  through  which 
venous  blood  flows ;  10,  11,  vessels  carrying 
the  venous  blood  which  has  passed  through 


FIG.  92.— Diagram  to  illustrate  the  struc 
ture  of  the  gill  of  Sepia  (after  Joubin).  1, 
Branchial  vein  (containing  arterial  blood) ;  2, 
branchial  canal ;  3,  branchial]  artery  (contain- 
ing venous  blood);  4,  special  branchial  vein 
(vas  efferens)of  each  leaflet ;  5,  special  branchial 
artery  (vas  afferens)  of  each  leaflet ;  0,  suspensor 
of  the  gill,  which  attaches  the  branchial  artery  1 
(3)  to  the  posterior  integument  of  the  visceral  terlor  edSe  of  each  leaflet  (that  facmg  tlie 
dome  (12) ;  7,  suspensor  of  each  leaflet  to  the  visceral  dome)  is  connected  with  this  mem- 
general  suspensor  (6) ;  8,  one  of  the  connecting  brane,  which  may  be  called  the  gill-suspen- 
vessels  between  the  branchial  artery  and  the  8Qr>  by  meang  of  another  triangular  mem- 
brane. A  special  vein  runs  along  the 
posterior  free  edge  of  each  leaflet,  and  enters 
the  "blood-making"  gland  back  to  the  venous  the  general  branchial  vein  at  its  base  ;  and 
sinus  at  the  base  of  the  gill.  The  arrows  in-  a  speciai  artery  runs  along  the  anterior  edge, 
dicate  the  direction  of  the  blood-stream.  .  ,  ,  c  ,  f  -,  a  ,  -.  .  -, • 

i.e.  along  that  edge  of  the  leaflet  which  is 

fastened  to  the  suspensor.  Each  leaflet  is  wrinkled  in  such  a  way  that  the  folds 
on  the  two  surfaces  alternate,  each  fold  being  creased  in  its  turn.  These  two  systems 
of  folds  cross  each  other  at  right  angles,  and  serve  to  increase  the  respiratory  surface. 
At  the  point  where  the  suspensor  of  the  gill  passes  into  the  integument  of  the 
visceral  dome,  it  contains  a  cellular  body,  which  is  traversed  by  a  system  of  inter- 
cellular blood-channels.  This  may  perhaps  be  a  blood-making  gland.  It  receives 
venous  blood  from  branches  of  the  principal  branchial  artery  and  of  the  special 
arteries  of  the  leaflets,  and  returns  the  same  along  two  veins  which  run  back  to  the 
base  of  the  gill,  there,  with  others,  to  open  into  the  venous  sinus  of  the  renal  organ  ; 
from  this  organ  the  blood  passes  for  the  second  time  along  the  branchial  artery  into 
the  gill.  We  thus  find  that  not  all  the  venous  blood  which  is  conducted  by  the 
branchial  artery  towards  the  gills  enters  the  leaflets  for  purposes  of  respiration  ;  part 


vii  MOLL USGA  —RESPIRATORY  ORGANS  97 

of  it  streams  through  the  "blood-making " gland,  and  returns  to  the  venous  branchial 
heart  still  unpurified.  There  are,  further,  certain  fine  branchings  of  the  branchial 
artery  which  serve  for  nourishing  the  gill  and  its  suspending  membranes.  The  blood 
in  these  returns  to  the  venous  sinus  through  a  special  vessel  which  runs  parallel  to 
the  branchial  artery  on  its  anterior  side. 

A  powerful  nerve  enters  the  gill  at  its  base  and  ramifies  through  it.  A  muscle 
spreads  over  the  surface  of  the  "blood-making"  gland,  and  a  special  musculature 
brings  about  the  contractions  of  the  principal  branchial  vein. 

The  gills  of  the  Octopoda  differ  considerably,  though  not  essentially,  in  structure 
from  those  of  the  Decapoda.  The  branchial  channel  is  much  larger,  and  the  leaflets 
are  not  only  folded,  but  have  on  each  side  alternating  lamellse,  which  in  their  turn 
may  cany  similar  lamellae  of  the  second  order,  and  so  on  till  in  some  cases  the 
seventh  order  of  subsidiary  lamellse  is  reached.  The  leaflet  is  thus  an  extremely 
complicated,  folded,  or  feathered  structure  with  its  surface  increased  to  an 
extraordinary  degree. 

Adaptive  Gills. 

The  Scaphopoda  and  many  Gastropoda  possess  no  true  ctenidia. 
In  the  Pulmonata  and  the  few  air-breathing  Prosobranchia,  the  ctenidia, 
as  organs  adapted  for  aquatic  respiration,  have  disappeared.  It  is, 
however,  at  present  difficult  to  determine  the  cause  of  their  dis- 
appearance in  Opisthobmnchia  which  inhabit  water,  and  in  the  gill-less 
forms  of  the  Pteropoda,  all  the  more  so,  as  in  most  Opisthobranchia 
they  are  replaced  by  adaptive  gills,  which  are  new  structures  in  no 
way  comparable  morphologically  with  ctenidia.  These  adaptive  gills 
may  even  appear  (Pneumoderma)  before  the  true  ctenidia  have  dis- 
appeared. The  Scaphopoda  and  many  Opisthobranchia  have  no  gills 
whatever,  and  in  these  respiration  evidently  takes  place  at  various 
suitable  parts  of  the  surface  of  the  body.  In  many  cases,  also,  where 
epipodial  or  parapodial  processes  are  developed  as  well  as  gills,  or 
the  mantle  possesses  extensions,  these  may  help  the  gills  in  the 
function  of  respiration. 

Adaptive  gills  are  found  in  most  Ascoglossa  and  in  the  Nudi- 
branchia ;  also,  as  mentioned  above,  in  the  gymnosomatous  Pteropoda. 
In  the  latter,  they  consist  of  small  fringed  or  plain  ridges  at  the 
posterior  end  of  the  body ;  these  may  be  of  various  shapes ;  a 
description  of  them  would  be  of  no  special  interest  to  the  comparative 
anatomist. 

The  principal  forms  of  adaptive  gills  of  the  Nudibranchia  are : 
(1)  the  anal  gills  of  the  Dorididce ;  (2)  the  longitudinal  rows  of 
branchial  leaflets  to  the  right  and  left  under  the  mantle  fold  of  the 
so-called  PhyllidHdw  ;  (3)  the  dorsal  appendages  or  eerata  of  the 
Nudibranchia  and  most  Ascoglossa. 

1.  The  Anal  Gills  (Fig.  93).— These  take  the  form  of  delicate  leaflets,  generally 
feathered  on  both  sides,  which,  in  the  Dorididce,  form  a  rosette  round  the  anus, 
which  has  a  median  dorsal  position  towards  the  posterior  half  of  the  body.  Cerata 
may  occur  with  the  anal  gills  (Poly  cer  idee).  The  view  that  these  gills  are  ctenidia 
has  as  yet  no  sufficient  foundation. 

VOL.  II  H 


98 


COMPARATIVE  ANATOMY 


CHAP. 


2.  The  Longitudinal  Rows  of  Branchial  Leaflets  (Fig.  20,  p.  13). — These  organs, 
which  lie  to  the  right  and  left  of  the  body  in  the  Phyllidiidce,  and  Pleurophyl- 
lidiidce,  bear  the  same  relation  to  the  (lost)  true  ctenidium  as  do  the  respiratory  struc- 
tures of  the  Patellidce  above  described  to 
the  same  organ,  which  in  them  is  some- 
times present,  sometimes  wanting.  The 
longitudinal  rows  consist  of  numerous 
small  lamellae  which  project  from  the  lower 
side  of  the  enveloping  mantle  fold  into  the 
shallow  pallial  cavity.  There  is  either  one 
long  row  of  these  lamellse  running  along 
the  whole  length  of  the  mantle  fold  and 
only  interrupted  anteriorly  (Phyllidia),  or 
a  row  interrupted  posteriorly  as  well 
(Pleurophyllidia)  ;  or  again,  the  rows  of 
lamellre  are  confined  to  the  posterior  end 
of  the  mantle  fold  (HypobrancMcea).  The 
genus  Dermatobranchus  has  no  gills. 

3.  Dorsal  Appendages  (Cerata)  (Fig. 
18,  p.  12).  —  These  processes  vary  very 
much  in  form,  being  sometimes  simple, 
and  sometimes  branched  ;  they  differ  also 
greatly  in  number  and  arrangement.  At 
their  tips  there  are  often  cnidophore  sacs  ; 
these  are  invaginations  of  the  ectoderm  in 
which  stinging  cells  with  stinging  capsules 
are  developed.  Diverticula  of  the  intestine 
(digestive  gland)  enter  the  cerata,  and  may 
open  outward  at  their  tips.  The  cerata  are 
generally  striking  and  beautiful  both  in 

93.  -  Respiratory  and  circulatory  colour  and  markings.  In  some  cases  they 
system  of  Doris,  after  Leuckart  ("  Wand-  may  serve  for  protection  and  concealment, 
tafeln  ").  a,  Rhinophore ;  ft,  posterior  edge  of  in  others,  where  the  brilliant  colouring  is 
the  visceral  dome ;  c,  end  of  the  foot ;  d,  plumose  combined  with  stinging  properties,  they 
gills  ;  dj,  two  gills  cut  off;  e,  anus  ;  /,  auricle  ;  &  * 

g,  ventricle ;  h,  aorta  ;  i,  circular  vein  around  may  serve  as  a  warning.  They  often  break 
the  anus,  which  receives  the  arterial  blood  from  off  easily  at  the  base  (as  a  protective  ar- 
the  gill,  and  sends  it  through  the  branchial  vein  rangement),  and  are  always  quickly  regen- 
into  the  auricle  ;fc,  circular  artery,  which  receives  erated  Th  no  doubt  ^^  nke  th(? 
the  venous  blood  coming  from  the  body :  x.  two  /•  ..  i  i  i  c  •  •  • 

vascular  trunks,  which  conduct  venous  blood  rest  °f  the  body  surface>  m  respiration, 
direct  to  the  heart.  especially  where  they  are  much  branched 

and  richly  supplied  with  blood-vessels. 

Certain  Opisthobranckia  are  altogether  gill-less,  e.g.  the  Elysiidw,  Limapontidce, 
and  Phyllirrhoidcc. 

Among  the  Pulmonata,  the  shell- less  genus  Onchidium  has  developed  adaptive 
gills.  The  species  of  this  genus  are  amphibious,  living  on  the  sea-coast,  within  reach  of 
the  tide.  Their  pulmonary  cavity  is  very  small  ;  respiration  therefore  takes  place  by 
means  of  the  richly  vascularised  dorsal  integument,  and  especially  of  the  simple  or 
branched  dorsal  papillae,  in  which  there  is  a  rich  vascular  network,  which  receives 
the  blood  from  an  afferent  vessel  and  gives  it  off  to  an  efferent  vessel. 


VII 


MOLLUSCA— RESPIRATORY  ORGANS 


99 


Lungs. 

The  total  disappearance  of  the  typical  molluscan  ctenidium  is 
characteristic  of  the  Pulmonata,  and  is  connected  with  their  terrestrial 
life  and  aerial  respiration.  Instead  of  water,  air  enters  and  escapes 
from  the  mantle  cavity  which  lies  either  anteriorly  or  laterally  on 
the  visceral  dome,  and  thus  the  mantle  cavity  becomes  a  pulmonary 
cavity.  The  free  edge 

of  the  man  tie  fold,  which  i.^t  f 

forms  the  roof  of   this  f, 

cavity,  unites  with  the 
nuchal  integument  be- 
neath it,  except  at  one 
point  on  the  right,  where 
the  respiratory  aper- 
ture, which  can  be  closed 
at  will,  allows  of  the 
entrance  and  egress  of 
air.  Along  the  line  of 
its  concrescence  with  the 
integument,  the  edge  of 
the  mantle  is  much 
thickened,  forming  the 
mantle  border,  and  is  FIG.  94.— Slightly  oblique  transverse  section  through  the 

very  rich  in  lime-secret-  body  and  sheu  of  Helix  *****  J1"* in  front  of  the  oommdia 

*  -  „,        .  (after  Howes),    pgl,  Pedal  gland  ;  fs,  lateral  pedal  blood  sinus  ; 

ing    glands.        Ine  inner    a0j   cephalic  aorta;    gd,   genital  duct  (uterus);    rp,  retractor 

delicate    Surface    Of     the    muscle  of  penis ;  plm,  pallial  muscle,  the  pallial  edge  having 

mantlp    wVnVh  if  rm«:  thp    Ullited  with  the  nuchal  integument ;  si,  salivary  gland  ;  cr,  crop, 

jie,  WHIG  ine    or  widenmg  of  the  oesophagus ;  s,  shell ;  ms,  floor  of  the  pulmon- 

1'Oof     of     the     Cavity,     is    ary  cavity  =  dorsal  integument  of  the  posterior  nuchal  region 

Overspread     by    a     close    WQich  is  covered  by  the  mantle;  sp,  spermatheca  =  stalk  of  the 

receptaculum  seminis  ;  pli,  pulmonary  cavity;  pv,  afferent  pul- 

'"    inonary  vessels  ;  re^  renal  duct ;  r,  rectum  ;  hgl,  hermaphrodite 

WOrk.      A  Circular  Vein    gland  or  ovotestis  ;  I,  digestive  gland  (liver) ;  hd,  hermaphrodite 
runs    alon0"    the    mantle    duct;  w»,  columellar  muscle ;  aj/Z,  albumen  gland ;  i,  intestine; 
st,  stomach. 

collar.     Jbrom  it  spring 

numerous  fine  anastomosing  vessels  which  ramify  on  the  mantle. 
These  vessels  are  again  collected  into  larger  trunks,  which  enter  the 
large  pulmonary  vein.  This  vein  runs  upwards  and  backwards, 
along  the  right  side  of  the  pulmonary  cavity,  to  the  left  of  and  almost 
parallel  with  the  rectum,  and  enters  the  auricle.  The  circular  vein 
contains  venous  blood,  but  the  pulmonary  vein  conducts  blood  which 
has  become  arterial  through  respiration  in  the  vascular  network,  to 
the  heart. 

Since,  in  most  Pulmonata,  as  in  the  Prosobmnchia,  the  respiratory 
organ  and  the  pallial  cavity  in  which  it  is  found  lie  in  front  of  the 
heart,  this  order  is  prosopneumonic.  An  account  of  the  opistho- 
pneumonic  condition  of  certain  Pulmonata,  which  results  from  the 


Jps-  u**%& 

T*  OF  THK         nr 

UNIVERSITY" 


100  COMPARATIVE  ANATOMY  CHAP. 

displacement  of  the  visceral  dome  and  mantle  to  the  posterior  end  of 
the  body,  will  be  found  in  Section  V.,  p.  76. 

Certain  Pulmonata  (Limnceidce)  have  become  readapted  to  aquatic  life,  but  their 
respiration  is  the  same  as  that  of  the  terrestrial  forms,  they  rise  periodically  to  the 
surface  of  the  water  to  take  in  air.  The  respiratory  cavity,  is,  however,  tilled  with 
water  when  the  animal  is  young,  and  it  is  then  a  water  breather.  In  Limncea 
abyssicola,  a  deep-water  form  found  in  the  lake  of  Geneva,  this  form  of  aquatic 


cr 

FIG.  95. — Helix.  The  roof  of  the  pulmonary  sac  cut  along  the  rectum,  and  along  the  edge 
uniting  with  the  nuchal  integument,  and  turned  back  to  show  the  arrangement  of  the  blood 
vascular  system,  after  Howes.  The  pulmonary  veins  are  of  a  lighter  shade  than  the  afferent 
pulmonary  vessels  and  the  venous  sinuses  ;  ««,  bl>,  show  the  cut  edges  which  belong  to  each  other  ; 
1,  afferent  pulmonary  vessels  which  draw  their  venous  blood  from  the  large  circular  venous 
sinus  (9);  this  latter  receives  its  blood  from  the  large  sinuses  of  the  body,  two  of  which, 
that  of  the  visceral  dome  (6)  and  that  on  the  right  side  of  the  foot  (7)  are  shown.  The  efferent 
pulmonary  vessels  collect  the  blood  which  has  become  arterial  on  the  roof  of  the  pulmonary 
chamber,  and  conduct  it  through  the  pulmonary  vein  (2)  to  the  auricle  (3) ;  4,  ventricle  ;  5,  renal 
circulatory  system. 

respiration  continues   throughout   life,  and  the   pulmonary  chamber,   in   no   way 
modified,  is  constantly  filled  with  water. 

In  certain  terrestrial  Prosobrandiia  (Cydostoma,  Cydophorus,  etc.)  the 
respiratory  cavity  becomes  transformed,  as  in  the  Pulmonata,  into  a 
pulmonary  chamber,  and  its  roof  is  covered  with  a  respiratory 
vascular  network.  But  there  is  here  no  concrescence  of  the  edge  of 
the  mantle  with  the  nuchal  integument.  Cydostoma  still  retains  a 
rudiment  of  a  prosobranchiate  gill,  but  this  is  lost  in  Cydophoi'us. 
The  amphibian  Ampullaria  possess  both  a  gill  and  a  pulmonary 
sac,1  and  can  breathe  either  water  or  air. 

1  See  note  ante,  p.  90. 


vii  MOLLUSCA— HYPOBRANCHIAL  GLAND,  HEAD  101 


VII.  The  Hypobranehial  Gland. 

(Slime  gland  of  the  Protobranchia,  epithelial  shield  of  the  Ptcropoda, 
etc.,  anal  gland,  etc.) 

This  is  an  organ  very  commonly  found  on  the  molluscan  mantle, 
always  occurring  near  the  ctenidium,  at  its  base  or  between  it  and 
the  rectum.  Cf.  on  its  position  and  occurrence  Section  V. 

The  hypobranchial  gland  varies  considerably  in  shape,  but  is 
never  a  multicellular,  acinose,  or  tubular  gland  wifch  efferent  ducts.  It 
is  originally  a  more  or  less  extended  area  of  the  epithelium  of  the 
mantle  cavity  (generally  of  the  inner  surface  of  the  mantle)  in  which 
epithelial  glandular  cells  are  particularly  numerous.  In  this  condition 
it  is  not  very  distinct  from  the  parts  around  it,  but  it  may  become 
more  definitely  localised,  and  may  assume  a  definite  shape ;  and  in 
this  latter  case,  the  glandular  epithelium  of  which  it  consists  may  also 
become  folded  in  order  to  obtain  a  larger  secretory  surface,  the  folds 
being  more  or  less  closely  crowded  together  and  projecting  into  the 
mantle  cavity.  This  gland  often  secretes  a  large  quantity  of  mucus. 
The  purple  gland  of  certain  Prosobranchia  (Pwpwra,  Murex,  Mitra)  is  a 
hypobranchial  gland,  the  slimy  secretion  of  which  is,  immediately 
after  ejection,  colourless  or  only  slightly  coloured,  but  under  the 
influence  of  light  becomes  violet  or  red.  In  Purpura,  the  gland  consists 
of  two  parts  which  differ  slightly  in  structure. 


VIII.  The  Head. 

If  by  the  word  head  is  meant  an  anterior  portion  of  the  body 
more  or  less  distinct  from  the  rest,  possessing  a  mouth  and  specific 
sensory  organs,  the  Lamellibranchia  must  be  considered  headless,  and 
as  such  have  been  distinguished  as  Acephala  from  other  Mollusca. 
This  absence  of  a  head  in  the  Lamellibranchia  cannot  be  regarded  as 
a  primitive  condition,1  but  is  to  be  accounted  for  by  their  general 
habit  of  living  in  mud,  and  by  the  strong  and  peculiar  development 
of  the  mantle  and  shell,  which,  by  cutting  off  the  anterior  portion  of 
the  body  (with  the  mouth)  from  direct  contact  with  the  outer  world, 
renders  specific  sensory  organs  useless.  In  those  Molluscs  which 
have  to  seek,  seize,  and  crush  their  food,  a  projecting  head  carrying 
sensory  organs  and  furnished  with  buccal  armature  is  of  great  use. 
Bivalves,  however,  feed  on  particles  brought  to  the  mouth  by  the 
water  which  by  the  motion  of  cilia  is  driven  through  the  mantle 
cavity ;  buccal  armatures  are  thus  unnecessary. 

In  the  Cephalopoda,  the  head  is  strengthened  by  the  incorporation 
with  it  of  the  foot,  here  transformed  into  a  circle  of  arm-like  prolonga- 

1  Hence  the  term  "  Lipocephala, "  suggested  by  Lankester. 


102  COMPARATIVE  ANATOMY  CHAP. 

tions  for  seizing  the  prey.  We  thus  have  a  combined  head  and  foot 
(Kopffuss),  on  each  side  of  which,  anteriorly,  lies  a  large  highly- 
developed  eye.  This  head  is  more  or  less  separated  from  the  rest 
of  the  body  (the  visceral  dome)  by  a  neck. 

The  Gastropoda,  with  very  few  exceptions,  possess  a  head  which  on 
its  anterior  lower  side  is  provided  with  an  oral  aperture,  on  its  upper 
side  with  eyes  and  tentacles,  and  often  asymmetrically  (generally  on 
the  right  side)  with  a  genital  aperture  or  a  copulatory  organ.  This  head 
is  distinctly  separated  ventrally  by  means  of  a  groove  or  furrow  from 
the  foot  behind  it ;  dorsally  it  passes  gradually  into  the  neck.  Further 
details  of  this  Gastropod  head  are  given  below. 

A.  Gastropoda. 

1.  Prosobranchia. 

The  head  in  this  order  always  carries  tentacles,  Avhich  are  solid,  simply  contractile 
(not  invaginable)  processes  of  the  cephalic  wall.  It  may  be  assumed  that  there 
were  originally  two  pairs  of  tentacles,  an  anterior  and  a  posterior  pair.  The 
posterior  are  called  ommatophores  and  carry  eyes  at  their  tips.  Most  Diotocardia 
possess  anterior  tactile  tentacles,  and  posterior  and  slightly  lateral  optic  tentacles. 

The  cephalic  tentacles  are  always  innervated  from  the  cerebral  ganglion,  and  are 
thus  distinguishable  from  the  tentacular  processes  which  may  occur  near  them  on 
the  head  or  neck,  but  belong  to  the  epipodium,  and  are  innervated  from  the  pedal 
or  pleural  ganglia. 

In  the  Docoglossa  and  most  Monotocardia  the  optic  tentacles  do  not  rise  separately 
from  the  head,  but  are  to  a  greater  or  lesser  extent  fused  with  the  tactile  tentacles. 
Starting  with  the  tentacular  arrangements  existing  in  Dolium,  Strombus,  Rostellaria, 

we  find  the  tactile  and  optic  tentacles 
fused  for  a  certain  distance  from  the 
base,  but  separating  later,  the  tips 
projecting  independently  (Fig.  96,  B). 
If  the  two  tentacles  were  of  the 
same  length,  and  were  fused  for  their 
whole  extent,  there  would  only  be  one 

FIG    96.-Relations  of  the  tactile   and   optic    tentacle  on  each  side  of  the  head,  which 
tentacles  in  the  Prosobranchia.    Description  in 

the  text.  would  carry  the  eye  at  its  tip  ( Terebra 

(7).     But  if  the  optic  tentacle  is  shorter 

than  the  tactile,  the  eye  might  be  met  with  at  any  point  between  the  base  and  tip 
of  the  latter,  on  a  projection  which  answers  to  the  tip  of  the  fused  optic  tentacle  (D 
and  E).  Finally,  the  eye  may  be  altogether  sessile,  i.e.  it  may  lie  near  the  base  of 
the  sensory  tentacle  in  the  integument  of  the  head  (F). 

The  snout,  which  carries  the  mouth  and  is  anterior  to  the  tentacles,  is  very 
variously  developed  in  the  Prosobranchia. 

1.  It  is  short  and  truncated  in  the  Diotocardia,  and  especially  in  the  herbivorous 
Tcenioglossa. 

2.  It  is  prolonged  like  a  proboscis  (rostrum),  but  is  only  contractile,  not  invagin- 
able (Capulidcc,  Strombidce,  Ctenopidce,   Calyptrceidce),  or  else  can  be  invaginated, 
commencing  at  the  tip  (Cyprccida',  Lamellaridce,  Naticidce,  Scalaridce,  Solaridce). 

3.  It  is  transformed  into  a  long  proboscis  with  the  mouth  at  its  anterior  end. 
This  proboscis  can  be  invaginated  in  such  a  way  that  the  invaginated  base  forms  a 
proboscidal  sheath  for  the  n  on -invaginated  anterior  portion  or  tip.     Gastropods 


vii  MOLLUSCA—HEAD  103 

with  such  proboscides  are  nearly  all  carnivorous  (the  Tritonidce,  Doliidce,  and  Cassi- 
didce,  among  the  Stenoglossa  the  Rachiglossa,  and  a  number  of  Toxiglossa). 

Most  male  Monotocardia  have  a  non-invaginable  penis,  which  varies  in  shape,  on 
the  right  (rarely  on  the  left)  side  of  the  head  or  neck,  near  the  tentacle  ;  this  organ 
in  most  cases  belongs  morphologically  to  the  foot,  being  innervated  from  the  pedal 
ganglion  ;  less  frequently  it  is  a  cephalic  appendage,  and  is  then  innervated  from  the 
cerebral  ganglion  (Fig.  71,  p.  73). 

The  head  of  the  Heteropoda  carries  two  tentacles  (occasionally  rudimentary  :  Ptero- 
trachea,  Firoloidect).  The  eyes  are  sessile  or  placed  on  small  prominences  near  the 
bases  of  the  tentacles  on  their  outer  posterior  sides.  That  part  of  the  head  which 
lies  in  front  of  the  tentacles  is  prolonged  to  form  a  large  proboscis-like  non-invagin- 
able snout. 

2.   Opisthobranchia. 

The  shape  of  the  head  in  this  order  varies  to  an  extraordinary  degree,  and  can 
here  be  only  generally  described.  It  usually  carries  two  pairs  of  tentacles  ;  the 
posterior  pair,  which  are  called  rhinophores,  are  perhaps  olfactory.  Their  surface  is 
often  increased  by  the  formation*of  circular  folds.  They  frequently  rise  from  the  base 
of  pits  into  which  they  can  be  withdrawn.  The  head  is  rarely  prolonged  into  a 
proboscidial  snout.  The  eyes  are  sessile. 

Among  the  Tectibranchia,  the  Cephalaspidce  are  distinguished  by  peculiarities  of 
the  head.  It  carries  dorsally  a  flat  fleshy  disc,  the  cephalic  or  tentacular  disc 
(Fig.  14,  p.  10),  which  is  regarded  as  the  result  of  the  fusion  of  the  tentacles,  and 
which,  by  its  shape,  recalls  the  propodium  of  the  Natitidce  or  Olividce  among  the 
Prosobranchia.  This  cephalic  disc  carries  the  sessile  eyes  on  its  dorsal  side,  and  its 
posterior  lobe,  which  is  sometimes  produced  in  the  shape  of  two. lateral  tentacular 
processes,  shifts  about  over  the  anterior  portion  of  the  shell.  The  shape  of  this  disc 
varies  considerably  in  details. 

Of  the  very  numerous  Nudibranchia  we  shall  only  notice  two  extreme  forms  : 
Tethys  and  Phyllirrhoe. 

In  Tcthys,  the  head  takes  the  form  of  a  large  flat  disc,  almost  semicircular  in 
shape  and  fringed  at  the  edge  ;  this  carries  on  its  upper  surface  two  conical  rhino- 
phores, which  can  be  retracted  into  large  sheaths. 

In  Phyllirhoe  (Fig.  19,  p.  12),  the  head  is  produced  into  a  short  proboscidial 
snout,  which  carries  only  two  very  long  curved  tentacles  ;  the  bases  of  these  are 
encircled  by  integumental  folds,  and  they  may  be  considered  as  rhinophores. 

Pteropoda  gymnosomata. — The  head  is  distinct,  and  carries  two  pairs  of 
tentacles,  one  labial  and  the  other  nuchal.  The  former  answers  to  the  anterior,  and 
the  latter  to  the  posterior  tentacles  or  rhiuophores  of  the  Tectibranchia,  especially 
those  of  the  Aplysiidce.  The  nuchal  tentacles  are  generally  small  or  rudimentary, 
the  rudiments  of  the  eyes  lying  at  their  bases. 

Xearly  all  the  Gymnosomata,  as  highly-developed  carnivorous  animals,  are  provided 
with  a  proboscidial  snout  which,  commencing  at  its  tip,  can  be  completely  invaginated, 
and  carries  at  its  base,  when  evaginated,  buccal  appendages  innervated  from  the 
cerebral  ganglion. 

Definite  compensatory  relations  exist  between  the  proboscidial  snout  and  the 
buccal  appendages  : — 

1.  When  the  proboscis  is   specially  long,  the   buccal  appendages  are  wanting 
(Clionopsis). 

2.  When  the  proboscis  is  of  median  length,  it  carries  suckers  at  its  base,  or  a 
pair  of  long  appendages  provided  with  suckers  (Pneumodcrmidce,  Fig.  76,  p.  79). 

3.  When  the  proboscis  is  short,  there  are  long  anterior  tentacles,  and  at  the  base 


104  COMPARATIVE  ANATOMY  CHAP. 

of  the  evaginated  proboscis  three  pairs  of  conical  processes  (cephalic  cones),  with 
special  nerve  endings  and  glands  whose  sticky  secretion  helps  in  the  capture  of  prey 
(Clionidce). 

4.  The  proboscis  may  be  wanting.  There  is  then  on  each  side  of  the  mouth  a 
long  extensible  buccal  appendage  carrying  at  its  base  the  labial  tentacle. 

Pteropoda  thecosomata. — The  head  is,  as  a  rule,  not  sharply  separated  from  the 
body,  and  has  no  invaginable  snout,  but  one  pair  of  tentacles  which  answer  to 
rhinophores,  and  sometimes  lie  in  sheaths  at  their  bases.  The  left  tentacle  may 
become  rudimentary.  In  the  Thecosomata  the  male  copulatory  organ  lies  on  the 
upper  side  of  the  head,  near  the  tentacle. 

3.  Fulmonata. 

The  head  is  here  distinct  from  the  foot  ventrally,  but  passes  dorsally  into  the 
neck.  It  carries  two  or  four  tentacles.  The  Stylom- 
liiatophora,  which  are  terrestrial,  have  four  tentacles 
(Fig.  97),  an  anterior  and  a  posterior  pair.  The 
posterior,  which  are  usually  the  longer,  carry  the  eyes 
on  their  tips.  The  tentacles  are  hollow  tubes  filled 
with  blood  and  connected  with  the  blood  spaces  of 
the  head.  They  can  be  invaginated  from  the  very 
tip  into  the  head,  special  muscles  acting  as  retractors 
which,  when  the  tentacle  is  evaginated,  run  from  the 
head  to  the  tip  of  the  tentacular  cavity. 

The   JSasommatophora,  which  are  aquatic,    have 
only  one  pair  of  tentacles  which  are  usually  triangular 
FIG.  97.-Helix,  front  view,  creep-    and  flat.     They  are  solid,  and  not  invaginable,  but 

ing  with  extended  tentacles  (after    merely  contractile.     The  eyes  lie  on  the  inner  side  of 

Howes),  s,  Shell ;  ti,  optic  tentacle ;    ^gjj.  bases 

;  Zl>  In  certain  Pulmonata  (Glandina,  Zonites,  Ond- 
dium)  the  upper  lip  may  be  drawn  out  into  a  lobe  or 
labial  palp  on  each  side.  This  labial  palp  in  Glandina  can  move  very  freely,  and  is 
the  seat  of  a  fine  sense  of  touch. 

On  the  right,  behind  the  right  tentacle,  lies  the  common  genital  aperture,  or,  in 
cases  where  the  male  and  female  apertures  are  distinct,  the  male  aperture. 

B.  Seaphopoda  (Fig.  101,  p.  113). 

In  this  order  the  non-invaginable  snout  is  ovoid  or  barrel-shaped, 
and  projects  from  the  body,  over  and  in  front  of  the  foot,  downwards 
into  the  mantle  cavity.  At  its  extremity  lies  the  mouth,  surrounded  by 
a  circle  of  dentate  oral  lobes  shaped  like  oak-leaves, — four  on  each  side. 

At  the  boundary  between  the  bases  of  the  foot  and  of  the  snout, 
to  the  right  and  left  of  the  cerebral  ganglion,  a  shield-shaped  lobe 
rises  from  the  body  on  each  side ;  this  is  attached,  at  the  centre  of  its 
inner  side,  by  a  short  slender  stalk  to  the  body  wall,  concrescence  also 
taking  place  at  its  lower  edge.  This  shield  carries  numerous  filamentous 
or  vermiform  glandular  tentacles,  which  move  very  freely  and  can  be 
protruded  far  beyond  the  mantle  aperture. 

The  ends  of  the  tentacles  are  swollen  into  the  shape  of  a  spoon,  and  can  become 
attached  to  foreign  objects  like  suckers.  Each  swelling  has  long  ciliary  hairs  on 


vii    MOLLUSCA—ORAL  LOBES  OF  THE  LAMELLIBRANCHIA  105 

its  concave  surface,  the  cilia  being  continued  in  a  baud  all  along  -tlite  tentacle  to  its 
base.  Tentacles  of  this  sort  are  found  in  all  stages  of  development ;  they  rise 
chiefly  from  the  inner  surface  of  ,the  shield,  an<|  easily  become  detached  or  broken 
off,  and  are  then  regenerated.  They^  aje  no  doubt  chiefly  useful  as  organs  of  touch, 
and  serve  for  seizing  particles  oftfood  (Foraminifera,  etc.).  They  may  further  assist 
respiration  in  the  absence  of  localised  gills,  by  causing  increase  of  surface.  The 
tentacles  are  innervated  from  the  cerebral  ganglion  through  the  stalk  of  the  shield 
on  which  they  stand. 

C.  Cephalopoda. 

In  Nautilus,  there  are  on  each  side  one  tentacle  above  and  one 
below  the  eye.  It  is  not  improbable  that  these  two  tentacles  cor- 
respond with  the  two  pairs  of  tentacles  in  the  Gastropoda. 


IX.  The  Oral  Lobes  of  the  Lamellibranehia. 

The  oral  aperture  of  the  Lamellibranehia  is  produced  right  and 
left  in  the  form  of  a  groove,  which  runs  backward  along  the  surface 
of  the  body  to  the  anterior  end  of  the  base  of  the  gill,  or  to  some 
point  near  it.  This  groove  is  bordered  by  two  projecting  ridges 
above  and  below  it.  The  two  upper  ridges,  at  the  point  where  they 
meet,  form  a  sort  of  upper  lip  over  the  mouth,  the  lower  ridges,  in 
the  same  way,  forming  a  lower  lip.  The  groove  between  the  ridges 
serves  for  conducting  to  the  mouth  the  particles  of  food  which  are 
swept  past  the  gills  by  the  cilia. 

The  length  of  the  groove  is  naturally  determined  by  the  distance 
between  the  anterior  ends  of  the  gills  and  the  mouth. 

The  two  ridges  just  described  are  continued  posteriorly  in  the 
shape  of  thin  lamellae,  which  hang  down  into  the  mantle  cavity.  These 
lamellae,  between  which  the  groove  becomes  a  deep,  narrow  cleft,  are 
the  oral  lobes  or  labial  palps  of  the  Lamellibranehia.  They  are  more 
or  less  triangular,  one  side  of  the  triangle  forming  the  base  by  which 
the  lobe  is  attached  to  the  body. 

In  cases  in  which  the  gills  lie  far  behind  the  oral  aperture,  the  bases  of  these 
lobes  are  long,  but  in  others,  where  they  begin  near  the  mouth,  the  bases  are  short, 
and  each  lobe  then  usually  forms  a  long,  free,  pointed  process.  The  surfaces  of  these 
two  oral  lobes  are  ciliated,  and,  further,  the  surfaces  which  face  each  other,  i.e. 
which  have  the  groove  between  them,  are  striated  at  right  angles  to  their  bases. 
This  striation  is  caused  by  parallel  ridges,  and  gives  the  lobes  a  superficial  resem- 
blance to  gills.  The  lobes  contain  blood  lacunae,  and  it  is  probable  that,  besides  their 
chief  function  of  conducting  food  to  the  mouth,  they  may  assist  in  respiration. 

In  certain  forms,  the  free  edge  of  the  upper  lip  folds  over  that  of  the  lower 
(Ostrra,  Tridacna)  ;  in  others,  the  two  edges  are  closely  apposed  and  interlocked  by 
means  of  processes  and  folds  (Pccfe/i.  Spondylus),  so  that  a  closed  cavity  rises  in 
front  of  the  mouth,  into  which  the  groove  brings  particles  of  food  from  each  side. 
The  edges  of  the  upper  and  lower  labial  palps  may  even  grow  together  (Lima). 

Nucula  (Fig.  21,  p.  14),  in  which  the  ctenidium  lies  far  back,  and  has  a  very 
small  respiratory  surface,  may  serve  as  an  example  of  very  highly  developed  oral  lobes, 


106  COMPARATIVE  ANATOMY  CHAP. 

which  were  formerly  considered  to  be  gills.  The  base  of  the  lobe  here  stretches 
along  the  whole  length  of  the  base  of  the  faot,  and  is  further  prolonged  posteriorly 
in  the  shape  of  a  free  appendage  with  a  groove  running  along  it.  This  process  can 
be  protruded  beyond  the  shell,  and  probably  assists  in  conducting  food  to  the 
mouth. 


X.  The  Foot  and  the  Pedal  Glands. 

The  ventral  side  of  the  body  in  the  Mollusca  is  characterised  by  the 
pronounced  development  of  its  musculature,  which  enables  the  animal 
to  creep,  a  fleshy  foot,  provided  with  a  flat  sole  suited  for  creeping, 
distinct  from  the  rest  of  the  body  and  especially  from  the  head,  being 
developed.  This  strong  ventral  musculature  must  be  considered  as  the 
remains  of  the  dermo-muscular  tube  of  the  racial  form,  which  attained 
greater  development  on  the  ventral  side  in  adaptation  to  a  creeping 
manner  of  life,  while  it  degenerated  on  the  dorsal  side,  being  rendered 
functionless  and  useless  by  the  hard  shell. 

The  flat  form  of  the  foot  with  a  sole  for  creeping  must  be  con- 
sidered the  primitive  form.  Such  a  foot  is  found  in  the  Chitonida> 
among  the  Amphineura,  in  most  Gastropoda,  and  in  certain  Lamelli- 
branchia,  especially  in  the  Protobfanchia,  which  for  other  reasons  also 
must  be  considered  the  most  primitive  form  of  Lamdlibranchia. 

The  musculature-  of  the  foot  and  of  all  parts  which  become  differ- 
entiated from  it  are  innervated  from  the  pedal  ganglia  or  pedal  nerve 
cords. 

The  foot  may  become  much  modified  in  adaptation  to  various 
methods  of  life  and  of  locomotion, — in  fact,  it  may  entirely  lose  all 
resemblance  to  the  primitive  organ.  It  may,  by  constriction  or  by 
the  formation  of  lobes  or  folds,  fall  into  several  parts,  of  which  the 
following  are  the  most  important : — 

1.  Proceeding  from  before  backward  we  have  the  propodium,  an 
anterior  portion  distinct  from  the  rest,  and  the  metapodium  behind 
the  former  and  seldom   very  distinct,  which   carries   the  operculum 
when  this  is  present. 

2.  From  below  upward  there  are  the  parapodia,  lobe-like  exten- 
sions of  the  edge  of  the  ventral  sole,  and  the  epipodium,  a  projecting 
ridge  or  fold  round  the  base,  i.e.  round  the  upper  portion  of  the  foot. 
Tentacular  processes  are  often  developed  on  this  ridge. 

Taking  the  different  groups  in  order,  the  following  variations  of 
the  foot  and  the  pedal  glands  (mucous  glands  and  byssus  gland)  are  to 
be  noted. 


A.  Amphineura. 

(Of.  Section  II.,  p.  29).     The  foot  is  here  not  divided  into  separate  consecutive 
portions,  and  there  are  no  parapodia  or  epipodia. 


vii  MOLLUSCA—THE  FOOT  AND  ITS  GLANDS  107 

B.  Gastropoda. 

1.  Prosobranchia. 

With  rare  exceptions,  which  will  be  described  later,  the  foot,  which  is  well 
developed  in  this  order,  has  a  simple  (undivided)  flat  sole  for  creeping. 

Propodium. — In  a  few  cases,  however,  the  anterior  portion  of  the  foot  forms  a 
propodium  well  marked  off  from  the  rest  of  the  organ.  This  is  especially  the  case 
in  the  Monotocardia  (Olividce,  Harpidcc,  certain  species  of  Pyrulidce,  Strombidce, 
Strambus,  Pterocera,  Tercbellum,  Rostellaria  [Fig.  6,  p.  6],  Xenophoridce  [Fig.  5,  p.  5], 
Xaricidcc,  Naticidce  [Fig.  98]). 

Among  the  above,  the  propodium  is  particularly  well  developed  in  Oliva,  sepa- 
rated from  the  rest  of  the  foot  by  a  transverse  furrow  and  forming  a  semicircular 
disc. 

In  the  large  foot  of  Natica  (Fig.  98),  the  propodium  is  also  very  distinct.  It 
has  an  anterior  lobe  which  bends  back  over  the  shell,  and  so  covers  the  head. 


FIG.  98.— Natica  Josephina,  with  protruded  proboscis,  from  the  right  side  (after  Schiemenz). 
1,  Propodium;  2,  sucker-like  boring  appendage  of  the  proboscis  (3)  with  boring  gland;  4,  siphon 
(here  formed  by  the  foot) ;  5,  tentacle  ;  6,  lobe  of  the  metapodium,  which  usually  covers  a  large 
part  of  the  shell  from  behind,  and  carries  the  operculum  on  its  inner  side ;  7,  metapodium. 

Sometimes  the  propodium  forms  a  sort  of  siphon  on  the  left  side,  and  in  other  cases 
the  lobe  which  bends  back  over  the  shell  shows  a  bulging.  Both  these  arrangements 
serve  to  conduct  water  to  the  respiratory  cavity.  The  metapodium  also,  which, 
when  swollen  and  expanded,  spreads  out  widely,  carries  on  its  dorsal  side  a  lobe 
which  bends  forward  over  the  shell,  and  carries  the  operculum  on  the  side  nearest 
the  shell. 

In  most  Prosobranchia  the  metapodium  carries,  on  its  dorsal  side,  a  horny  or 
calcareous  operculum  which  serves  to  close  the  shell. 

Epipodium. — The  epipodiuin  is  very  commonly  present  in  the  Diotocardia.  It 
is  most  strongly  developed  in  Haliotis  (Fig.  105,  p.  121),  where  it  surrounds  the  base 
of  the  foot  in  the  form  of  a  large  integumental  fold.  This  fold,  which  may  aptly 
be  called  the  ruff,  has  fringed  or  digitate  appendages  as  well  as  long  contractile 
tentacular  processes.  The  tentacles  here,  as  in  other  Prosobranchia,  are  organs  of 
touch,  and  may  be  provided  at  their  bases  with  so-called  lateral  organs.  In  the 
Fissurellidce  this  epipodial  ruff  is  replaced  by  a  row  of  numerous  tentacles  or 
papillae,  rising  on  each  side  from  the  base  of  the  groove  between  the  base  of  the  foot 
and  the  visceral  dome.  Among  the  other  Diotocardia  also,  the  epipodium  is  well 


108  COMPARATIVE  ANATOMY  CHAP. 

developed  as  a  simple  or  fringed  border,  which  carries  a  few  tentacles  (usually  four 
on  each  side)  of  varying  length  (Fig.  3,  p.  4).  At  the  base  of  each  tentacle  there 
is  a  lateral  organ.  Eyes  are  said  to  occur  at  the  bases  of  the  epipodial  tentacles  in 
Eumargerita  and  Scissurella. 

The  epipodium  is,  as  a  rule,  wanting  in  Docoglossa,  but  one  is  found  beset  with 
papillae  in  the  genus  Helcion,  and  in  Patinella  and  Nacella  it  is  fringed  ;  these 
epipodia  correspond  in  position  with  those  of  other  Diotocardia. 

A  well-developed  epipodium  rarely  occurs  among  the  Monotocardia,  but  lanthina 
has  a  typical  epipodial  border,  and  the  Litiopidce  and  many  Rissoidce  have  an 
epipodium  with  several  (1-5)  tentacles  on  each  side.  Many  other  Monotocardia 
have  retained  either  the  anterior  or  posterior  portions  of  the  epipodium. 

(a]  Anterior  vestiges  of  the  epipodium  are  found  in  Vcrmetus  in  two  anterior 
pedal  tentacles,  and  in  Paludina  and  Ampullaria  in  two  nuchal  lobes,  which 
must  not  be  confounded  with  .true  cephalic  tentacles.  In  Paludina,  the  right 
nuchal  lobe,  and  in  Ampullaria  the  left,  forming  a  longitudinal  groove,  becomes  a 
sort  of  siphon.  Oalyptrcea  possesses  on  each  side  under  the  neck  a  semicircular 
epipodial  fold. 

(6)  Posterior  vestiges  of  the  epipodium  are  found  in  Lacuna  in  the  form  of  an 
epipodial  fold  with  a  process  on  each  side  above  the  foot.  Narica  has,  above  the 
metapodium  on  each  side,  a  wing-like  epipodial  lobe. 

(c)  Median  and  posterior  vestiges  of  the  epipodium  are  found  in  Choristes,  where 
there  is  a  median  papilla  on  each  side,  and  posteriorly  a  pair  of  tentacles  below  the 
operculum. 

The  epipodium  is  always  innervated  from  the  pedal  nerve  cords  or  the  homolo- 
gous pedal  ganglia,  or  from  the  pleural  ganglia  which  separate  off  from  the  latter. 

The  foot  of  Hipponyx  undergoes  a  curious  transformation.  Hipponyx  is  a 
Monotocardian  genus,  with  a  conical  shell  ;  the  animal  attaches  itself  firmly  to 
rocks  or  the  shells  of  other  Molluscs,  which  it  excavates,  either  directly  or  by 
means  of  a  shell  plate,  which  probably  answers  to  the  operculum.  The  median  part 
of  the  sole  of  the  foot  has  lost  its  muscle  layer,  and  its  edge  has  united  with  the 
edge  of  the  mantle,  leaving  only  an  anterior  aperture  through  which  the  head  can 
be  protruded.  On  the  lower  side  of  the  foot,  the  columellar  muscle  which  descends 
from  the  shell  gives  rise  to  a  horseshoe  -  shaped  muscular  area  surrounding  the 
central  non-muscular  part. 

Without  going  into  details  as  to  the  method  of  locomotion  of  the  Prosobranchia, 
it  may  be  stated  that  most  of  them  creep  or  attach  themselves  by  means  of  the  flat 
sole  of  the  foot. 

Heteropoda. — The  Heteropoda  are  pelagic  Prosobranchia  (Monotocardia},  which 
have  exchanged  the  creeping  for  the  swimming  manner  of  life.  The  foot  has  in 
them  become  peculiarly  adapted  to  this  new  method  of  locomotion.  The  propodium 
has  become  changed  into  a  narrow  vertical  rowing  fin  (carinate  foot),  which  when 
the  animal  is  in  its  swimming  position  is  turned  upward. 

The  development  of  this  vertical  fin  can  be  traced  almost  step  by  step  within 
this  division,  starting  with  Oxygyrus,  and  proceeding  through  Atlanta  and  Carinaria 
to  Pterotrachea.  In  this  series,  the  typical  outer  appearance  of  the  Prosobranchiate 
(its  shell,  visceral  dome,  mantle,  and  gills,  which  are  still  retained  in  Oxygyrus  and 
Atlanta],  gradually  disappears  owing  to  development  in  another  direction. 

Oxygyrus  (Fig.  99,  A)  still  has  the  characteristics  of  a  Prosobranchiate.  The 
foot  consists  of  (1)  a  propodium,  the  creeping  sole  of  which  has  been  somewhat 
hollowed  out  or  deepened  ;  anteriorly  it  possesses  a  fin-like  outgrowth,  which  is  used 
as  a  propelling  organ  in  swimming  ;  and  (2)  a  distinct  metapodium  directed  backwards 
like  a  tail,  and  bearing  an  operculum.  The  derivation  of  such  a  foot  from  that  of 
certain  Prosobranchia,  which  have  distinct  propodia  and  metapodia,  such  as  the 


VII 


MOLLUSC  A— THE  FOOT  AND  ITS  GLANDS 


109 


saltatory  Strombidce,  is  clear.  The  sole  of  the  foot  in  Oxygyrus,  although  it  can  be 
used  for  creeping,  is  looked  upon  as  a  sucker. 

In  Atlanta  (B),  the  arrangements  of  the  foot  are  similar  to  those  in  Oxygyrus, 
but  the  fin-like  outgrowth  of  the  propodium  has  become  its  most  important  part, 
the  comparatively  reduced  sole  or  sucker  appearing  merely  as  an  appendage  to  it. 

In  Carinaria  (C)  both  the  foot  and  the  general  external  appearance  of  the  whole 


FIG.  99.— Comparative  Morphology  of  the  Heteropoda.  A,  Oxygyrus.  B,  Atlanta.  C', 
Carinaria.  D,  Pterotrachea  9  ,  adapted  from  figures  by  Souleyet.  1,  Visceral  dome  and.  shell ; 
2,  head  with  eyes  and  tentacles  and  proboscidal  snout  (3)  ;  4,  gills  ;  5,  foot  with  sole,  which  latter  in 
B  and  C  is  reduced  to  a  sucker,  and  in  D  is  wanting ;  6,  fin-like  appendage  of  the  foot ;  7,  meta- 
podium  with,  8,  operculum. 

animal  are  much  changed.  The  metapodium,  which  here  has  no  operculum,  appears 
as  a  mere  tail-like  posterior  prolongation  of  the  body.  The  fin  is  much  broader  and 
longer,  and  the  sucker  seems  to  have  shifted  backward  along  its  free  edge. 

Finally,  in  the  PterotracJiea  (D),  the  sucker  (the  original  sole  of  the  foot)  is  still 
further  reduced,  and  only  present  in  the  male. 

The  Heteropoda  are  said  to  attach  themselves  occasionally  by  means  of  the  sucker. 


2.  Pulmonata. 

The  foot  is  here  almost  always  undivided,  and  provided  with  a  large  flat  sole 
for  creeping.  In  a  few  Auriculldce,  however  (Melampus,  Leuconia,  Blauneria, 
Pedipes),  it  is  divided  into  two  portions  by  a  temporary  or  permanent  transverse 
groove. 

3.  Opisthobranchia. 
In  almost  all   Opisthobrauchia  the  foot  has  a  well-developed   sole   for  creep-- 


110 


COMPARATIVE  ANATOMY 


CHAP. 


ing.     There  is  no  division   into   parts,  and    the  adult  rarely  (Adceon]  carries  an 

operculum. 

The  epipodium  is  wanting. 

The  parapodia,  on  the  contrary,  i.e.  lateral  lobes  or  fold-like  extensions  of  the  edges 

of  the  sole,  are  highly  developed  in  many  Opisthobranchia  (e.g.  the  Elysiadce  among  the 

Ascoglossa,  and  very  many  Tectibranchia,  such  as  the  Scaphaiidridce,  Bullidm,  Aplus- 
Gastropteridce  (Fig.    14,  p.  10),  Philinidce,  Doridiidcc,  Aplysiidce  (Fig.    75, 
p.   78),   Oxynoeidce).     The  parapodia  are  often   bent 
•A  back  over  the  shell,  their  edges  sometimes  touching, 

so  that  the  shell  may  be  entirely  roofed  over  by  them. 
In  many  forms  which  are  provided  with  parapodia 
(Gastropteridce,  Philinidce,  Doridiidce,  Aplysiidcc)  the 
mantle  also  bends  back  over  the  shell,  more  or  less 
completely  covering  it.  In  these  cases  the  shell  is 
to  some  extent  doubly  internal,  being  covered  first 
by  the  mantle  and  then  (not  in  Philine  and  Doridium} 
by  the  parapodia  (Fig.  100). 

The  parapodia  may  fuse  posteriorly  along  their 
upturned  edges  (Aplysiidce,  Oxynoe}.  In  Lobiger  each 
parapodium  is  transversely  slit,  so  that  two  long 
wing-like  processes  are  formed  on  each  side.  Many 
Opisthobranchia  (Aplysiidce,  Oxynoe,  Gastropteridce) 
can  propel  themselves  through  the  water  by  means  of 
the  waving  motion  of  their  parapodia.  Phyllirhoe 
is  a  Nudibranch  which  appears  to  have  become 
adapted  to  a  pelagic  swimming  manner  of  life  by  the 
compression  of  its  body  into  the  shape  of  a  long 
narrow  leaf  with  sharp  dorsal  and  ventral  edges  ;  it 
travels  through  the  water  with  an  undulating  motion 
(Fig.  19.  p.  12).  The  foot  has  disappeared. 

Pteropoda. — The  Pteropoda,  which  are  Tecti- 
branchiate  Opisthobranchs,  have,  like  the  Proso- 
branchiate  Heteropoda,  become  pelagic  animals  adapted 
for  swimming. 

While  in  the  Heteropoda  the  propodium  becomes 
transformed  into  a  medio-ventral  vertical  rowing  fin, 
in  the  Pteropoda  the  paired  Tectibranchiate  parapodia 


FIG.  100.— Diagrammatic  trans- 
verse sections  of  Gastropods,  to 
illustrate  the  arrangement  of  the 
shell  (black,  1),  visceral  dome  and 
mantle  (dotted,  2),  and  foot  (streak- 
ed, 3).  A,  Prosobranchiate  with  which,  as  we  have  already  seen,  can  be  used  for  swim- 
outer  shell  and  epipodium  (4).  B,  min&  develop  into  the  paired  fins  or  wings  of  these 
animals  (Figs.  16  and  17,  p.  11  ;  87,  p.  91). 

In  the  Thecosomata  (Fig.  87,  p.  91),  which  must 
be  derived  from  Cephalaspidce  (Bulloidea),  in  which 
the  parapodia  lie  on  each  side  as  direct  prolongations 
of  the  reptant  surface  of  the  foot,  this  organ,  i.e.  the 
foot,  has  become  confined  to  the  anterior  end  of  the 
body,  and  consists  of  three  portions — the  median  un- 
paired mesopodium  and  the  two  lateral  parapodia  or  fins.  The  mesopodium  is  small, 
and  the  ventral  side  of  it  (which  corresponds  with  the  sole  of  the  Cephalaspidce,  but 
can  no  longer  be  used  for  creeping)  is  strongly  ciliated.  The  ciliary  movement  is  from 
behind  forward,  i.e.  towards  the  oral  aperture  which  lies  anteriorly  on  the  foot, 
and  no  doubt  serves  for  conveying  to  it  the  minute  marine  animals  on  which  the 
creature  feeds.  On  the  dorsal  side  of  the  mesopodium,  which  projects  freely  back- 
wards, the  Limacinidcc  carry  a  delicate  transparent  operculum,  which  often  becomes 


Tectibranchiate  with  lobes  (6)  of 
the  mantle  turned  back  over  the 
outer  surface  of  the  shell.  Dorsally 
the  shell  is  still  uncovered ;  5,  para- 
podia ;  7,  ctenidium.  C,  Tecti- 
branchiate with  internal  shell,  i.e. 
completely  overgrown  by  the  lobes 
of  the  mantle. 


vii  MOLLUSCA—THE  FOOT  AND  ITS  GLANDS  111 

detached.1  The  parapodia  are  large,  fin-  or  wing-like,  and  anteriorly  inserted  on  each 
side  of  the  median  portion  of  the  foot ;  they  unite  in  front  of  and  above  the  mouth. 

The  Gymnosomata  (Fig.  16,  p.  11)  are  to  be  derived  from  the  Aplysiidce,  in 
which  the  parapodia  are  not  exactly  lateral  extensions  of  the  sole  of  the  foot,  but 
arise  somewhat  above  the  edge  of  the  sole  on  each  side.  This  may  be  explained  by 
supposing  that  they  are  fused  for  a  certain  distance  from  their  bases  with  the  lateral 
wall  of  the  body.  In  the  Gym  nosomata,  also,  the  foot  is  distinctly  separated  from 
the  two  lateral  fins  or  parapodia.  The  mesopodium  and  the  fins  lie  anteriorly  on 
the  ventral  side  of  the  body,  behind  the  head. 

The  foot  itself,  which  is  distinct  from  the  head,  consists  of  three  parts — a  pair  of 
anterior  lobes,  which  converge  anteriorly  till  they  unite,  and  a  median  posterior  lobe 
drawn  out  to  a  point  posteriorly.  The  fins  never  unite  in  front  of  or  above  the  head. 

% 

Pedal  glands  of  the  Gastropoda. — Many  Gastropods,  and  especially 
most  Prosobranchia  and  Pulmonata,  possess,  besides  the  various  unicellular 
glands  scattered  over  the  upper  and  lower  sides  of  the  foot,  larger  multi- 
cellular  localised  pedal  glands.  These  belong  to  two  morphologically 
distinct  groups. 

1.  In  the  Prosobranchia  an  anterior  pedal   gland  opens  at   the 
anterior  edge  of  the  foot.     In  those  forms  in  which  this  anterior  edge 
is  divided  into  an  upper  and  a  lower  lip,  this  "  labial  gland "  opens 
between  the  lips.     In  the  Pulmonata  it  opens  externally  between  the 
head  and  the  foot.      It  consists  of  an  epithelial  tube  of  varying  length, 
not  infrequently  as  long  as  the  foot  itself ;  this  tube  runs  backward 
in  the  median  line  mostly  through  the  base  of  the  foot ;  less  frequently 
it  lies  upon  this  base,  projecting  into  the  body  cavity. 

This  tube  serves  both  as  reservoir  and  duct  for  the  numerous 
unicellular  mucous  glands  which  lie  in  the  surrounding  tissue  of  the  foot 
and  open  on  its  walls.  It  secretes  mucus,  though  it  has  been  incorrectly 
described  as  an  olfactory  organ.  It  undergoes  considerable  modifica- 
tions with  regard  to  its  size,  the  form  of  its  lumen,  and  the  number  and 
arrangement  of  its  glandular  cells. 

2.  Among  the  ProsobrancMa,  opening  on  the  sole  of  the  foot,  there 
is  commonly  found  an  unpaired  gland.     Its  outer  slit-like  aperture  is 
median,  and  lies  behind  the  anterior  edge  of  the  foot.     It  leads  into  a 
cavity  in  the  foot  which  serves  as  a  reservoir ;  the  epithelial  wall  of 
this  cavity  projects  in  the  form  of  folds  into  its  lumen.     As  in  the 
former  case,  unicellular  glands  pour  their  secretions  into  it  through 
ducts  which  pass  between  the  epithelial  cells.     This  sole  gland  in  the 
Prosobranchia  has  rightly  been  considered  homologous  with  the  byssus 
gland  of  the  Lamdlibranchia.     It  is  developed  in  varying  degrees,  and 
not  infrequently   is   altogether  wanting.      Its  slimy   secretion  forms 
threads  by  means  of  which  many  Prosobranchia  attach  themselves  to 
objects  in  the  water.      Some  terrestrial  Pulmonata  also  lower  themselves 
from  a  height  (from  plants)  by  means  of  the  tough  threads  which  they 
secrete. 

1  With  regard  to  the  derivation  of  the  Thecosanuita  from  the  Cepkalaspidce,  which,  like 
other  Opisthobrauchia,  have  as  a  rule  no  operculum.  it  must  be  noted  ih&tActceon,  which 
is  iu  many  respects  a  primitive  CepJudaspid  genus,  possesses  an  operculum. 


112  COMPARATIVE  ANATOMY  CHAP. 

Besides  these  two,  other  pedal  glands  are  occasionally  found.  Only  one  need  be 
mentioned,  which  is  found  in  some  Opisthobranchia  (Pleurobranchus,  Pleurobranchcea, 
Pleurophyllidia).  It  lies  at  the  posterior  end  of  the  sole,  and  consists  of  glandular 
caeca,  each  of  which  opens  separately. 


C.  Seaphopoda. 

The  foot  of  Dentalium  (Fig.  101)  is  almost  cylindrical;  it  projects 
downwards  into  the  tubular  mantle  cavity,  and  can  be  protruded 
through  its  lower  aperture.  The  free  end  of  the  foot  is  conical ;  the 
base  of  the  cone  carries  on  each  side  a  fold  or  ridge  which  has  been 
compared,  with  questionable  propriety,  to  an  epipodium.  These  two 
lateral  folds  or  ridges  encircle  the  base  of  the  conical  end  without 
uniting  either  anteriorly  or  posteriorly.  A  groove  runs  along  the 
anterior  middle  line  of  the  foot. 

In  Siphonodentalium  both  this  groove  and  the  lateral  lobes  are 
wanting,  and  the  anterior  end  of  the  foot  is  broadened  into  a  round 
disc  carrying  on  its  edge  small  conical  papillae. 


D.  Lamellibranehia. 

The  foot  in  this  class  is,  as  a  rule,  laterally  compressed,  and  has  a 
sharp  edge  directed  downwards  and  forwards,  which  can  be  stretched 
out  beyond  the  shell.  It  may  be  called  hatchet-shaped  (Pelecypoda)  or 
linguiform,  and  is  especially  suited  for  forcing  its  way  into  mud  by 
means  of  alternate  contraction  and  expansion. 

This  peculiar  shape  must  be  considered  as  acquired.  Originally 
the  foot  of  the  Lamellibranehia  also  possessed  a  flat  sole  for  creeping. 
The  Protobranchia,  in  fact,  have  a  foot  with  a  ventral  disc  (Fig.  21, 
p.  1 4),  and  so  has  Pectunculus.  The  edge  of  this  pedal  disc  is  notched 
or  toothed.  When  the  foot  is  retracted,  this  disc  folds  down  the 
middle  line. 

The  foot  in  the  Lamellibranehia  varies  much  in  details,  according 
to  the  manner  of  life  or  of  locomotion  of  the  animal,  and  according  to 
the  development  of  the  byssus.  One  of  the  special  characteristics  of 
the  Lamellibranchiate  foot  is  the  gland  which  secretes  the  byssus,  the 
latter  being  a  bundle  of  tough  threads  varying  in  thickness,  and 
resembling  horn  in  their  physical  properties.  The  Lamellibranch,  with 
these  threads,  anchors  itself  to  foreign  objects.  The  byssus  can 
generally  be  thrown  off  and  replaced  by  a  new  one,  and  many  forms 
can  move  about  on  a  smooth  perpendicular  pane  of  glass  by  means  of 
alternate  attachment  and  rejection  of  portions  of  the  byssus  applied 
by  means  of  the  foot. 

Stationary  bivalves,  i.e.  those  attached  by  one  of  the  shell  valves, 
are  in  the  first  instance  attached  by  means  of  the  byssus,  for  a  byssus 
is,  as  a  rule,  present  in  the  young  stages  of  those  bivalves  which  do 
not  possess  it  as  adults. 


VII 


MOLLUSCA—THE  FOOT  AND  ITS  GLANDS 


. 
OF 


FIG.  101.— Anatomy  of  Dent- 
alium  entale,  after  Leuckart 
(Wandtafeln)  and  Lacaze-Du- 
thiers.  The  riglit  half  of  the 
shell  and  the  lower  portion  of 
the  mantle  are  removed.  a, 
Pallial  nerve  running  up  from 
the  visceral  ganglion  ;  b,  shell ; 
c,  space  between  the  mantle  and 
shell ;  d,  anus ;  e,  visceral  gan- 
glion ;  /,  mantle  cavity ;  g, 
mantle  ;  h,  lower,  t,  upper  buccal 
ganglion  ;  i,  auditory  organ  ;  A-, 
pedal  ganglion,  m,  lateral  folds 
of  the  foot ;  n,  terminal  pedal 
cone  ;  o,  filamentous  tentacles  ; 
I,  lower  edge  of  the  mantle ;  p, 
leaf -like  oral  appendages;  q, 
snout ;  r, '.  cerebral  ganglion ;  s, 
shell  or  columellar  muscle,  cut 
through  ;  i(,  right  nephridial  (and 
genital)  aperture ;  v,  digestive 
gland  (liver) ;  w,  gonad  ;  x,  upper 
end  of  the  columellar  muscle  ;  y, 
upper  open  end  of  the  mantle. 


VOL.  II 


114 


COMPARATIVE  ANATOMY 


CHAP. 


The  complete  byssus  apparatus  (Fig.  102)  consists  of :  (1)  a  cavity 
in  the  foot,  into  which  the  byssus  gland  opens ;  (2)  a  duct  connecting 
this  cavity  with  the  exterior ;  (3)  a  groove  which  runs  from  the 
aperture  of  the  duct  along  the  ventral  edge  of  the  foot  to  its  anterior 
end ;  and  (4)  a  crescent-shaped  or  cup-like  widening  of  the  groove  at 
its  anterior  end. 

(1)  The  byssus  cavity  is  divided  into  narrow  shelves  by  numerous  folds,  which 
project  from  each  side  into  its  lumen.  A  septum,  descending  from  its  roof,  further 
divides  it  into  two  lateral  parts.  The  byssus  secretion  is  yielded  partly  by  the  cells 
of  the  epithelial  walls,  and  partly  by  glandular  cells  which  lie  in  the  surrounding 
tissue,  their  ducts  passing  between  the  epithelial  cells.  The  secretion  takes  the 
form  of  the  cavity,  and  is  thus  held  fast  as  with  roots  by  the  numerous  lamellse 
which  occupy  the  shelves.  As  the  amount  of  the  secretion  in  the  cavity  increases, 
these  lamellae  are  pressed  into  the  duct  (2),  where  they  unite  to  form  the  main  stem 
of  the  byssus. 

The  walls  of  the  groove  (3)  and  its  terminal  expansion  (4)  are  also  glandular. 
When  a  bivalve  attaches  itself  it  forms  a  byssus  thread 
in  this  groove,  which  fuses  with  the  end  of  the  main 
stem.  The  tip  of  the  foot  presses  against  some  surface, 
such  as  a  rock,  and  attaches  the  thread  by  means  of  a 
cement  secreted  by  the  widened  terminal  portion  (4)  of 
the  groove.  In  this  way  the  main  stem  of  the  byssus 
may  be  fastened  to  a  rock  by  means  of  numerous  threads 
successively  secreted  in  the  groove. 

The  relation  existing  between  the  development  of 
the  foot  and  that  of  the  byssal  apparatus  may  be 
sketched  as  follows  : — 

1.  The  foot  in  its  primitive  form,  with  a  flat  sole 
and  no   groove,   has   a   simple   invagination   without 
byssus  (Solenomya). 

2.  With  the  same  foot,  a  small  lamella  rises  from 
the  base  of  the  simple  invagination  ;    the  byssus  is 
very  slightly  developed  (Nucula,  Leda). 

3.  The  invagination  becomes  differentiated  into  a 
cavity  and  a  duct,  and  the  byssus  and  its  glands  are 
strongly  developed.     In  consequence  of  this  the  foot 
ceases    to    be    a    locomotory    organ  ;     its    flat    sole 
disappears,  and  it  becomes  finger-  or' tongue-shaped, 
often   more  or  less  reduced    in  size,  and    serves  for 

attaching  the  byssus.  In  very  many  cases  the  groove  is  formed  from  the  end  of  the 
duct,  widening  at  the  tip  of  the  foot  as  above  described.  This  is  especially  the  case  in 
forms  which  anchor  themselves  by  the  byssus  to  stones,  plants,  or  the  shells  of  other 
Molmscs.  This  attachment  may  be  more  or  less  firm,  and  may  be  temporary  or 
permanent  (Limidoe,  Spondylidce,  Pectinidcc,1  Mytilidce,  Arcidce,1  Carditidcc,1  Ery- 
cinidce,  Galeommidce,  Tridacnidce,  Cyprinidce,1  Fenendce,1  Glycymcridce,  My  idee,1  etc.) 

When  the  byssus  is  very  highly  developed,  some  of  the  pedal  muscles  become 
attached  to  the  byssus  gland  and  form  the  retractors  of  the  byssus. 

4.  Many  Lamellibranchs,  in  the  adult  state,  have  neither  byssus  nor  byssus 
glands,  but  the  cavity,  the  duct,  and  even  the  retractors  (e.g.  Trigonia]  may  be 


FIG.  102.  —Byssus  of  a  Lamel- 
libranch  with  its  cavity  and 
duct.  1,  Diagrammatic  trans- 
verse section  through  the  foot ;  2, 
main  stem ;  3,  terminal  threads 
ie  byssus  to  a  foreign 


1  Proparte. 


vii  MOLLUSCA—THE  FOOT  AND  ITS  GLANDS  115 

retained.^  The  byssal  apparatus  may  be  found,  in  closely-related  forms,  sometimes 
with  and  sometimes  without  the  byssus  itself.  In  the  latter  case  the  foot  is 
generally  more  strongly  developed,  and  serves  for  locomotion,  i.e.  for  forcing  a  way 
forward  into  sand  or  mud,  which  most  of  these  forms  inhabit,  or  for  the  saltatory 
motion  of  Trigonia.  In  these  cases  it  is  linguiform,  or  wedge-  or  hatchet-shaped 
(A  re  idee,1  Carditidte,1  Cyprinidce,1  Tellinidce,  Scrobiculariidce,  Myidce?  Cardiidce,1 
Lucinidm  (foot  vermiform),  Donacidce,  etc.). 

5.  When  the  linguiform,  or  hatchet-shaped,  and  often  bent,  foot  becomes  more 
strongly  developed  as  a  fleshy  and  extensible  organ,  every  trace  of  the  byssus  and  its 
apparatus  disappears,  at  least  in  the  adult  (Unionidce,  many  Veneridce,  Cyrenidce, 
Psammobiidce,  Mesodermatidce,  Solenidce,  Mactridce).  All  these  live  in  mud.   The  fleshy 
foot  of  the  Solenidce,  which  is  directed  forwards,  is  so  strongly  developed  that  it  can 
often  no  longer  be  wholly  withdrawn  into  the  shell,  which  therefore  gapes  anteriorly. 
The  foot  is  thick  and  linguiform  in  Solenocurtus ;  club-shaped  and  truncated  at  the 
tip  in  Pharus,  Cultellus,  Siligua,  and  Ensis ;  and  cylindrical,  with  an  egg-shaped 
tip,  in  Solen. 

6.  In   forms  where  one  of  the  valves  has  become  firmly  attached  to  some  hard 
substance,  the  foot  (the  byssus  being  absent)  may  become  rudimentary  (Chamacea), 
or  may  altogether  disappear  (Ostrcidce).     In  forms  which  inhabit  mud  or  excavations 
made  by  themselves  in  stone,  etc.,  and  which  surround  the  body  with  an  accessory 
calcareous  tube  (Gastrochcenidce,  Clavagellidce),  the  foot  is  also  reduced  to  a  small, 
usually   finger-shaped    rudiment.     The    series   of    boring    Pholadidce    is   specially 
interesting.     Pholas  has  a  pestle-  or  sucker-shaped  foot,  which,  projecting  through 
the  shell  cleft,  serves  to  attach  the  animal  while  boring.     In  Pholadidea  and 
Jouannelia  only  the  young  while  boring  their  habitations   possess  such  a  foot ; 
as  soon  as  they  have  finished  this  work  the  pedal  aperture  of  the  mantle  closes,  the 
anterior  cleft  of  the  shell  is  also  closed  by  means  of  an  accessory  "shell-piece  called 
the  callurn,  and  the  foot  completely  atrophies,  so  that  the  animals  are  no  longer 
capable  of  locomotion. 

In  the  attached  Anomia,  also,  the  foot  is  small :  it  is  of  great  importance,  how- 
ever, as  bearer  of  the  byssal  apparatus.  The  shelly  plug  (see  p.  63),  by  means  of 
which  the  animal  is  fastened  to  the  ground,  and  which  occupies  the  deep  notch  cut 
by  the  byssus  into  the  right  or  under  valve,  must  be  regarded  as  a  calcified  byssus. 

Many  Lamellibranchs  (Crenella,  Lima,  Modiola)  weave  a  byssus  web  which  they 
inhabit  like  a  nest,  and  which  they  strengthen  by  the  addition  of  foreign  bodies 
attached  by  byssus  threads. 

E.  Cephalopoda. 

The  question,  what  part  of  the  body  in  Cephalopoda  corre- 
sponds with  the  foot  of  other  Mollusca,  has  led  to  much  discussion  and 
careful  investigation.  It  may  now  be  considered  as  pretty  well  estab- 
lished that  the  foot  in  Cephalopoda  forms  :  (1)  the  arms,  (2)  the  siphon. 

The  arms  are  considered  as  lateral  processes  of  a  Molluscan  foot 
which  have  pushed  past  the  head  to  the  right  and  left,  and  have 
united  in  front,  so  that  the  head  is  entirely  encircled  by  the  foot,  and 
the  mouth  has  come  to  lie  in  the  middle  of  the  ventral  pedal  surface, 
i.e.  at  the  centre  of  the  circle  of  arms  or  brachial  umbrella.  That  this 
circle  of  arms  is  a  derivative  of  the  foot  is  supported  by  important 
anatomical  and  ontogenetic  facts:  (1)  The  arms  are  innervated  from 

1  Pro  parte. 


116  COMPARATIVE  ANATOMY  CHAP. 

the  brachial  ganglion,  which  lies  under  the  oesophagus,  and  is  an 
anterior  division  of  the  pedal  ganglion.  (2)  The  arms  do  not 
occupy,  in  the  embryo,  their  definitive  position  round  the  mouth,  but 

rise  on  the  ventral  side  behind  the 
mouth,  between  it  and  the  anus,  in  a 
row  on  each  side.  These  two  rows  shift 
secondarily  forward  to  form  the  circle  of 
arms  round  the  mouth.  (According  to 
another  view,  the  arms  are  cephalic  ap- 
pendages, comparable  with  the  cephalic 
tentacles  of  the  Pteropoda.) 

The  pedal  nature  of  the  siphon  or 
funnel  has  rarely  been  doubted.  It  is 
innervated  from  the  pedal  ganglion.  Its 
two  lateral  lobes,  which  in  the  Nautilus 
remain  separate  throughout  life,  but  in 
the  Dibranchiata  overlap,  may  be  con- 
sidered as  epipodia.  The  accompany- 
ing figure  of  a  Cephalopod  embryo  con- 
FIO.  los.-Embryo  of  a  Cephaiopod,  fi  thi  opinion  .  the  rudimentary 

seen  obliquely  from  the  left  posterior  side  *   .  >  .  .   .          J 

(after  Grenacher).  1,  Mantle;  2,  anus ;  Siphon  IS  seen  in  the  typical  position  Ol 
3,  right  ctenidium  ;  4,  rudimentary  epipodia  in  the  shape  of  tWO  lateral  f  olds 

y^c;\^e°yey.0rSan;  "'  ^ '  ''  running  backward  above  the  foot  and 

under  the  visceral  dome. 

In  Nautilus  and  the  Decapoda  (excluding  the  Loligopsidce)  a  valve 
is  present  within  the  siphon.  For  the  form  of  the  siphon,  see  p.  38. 

1.  The  Arms  of  the  Tetrabranchia  (Nautilus). 

The  "head"  of  the  Nautilus  (Fig.  104)  carries  numerous  tentacles  placed  in  a 
circle  round  the  mouth  ;  these  do  not  rise  directly  from  the  integument  around  the 
mouth,  but  stand  upon  special  lobes  which  are  differently  developed  in  the  two 
sexes.  These  lobes  may  be  compared  with  the  arms  of  the  Dibranchia,  and  the 
tentacles  they  carry,  perhaps,  with  the  suckers  on  those  arms.  Each  tentacle  can  be 
retracted  into  its  own  basal  portion  as  into  a  sheath. 

If  the  head  be  viewed  from  the  ventral  side,  so  that  the  mouth  appears  lying  in 
the  centre  of  the  extended  lobes  and  tentacles,  we  see  in  the  female  (lower  figure) 
three  inner  lobes  close  to  the  mouth,  two  lateral  and  one  posterior.  The  posterior 
inner  lobe  consists  of  two  fused  lateral  lobes,  the  line  of  fusion  being  indicated  by  a 
lamellated  (olfactory  ?)  organ.  It  carries  twenty-eight  tentacles,  fourteen  on  each  side. 
Each  lateral  inner  lobe  carries  twelve  tentacles.  Besides  these  three  inner  lobes, 
the  foot  develops  a  muscular  circular  fold  ;  this  is  particularly  thick  anteriorly,  and 
here  forms  a  lobe,  the  so-called  hood  (Fig.  32,  a,  p.  22),  which,  when  the  head  is- 
retracted,  covers  the  aperture  of  the  shell  like  an  operculum.  The  outer  circular 
fold  carries  nineteen  tentacles  on  each  side. 

Besides  these  tentacles  which  belong  to  the  foot,  there  are  two  more  on  each  side- 
which  probably  belong  to  the  head,  one  lying  above  and  the  other  below  the  eye. 

In  the  male  Nautilus  (upper  figure)  the  posterior  inner  lobe  is  rudimentary. 
Each  of  the  lateral  inner  lobes  is  divided  into  two  portions.  In  the  right  lobe,  the 
anterior  portion  carries  eight  tentacles  and  the  posterior  (antispadix)  four,  three  of 


VII 


MOLLUSCA—THE  ARMS  OF  THE  CEPHALOPODA 


117 


which  have  a  common  sheath.     The  anterior  portion  of  the  left  lobe  also  carries 
eight  tentacles,  and  the  posterior  portion  forms  the  conical  spadix,  which,  instead 


t  s 


FIG.  104.— Circumoral  ring  of  tentacles  in  Nautilus  pompilius  (after  Lankester  and  Bourne). 
From  the  oral  or  ventral  side.  Upper  figure  male,  lower  female,  a,  Shell ;  b,  circular  fold  or  hood 
with  its  tentacles,  g ;  c,  the  two  lateral  inner  lobes,  in  the  male  the  left  inner  lobe  forms  the  spadix 
or  hectocotylus  p,  and  the  right  the  antispadix  q  ;  d,  the  posterior  inner  lobe,  reduced  in  the  male  ; 
n,  lamellated  organ  (olfactory?) ;  e,  jaws  in  the  buccal  cone  ;  /,  the  tentacles  of  the  outer  muscular 
circular  fold  ;  I,  eye  ;  m,  paired  lamellated  organ  ;  o,  siphon  or  funnel. 

of  tentacles,  carries  imbricated  lamellae.  This  spadix  is  looked  upon  as  the  hecto- 
cotylised  limb  of  the  Nautilus,  and  probably  takes  some  part  in  copulation  (see 
the  Copulatory  Apparatus,  p.  242). 

2.  The  Arms  of  the  Dibranchia. 

The  Dibranchia  have  either  eight  or  ten  arms,  which  stand  in  a  circle  round  the 
mouth  and  carry  two  longitudinal  rows  of  suckers  (acetabula)  ;  rows  of  cirri  may 
accompany  the  suckers,  and  the  cirri  may  here  and  there  become  transformed  into 
hooks  or  claws  (e.g.  Onychotcuthis). 


118  COMPARATIVE  ANATOMY  CHAP. 

In  many  Octopoda,  the  long  arms  are  connected  by  means  of  membranes  near 
their  bases,  and  occasionally  as  far  as  their  tips.  In  the  latter  case  the  circle  of 
arms  has  the  appearance  of  an  umbrella,  of  which  the  arms  are  the  ribs.  The 
mouth  lies  in  the  centre.  The  Octopoda  can  creep  by  means  of  their  circle  of  arms, 
the  visceral  dome  standing  erect.  In  this  position  they  may  best  be  compared 
with  snails,  the  ventral  side  of  the  circle  of  arms  functioning  like  the  sole  of  the 
snail's  foot. 

The  Decapoda  have  ten  arms  ;  eight  of  these  correspond  with  the  eight  arms  of 
the  Octopoda,  but  are  shorter  and  are  never  connected  by  membranes.  The  two 
others,  the  prehensile  tentacles,  are  inserted  between  the  third  and  fourth  Octopodan 
arms  on  each  side  and  differ  from  the  latter  in  structure,  being  long  and  vermiform, 
with  swollen  ends  armed  with  suckers,  hooks,  etc.  The  prehensile  tentacles  are 
very  contractile,  and  in  many  Decapoda  (e.  g.  Sepia)  are  concealed  in  special  cavities 
of  the  head  when  the  animal  is  at  rest.  These  cavities  probably  correspond  morpho- 
logically with  the  water  pores,  which  often  occur  elsewhere  at  the  bases  of  the  arms 
or  on  the  head.  When  pursuing  prey  the  Decapods  dart  these  tentacles  out  of  their 
cavities  with  great  force. 

One  (less  frequently  two)  of  the  eight  or  ten  arms  of  the  male  Dibranchia  is 
almost  always  transformed  (hectocotylised)  to  assist  in  copulation.  In  some 
Octopoda  it  even  becomes  detached  from  the  body  and  is  regenerated. 

The  hectocotylised  arm  is,  in  the  Octopoda,  usually  the  third  arm  on  the  right 
side,  and  in  the  Decapoda  the  fourth  on  the  left.  (The  arms  are  counted  from 
before  backward.) 

In  the  female  Argonaut,  each  arm  of  the  first  pair  is  widened  into  a  sail-like 
expansion,  which  stretches  back  over  the  outer  surface  of  the  shell. 

All  Cephalopods,  even  the  more  massive  Octopoda,  are  good  swimmers.  In  swim- 
ming, the  mantle  and  funnel  play  the  chief  parts.  "Water  is  alternately  taken  into 
the  mantle  cavity  through  the  mantle  cleft,  and  expelled  from  it  forcibly  through 
the  funnel,  the  reaction  propelling  the  animal  backwards.  When  the  water  is  being 
ejected,  the  mantle  cleft  is  closed  by  the  locking  apparatus,  so  that  all  the  water  in 
the  mantle  cavity  has  to  pass  out  through  the  funnel.  Many  Decapoda  can  also 
swim  with  the  head  directed  forward,  the  lower  (distal)  end  of  the  funnel  being  bent 
round,  so  that  the  water  is  expelled  in  the  direction  of  the  visceral  dome.  In 
swimming  the  arms  are  apposed  to  one  another,  so  as  to  diminish  the  friction  as 
much  as  possible.  Some  Octopoda,  especially  those  which  have  interbrachial 
membranes,  assist  themselves  in  swimming  by  opening  and  shutting  their  circle  of 
arms  like  an  umbrella. 


XI.  Swelling1  of  the  Foot  (Turgescence). 

Imbibition  of  Water. 

The  foot  in  many  Lamellibranchia  and  Gastropoda  may  swell  when 
it  has  to  be  protruded  from  the  shell  and  used  for  locomotion.  Until 
recently  opinions  varied  very  much  as  to  the  way  in  which  this  swell- 
ing or  expansion  took  place.  Many  believed  that  water  was  taken 
up  from  without  into  the  blood  vascular  system  or  into  a  special 
water  vascular  system,  but  there  was  difference  of  opinion  as  to  the 
manner  in  which  it  was  taken  in.  On  the  one  hand  it  was  said  to 
enter  through  apertures  or  pores  in  the  foot,  which,  however,  do  not 
exist,  the  only  pores  found  being  the  apertures  of  the  pedal  glands 


vii  MOLL  USCA— MUSCULATURE  1 1 9 

(byssus  and  sole  glands).  On  the  other  hand,  the  water  was  sup- 
posed to  enter  the  foot  through  intercellular  ducts  between  the 
epithelial  cells,  but  this  theory  has  also  been  disproved. 

Others,  again,  maintained  that  the  water  was  conducted  by  the 
nephridia  to  the  pericardium,  and  conveyed  thence  through  the  blood 
vascular  system ;  but  the  pericardium  has  been  shown  to  be  entirely 
separated  from  the  vascular  system.  Indeed,  many  theories  on  this 
subject  have  been  put  forward  and  disproved. 

It  is  now  the  received  opinion  that,  except  in  the  case  of  one 
animal,  which  will  be  presently  described,  the  foot  is  swelled  by  a 
rush  of  blood  which,  flowing  into  the  foot,  is  prevented  from  returning 
to  the  body  by  sphincter  muscles. 

The  exceptional  case  is  that  of  Natica  Josephina.  In  this  animal 
there  can  be  no  doubt  that  water  is  taken  in  to  swell  the  foot.  The 
swelling  takes  place  very  quickly — in  less  than  five  minutes.  When 
the  foot  is  stimulated  it  gives  out  an  amount  of  water  which  would 
fill  the  empty  Natica  shell  two  or  three  times.  The  water  is  taken  in 
through  very  small  slits,  invisible  to  the  naked  eye  (probably  indeed 
through  a  single  very  narrow  slit,  lying  at  the  edge  of  the  foot),  and 
finds  its  way  to  a  system  of  water  sinuses,  quite  distinct  from  all 
other  cavities  of  the  foot,  and  also  distinct  from  the  blood  vascular 
system  (which  in  Natica  is  closed).  There  can  thus  be  no  question  of 
a  direct  taking  in  of  water  into  the  circulatory  system.  The  water 
slits  at  the  edge  of  the  foot  can  be  closed  by  muscles,  which  extend 
from  their  upper  to  their  lower  edges. 


XII.  Musculature  and  Endoskeleton. 

This  chapter  has  for  its  subject  simply  the  general  musculature  of  the  body.  It 
would  be  impossible  to  describe  in  detail  the  musculature  of  special  organs,  such  as 
the  intestine,  the  heart,  and  the  copulatory  organs,  that  of  the  cutis,  or  even  that  of 
the  most  muscular  of  all  the  organs — the  foot ;  since,  owing  to  the  varied  development 
and  functions  of  this  organ,  its  musculature  is  liable  to  innumerable  modifications. 

The  character  of  the  general  body  musculature  of  the  Mollusca  is 
determined  by  the  degree  of  development  of  the  shell,  whose  function 
is  to  protect  the  soft  portions  of  the  body.  In  order  to  make  this 
protection  complete,  the  Molluscan  body  is,  as  a  rule,  though  differing 
greatly  in  details,  so  arranged  that  the  soft  parts  can  be  entirely  con- 
cealed in  the  shell,  which  can  itself  in  many  cases  be  closed.  The  shell 
thus  functions  as  skeleton  and  passive  locomotory  organ,  to  which  are 
attached  such  muscles  as  draw  the  body  into  the  shell  by  their  contrac- 
tion, and  such  as  partially  or  wholly  close  the  shell. 

It  is  obvious  that  the  arrangement  of  the  musculature  becomes 
much  modified  secondarily  in  cases  where  the  shell  aborts  or  altogether 
disappears. 

The  musculature  of  the  Mollusca  is  not  transversely  striated. 


120  COMPARATIVE  ANATOMY  CHAP. 


A.  Amphineura. 

The  musculature  of  the  Chitonidce  has  neither  been  sufficiently 
investigated  nor  systematically  described.  According  to  the  figures 
of  various  writers  on  the  subject  there  are — (1)  a  large  longitudinal 
muscle  mass  on  each  side  above  the  foot ;  (2)  numerous  muscle  fibres 
which  run  down  from  the  latero-dorsal  region  and  radiate  into  the 
sole ;  and  (3)  the  special  fibres  of  the  foot,  which  run  through  it  in 
various  directions.  The  muscle  fibres  mentioned  under  (2)  no  doubt 
correspond  with  the  shell  muscles  of  the  Fissurellidce,  etc.,  and  the 
columellar  muscle  of  other  Gastropods.  Some  of  the  fibres  descending 
from  one  side  cross  those  from  the  opposite  side.  These  crossings  are 
very  marked  in  the  median  plane  between  the  two  pedal  nerve 
cords. 

Among  the  Solenogastres,  the  muscular  system  of  Proneomenia  has 
been  the  most  thoroughly  investigated.  In  connection,  no  doubt, 
with  the  degeneration  of  the  foot  and  the  vermiform  development 
of  the  body,  a  kind  of  dermo-muscular  tube  has  been  formed ;  its 
layers,  consisting  of  muscle  fibres  running  in  various  directions,  are 
very  thin  in  comparison  with  the  thick  epidermis.  This  muscular 
tube  lies  immediately  under  the  epidermis.  Its  outer  layer  consists 
of  circular  muscle  fibres,  then  follows  a  layer  of  diagonal  fibres,  cross- 
ing each  other  at  right  angles,  but  crossing  the  circular  and  longi- 
tudinal fibres  at  an  angle  of  45°.  The  innermost  layer  consists  of 
longitudinal  fibres,  and  is  most  strongly  developed  on  the  ventral 
surface  on  each  side  of  the  ventral  groove.  Groups  of  fibres  are 
detached  from  the  circular  layer  on  both  sides,  and  converge  towards  the 
base  of  the  rudimentary  foot,  some  of  them  crossing  above  it.  The 
bundles  which  arise  from  the  lateral  and  upper  walls  of  the  body  run 
within  the  septa  which  separate  the  consecutive  lateral  diverticula  of 
the  intestinal  canal. 

So  far  as  a  comparison  between  these  animals  and  the  Chitonidce  is  possible,  the 
abortion  of  the  foot  and  vermiform  shape  of  the  body  being  taken  into  account,  and 
Chitonellus  taken  as  the  transition  form,  it  may  be  assumed  that  the  circular  muscle 
layer,  and  in  particular  the  groups  of  fibres  converging  towards  the  foot,  correspond 
with  the  dorso-ventral  muscles  of  Chiton,  and  the  longitudinal  layer  with  their  lateral 
longitudinal  muscle  masses. 

B.  Gastropoda. 

The  only  important  muscle  to  be  considered  in  this  class  is  the 
columellar  muscle.  This  muscle  is  attached  inside  the  shell  to  the 
columella,  along  which  it  runs  on  the  right  side  of  the  visceral  dome 
and  along  the  right  edge  of  the  mantle  cavity;  it  then  enters  the  dorsal 
side  of  the  foot  in  which  it  spreads  out.  The  columellar  muscle  acts 
as  a  retractor  to  withdraw  the  animal  into  its  shell. 


VII 


MOL  L  USOA— MUSCULATURE 


121 


1.  Prosobranehia. 

The  columellar  muscle  is  here  always  developed  in  its  typical  form. 
It  is  attached  at  one  end  to  the  columella  in  the  last  coil  of  the  shell, 
and  at  the  other  to  the  operculum,  which  lies  on  the  dorsal  side  of  the 
metapodium. 

A  few  Prosobranehia,  such  as  most  Fissurellidce,  Haliotidce,  and  Docoglossa,  use 
their  foot  chiefly  as  a  sucker  for 
attaching  themselves  to  some  firm 
surface.  These  forms  have  no 
operculum.  The  columellarmuscle 
descends  vertically  into  the  foot, 
and  by  its  contraction  presses  the 
shell  against  the  surface  to  which 
it  is  attached.  In  Haliotis  (Fig. 
105),  the  ear-shaped  shell  of  which 
is  coiled,  this  muscle  is  cylindrical 
and  is  very  highly  developed  ;  it 
runs  somewhat  to  the  right  of  the 
median  plane,  at  right  angles  to 
the  pedal  disc,  thereby  pushing  the 
mantle  cavity  and  the  viscera  to 
the  left.  In  many  Fissurellidce 
and  the  Docoglossa,  the  shell  has 
become  cup  -  shaped  and  sym- 
metrical ;  the  columellar  muscle, 
which  is  very  much  shortened, 
descends  direct  from  the  inner 
surface  of  the  shell  to  the  foot, 
and  is  no  longer  cylindrical.  The 
whole  muscle  has  the  form  of  a 
short  truncated  hollow  cone,  open 
anteriorly,  which  is  attached  to 
the  shell  by  its  upper  horseshoe-  FlG-  105. -Haliotis,  from  above,  after  removal  of  the 
, .  i  ,.  j  i.  shell,  the  mantle,  and  the  entire  dorsal  integument  (after 

shaped  sectional  surface,  and,  by  Wegmann).  f)Snout;  sand  p,  salivary  glands  ;  ft,  lateral 
its  base  of  the  same  shape,  to  the  pockets  of  the  oesophagus  ;  i,  mid-gut ;  a,  oesophagus ;  r, 
sucker-like  foot.  The  viscera  are  rectum ;  e,  stomach  with  caecum  (c) ;  h,  digestive  gland 
contained  in  its  hollow  axis  (Fig.  (liver>'  its  right-hand  portion  which  lies  next  the  large 
106).  The  same  arrangement 
occurs  in  all  cases  where  the  shell 
is  flatly  conical,  cup-  or  plate-shaped,  as  in  the  Hipponycidce  and  the  Capulidce 
among  the  Monotocardia. 

Heteropoda. — In  this  order,  in  which  the  atrophy  of  the  shell,  the  transformation 
of  the  foot,  and  the  gradual  obliteration  of  all  resemblance  to  a  Gastropod  can  be 
traced,  step  by  step,  the  musculature  deserves  special  attention. 

In  Atlanta,  where  the  head  and  foot  can  still  be  completely  withdrawn  into  the 
shell,  the  columellar  muscle  retains  its  typical  form.  It  descends  from  the  shell, 
dividing  into  three  strands ;  the  strongest  median  strand  stretches  out  into  the  fin  and 
the  sucker,  the  posterior  into  the  operculum-bearing  metapodium,  and  the  anterior, 
which  is  the  smallest,  into  the  head  and  snout. 


columellar  muscle  (m)  is  covered  by  the  genital  gland. 
A  fringed  epipodium  encircles  the  body. 


122 


COMPARATIVE  ANATOMY 


CHAP. 


ecr 


The  cutis  in  Atlanta  is  still  comparatively  thin.     The  network  of  muscles  lying 
immediately  beneath  it  is  not  more  strongly  developed  than  in  other  Gastropods. 
A  special  system  of  crossing  muscle  fibres  independent  of  the  other  dermal  muscula- 
ture lies  on  each  side  under  the  cutis  of  the 
fin.     This  is  the  case  in  all  Heteropoda. 

The  integument  greatly  increases  in 
thickness  in  the  typical  Hetcropoda 
(Carinaria,  Pterotrachea),  and  the  sub- 
cutaneous muscular  tube  also  grows 
thicker.  Over  the  body  the  latter  con- 
sists of  two  superimposed  layers  of  fibres 
crossing  one  another  diagonally.  In  the 
outer  layer,  the  fibres  run  from  above 
anteriorly  downwards  posteriorly,  and  in 
the  inner  from  below  anteriorly  to  above 
posteriorly.  On  the  head  and  snout,  the 
visceral  dome  and  the  tail-like  metapo- 
dium,  the  diagonal  fibres  of  both  layers 
run  longitudinally.  In  addition,  an  ex- 
ternal circular  musculature  is  found  in 
Carinaria  nearly  all  over  the  body,  in 
Pterotrachea  only  in  the  snout. 

Turning  now   to    Carinaria,    which 


env 


FIG.  106.— Patella,  from  above,  after  removal 
of  the  shell  (after  Lankester).  c,  The  separate 
bundles  of  the  shell  muscle,  the  section  of  which 
is  horseshoe-shaped  ;  I,  pericardium  ;  Ix,  fibrous 


septum  behind  the  same ;  n,  digestive  gland  ;  int,  still  possesses  a  delicate,  easily  detachable 
intestine ;  fc,  larger  right  nephridium  ;  i,  smaller  sheii  covering  the  visceral  dome,  but  un- 
left  ditto  ;  e,  mantle  border,  widening  anteriorly  We  to  ^  other  t  Qf  the  bod 

into  the  mantle  fold  ;  ecr,  em,  edge  of  mantle.  * 

we  find  the  columellar  muscle  still  per- 
sisting in  the  form  of  two  bands  descending  from  the  visceral  dome  into  the  fin  and 
radiating  out  to  its  edge. 

In  Pterotrachea,  where  the  shell  is  wanting  and  the  visceral  dome  rudimentary, 
the  columellar  muscle. is  also  reduced.  It  has  now  no  connection  with  the  visceral 
dome,  and  commences  half-way  up  the  body  wall  as  three  short  bands  running  down 
into  the  fin  and  radiating  out  to  its  edge. 

The  columellar  muscle,  which  originally  served  for  drawing  the  foot  back  into 
the  shell,  now  serves  chiefly  to  bring  about  the  lateral  movements  of  the  vertical 
rowing  fin  into  which  the  foot  has  been  transformed. 


2.  Opisthobranchia. 

The  columellar  muscle  is  well  developed  in  forms  possessing  a  shell  into 
which  the  body  can  be  partly  or  wholly  withdrawn.  Where,  however,  the  shell 
is  rudimentary  or  wanting,  as  is  the  case  with  most  Opisthobranchia,  the 
columellar  muscle  atrophies  or  perhaps  forms  part  of  the  pedal  musculature.  The 
subcutaneous  dermo  -  muscular  tube,  on  the  other  hand,  develops  in  proportion 
to  the  activity  of  the  animal.  It  consists  of  longitudinal,  circular,  and  diagonal 
muscle  fibres,  which  occasionally  form  a  regular  network.  The  pedal  musculature 
is  merely  a  thickened  portion  of  this  dermo-muscular  tube  in  which  longitudinal 
fibres  predominate.  The  development  of  the  musculature  varies  much  in  detail. 
Where  movable  or  contractile  dorsal  appendages,  gills,  oral  lobes,  oral  discs,  para- 
podia,  etc.,  are  developed,  their  musculature  is  detached  from  the  dermo-muscular 
layer,  and  the  latter,  in  combination  with  the  occasionally  tough  skin,  forms  a 
passive  organ  of  support  for  the  former. 

A  columellar  muscle  is  further  found  in  the  Ptcropoda  thecosomata.    It  is  ventral 


VII 


MOLL  USCA—MUSCULA  TUBE 


123 


in  the  Limacinidce,  but  dorsal  in  the  Cavoliniidce,  in  which  family  the  body,  as 
compared  with  the  head,  seems  to  have  been  twisted  through  an  angle  of  180° 
(p.  80).  The  muscle  divides  anteriorly  into  two  lateral  branches,  which  radiate 
out  into  the  fins. 


3.  Pulmonata. 

In  the  shell-bearing  Pulmonata,  the  columellar  muscle  is 
developed.  It  is  paired,  and  attached 
at  one  end  by  many  roots  to  the  foot, 
behind  the  buccal  mass,  and  at  the 
other  to  the  columella  of  the  first  coil 
or  whorl  of  the  shell.  It  gives  off 
three  subsidiary  branches — (1)  the 
retractor  muscles  of  the  optic  and 
other  tentacles ;  (2)  the  retractors  of 
the  buccal  mass  ;  (3)  muscles  running 
to  the  viscera. 


strongly 


In  the  Daudebardia  and  Testacellidce,  in 
which  the  dwindling  visceral  dome  with  the 
shell  which  covers  it  have  shifted  to  the 


FIG.  107.— Shell  of  Helix,  in  longitudinal 
section  through  the  columellar  axis  (after 
Howes).  c,  Columella  ;  mi,  columellar 
posterior  end  of  the  body,  and  m  which  all  muscle;  p>  edge  of  oral  aperture  (peritreme). 
possibility  of  the  retraction  of  the  body  into 

the  shell  has  ceased,  only  parts  of  the  columellar  muscle  are  retained,  and  naturally 
those  parts  which  are  still  functional.  In  the  Daudebardia  and  Testacellidce  these 
are  the  retractors  of  the  tentacles,  and  in  Daudebardia  the  retractors"  of  the  pharynx. 
The  tentacular  and  pharyngeal  retractors  are  distinct. 

The  retractors  of  the  tentacles,  in  Daudebardia  rufa,  run  back  separately  to  the 
base  of  the  visceral  dome,  not  entering  it,  but  fusing  with  the  body  wall  on  each 
side  of  it.  In  D.  saulcyi  the  retractors  do  not  run  so  far  back,  but  the  two  on  the 
right  fuse  with  the  two  011  the  left,  and  pass  into  the  pedal  musculature  in  the 
anterior  half  of  the  body.  The  same  is  the  case  in  the  Testacellidce. 

The  Retractors  of  the  Pharynx.  — In  Daudebardia  rufa  there  are  found,  attached 
to  the  pharynx,  two  retractors,  which,  passing  through  the  cesophageal  nerve  ring, 
fuse  to  form  one  muscle,  which  runs  back  along  the  base  of  the  pharyngeal  cavity 
somewhat  to  the  left,  then  ascends  in  the  visceral  dome  to  be  attached  to  the 
columella  of  the  last  coil  of  the  shell.  In  D.  saulcyi,  where  there  is  no  projecting 
visceral  dome,  and  the  shell  merely  covers  a  mantle  cavity,  the  cesophageal  re- 
tractors, which  are  not  in  this  case  fused  together,  no  longer  run  up  into  the  shell, 
but  end  in  the  middle  of  the  body,  where  they  enter  the  pedal  musculature. 

The  numerous  cesophageal  retractors  which,  in  Testacella,  are  arranged  in  two 
asymmetrical  rows,  cannot  for  several  reasons  be  considered  as  the  remains  of  a 
columellar  muscle. 

Oncidium  when  adult  has  neither  shell  nor  columellar  muscle,  but  its  shell- 
bearing  larva  also  possesses  a  columellar  muscle. 


In  Dentolium    (Fi< 


C.  Seaphopoda. 
101,  p.   113)  two  closely  contiguous  muscle 


bands  run  on  each  side  along  the  anterior  side  of  the  body,  and  are 
attached  anteriorly  to  the  dorsal  end  of  the  tubular  shell.     At  the 


124  COMPARATIVE  ANATOMY  CHAP. 

base  of  the  foot  these  two  bands  unite  to  form  a  single  muscle  on  each 
side,  which  enters  the  foot  and  radiates  out  through  it  in  the  form  of 
numerous  longitudinal  bundles.  This  then  is  a  paired  eolumellar 
muscle  which  retracts  the  foot,  and  draws  the  whole  of  the  lower 
portion  of  the  body  back  into  the  upper  part  of  the  shell. 

D.  Lamellibranehia. 

The  two  principal  groups  of  muscles  to  be  considered  in  this 
class  are : — 

1.  The  pallial  musculature. 

2.  The  pedal  musculature. 

The  former  is  principally  developed  near  the  free  edge  of  the 
mantle,  and  consists  of  three  systems  :  (1)  Fibres  which  run  in  the 
plane  of  the  mantle  fold  towards  and  at  right  angles  to  its  edge. 
These  are,  in  the  narrower  sense,  the  muscles  of  the  pallial  edge, 
and  leave  on  the  shell  the  scar  known  as  the  pallial  line.  (2) 
Fibres  running  parallel  with  the  edge  of  the  mantle.  (3)  Short  trans- 
verse fibres  running  more  or  less  straight  between  the  inner  and  the 
outer  surfaces  of  the  mantle.  In  the  siphons,  which  are  formed  from 
the  mantle,  these  three  systems  become  circular,  longitudinal,  and 
radial  layers.  The  retractors  of  the  siphons  are  a  special  differentia- 
tion of  the  pallial  musculature  ;  their  development  is  in  direct  relation 
to  the  size  of  the  siphons ;  their  crests  of  attachment  to  the  shell 
valves  cause  the  scar  known  as  the  pallial  sinus  (cf.  p.  64). 

The  important  adductor  muscles  for  closing  the  shell  must 
also  be  regarded  as  differentiations  of  the  pallial  musculature.  These 
are  exceedingly  thick  and  powerful  and  run  transversely  from  the 
inner  surface  of  one  valve  to  the  corresponding  surface  of  the  other 
valve.  They  counteract  the  ligament  at  the  hinge,  their  contraction 
causing  the  two  valves  to  approach  one  another,  till  the  shell  is  closed. 
These  adductors  leave  scars  on  the  inner  surfaces  of  the  valves. 
Typically,  there  are  two  adductors,  an  anterior  and  a  posterior 
(Dimyaria),  situated  nearer  the  dorsal  than  the  ventral  edge  of  the 
valves.  In  the  Mytilacea,  the  posterior  adductor  "is  larger  than  the 
anterior  (Heteromyaria  as  opposed  to  Isomyaria).  In  one  large 
series  of  forms  the  anterior  adductor  completely  atrophies,  and -the 
posterior  adductor,  which  is  all  the  more  strongly  developed,  shifts 
forwards  towards  the  middle  of  the  shell.  These  forms  are  grouped 
together  as  Monomyaria ;  but  this  is  no  natural  group,  since  nearly- 
related  forms  (e.g.  within  the  Muelleriacea)  may  possess  either  one  or 
two  adductors,  and  widely  different  forms  (e.g.  Tridacna,  Anomia, 
Muelleria,  Aspergillum)  agree  in  having  only  one.  The  Anomiidce, 
Ostreidce,  Spondylidce,  Limidce,  Pectinidce,  Aviculidce,  Muelleridce,  etc.,  are 
Monomyarian. 

The  adductor  often  (e.g.  Pecten,  Ostrea,  Nueula)  consists  of  two  apparently 
different  parts,  one  containing  smooth  fibres  and  the  other  fibres  which  appear 


vii  MOLLUSCA— MUSCULATURE  125 

transversely  striated,  although   their  striation   does  not  correspond  with  that   of 
Arthropod  and  Vertebrate  muscles. 

The  pedal  musculature,  taken  as  a  whole,  answers  to  the 
columellar  muscle  of  other  Molluscs,  especially  of  the  Gastropoda. 
It  consists  of  symmetrical  pairs  of  muscles  attached  at  one  end  to  the 
inner  surface  of  the  shell  on  which  they  leave  impressions,  the  other 
ends  entering  the  foot.  The  correspondence  of  this  musculature  with 
the  columellar  muscle  of  the  Gastropoda  is  best  seen  by  comparing  a 
Protobmnchiate  with  Patella  or  Fissurella.  In  Nucula  or  Leda,  for 
example,  there  is  an  almost  continuous  series  of  muscle  bundles 
running  down  to  the  foot  on  each  side  between  the  anterior  and 
posterior  adductors.  The  two  series  taken  together,  seen  from  above 
or  below,  have  an  oval  outline  answering  to  the  horseshoe-shaped  or 
almost  oval  form  of  the  section  of  the  columellar  muscle  in  Patella 
(Fig.  106)  or  Fissurella. 

In  most  cases  in  which  the  foot  is  developed,  the  following  muscles  on  each  side 
are  distinguished  in  order  from  before  backwards  (cf.  Fig.  108) :  (1)  the  protractor 


.-OA 


-OB 


FIG.  10S.— Pliodon  Spekei,  from  the  left  (after  Felseneer).  The  shell,  mantle,  gills,  and  oral 
lobes  of  the  left  sides  removed.  AA,  Anterior  ;  AP,  posterior  adductor  ;  OA,  anal ;  OB,  branchial 
aperture  of  the  siphon  ;  V,  visceral  mass  ;  p,  foot ;  1,  protractor  pedis ;  2,  retractor  pedis  anterior; 
3,  elevator  pedis  ;  4,  retractor  pedis  posterior. 

pedis  ;  (2)  the  anterior  retractor  pedis  ;  (3)  the  elevator  pedis,  and  (4)  the  posterior 
retractor  pedis. 

AY  here  there  is  a  byssus,  the  posterior  retractor  becomes  the  byssus  muscle.  It 
is  then  usually  highly  developed,  runs  far  forward,  and  may  break  up  into  several 
bundles. 

In  those  cases  in  which  the  foot  is  rudimentary  and  the  byssus  wanting,  the  pedal 
muscles  degenerate. 

In  Pecten  the  pedal  retractors  are  asymmetrically  attached,  i.e.  only  to  the 
left  valve.  The  same  is  the  case  in  Anomia,  where  the  shelly  plug  which  lies  in 
the  byssus  notch  of  the  right  valve,  and  corresponds  with  the  byssus,  is  attached  to 
the  left  (or  physiologically  upper)  valve  by  two  highly-developed  retractors.  These 
two  muscles  leave  scars  near  that  of  the  adductors.  This  fact  gave  rise  to  the 
erroneous  opinion  that  the  Anomia  were  Trimyaria. 


126  COMPARATIVE  ANATOMY  CHAP. 


E.  Cephalopoda. 

In  the  Cephalopoda,  a  cartilaginous  endoskeleton  is  developed. 
This  not  only  serves  for  the  attachment  of  various  muscles  and 
muscular  membranes,  but  is  also  a  protection  for  important  organs, 
especially  for  the  central  portion  of  the  nervous  system  and  the  eyes. 
Of  the  different  cartilages  forming  this  endoskeleton  the  only  constant 
one  is  the  cephalic  cartilage. 

1.  Tetrabranehia  (Nautilus). 

Nautilus  possesses  only  the  cephalic  cartilage.  This  is  shaped 
somewhat  like  an  X,  with  thick  limbs.  The  oesophagus  runs  up 
between  the  one  pair  of  limbs,  the  other  pair  serving  as  supports  for 
the  funnel  and  as  surfaces  of  attachment  for  its  muscles. 

The  most  important  of  the  muscles  is  the  large  paired  shell 
muscle,  which  corresponds  with  the  columellar  muscle  of  other 
Molluscs.  It  arises  from  the  cephalic  cartilage,  and  runs  on  each 
side  into  the  band  (annulus),  by  which  the  body  of  the  Nautilus  is 
attached  to  the  inner  wall  of  the  body-chamber  (cf.  Fig.  32,  p.  22),  and, 
like  the  band  itself,  is  attached  to  the  shell.  The  muscle  leaves  a 
deep  scar  on  the  shell  (the  lobate  sutural  line).  From  the  lateral 
edges  of  the  cephalic  cartilage,  especially  that  portion  of  it  which 
supports  the  funnel,  a  broad  muscle-band,  the  museulus  collaris,  runs 
forward  on  each  side  embracing  the  nuchal  part  of  the  body.  The 
two  unite  on  the  neck  to  form  the  muscular  nuchal  plate.  The 
ventral  lower  side  of  the  cephalic  cartilage  serves  for  the  attachment 
of  the  musculature  of  the  tentacles. 

2.  Dibranehia. 

The    cartilaginous    skeleton    is    much    more    developed    than    in 

Nautilus,  owing  perhaps,  to  some  ex- 
tent, to  the  atrophy  of  the  shell.  Fins, 
with  their  supporting  cartilages,  for 
example,  are  developed  only  in  those 
forms  with  internal,  degenerated 
shells. 

The  cephalic  cartilage  (Fig.  109)  is  every  - 
Fio.  109.— Cephalic  cartilage  of  Sepia,    where  well  developed.     It  encloses  all  those 
1.  Central  aperture  through  which  the  ceso-   central  portions  of  the  nervous  system  which 
phagus  passes  ;   2,  preorbital  cartilage  ;  3,    are  crowded   round  the  oesophagus,  being  in 
chamber    for    the    eye ;    4,    cartilaginous    ,-,     ?  /•     i     -n          •       i 

auditory  capsule.  the  form  of  a  hollow  circular  capsule  traversed 

by  the  oesophagus.    Processes  of  this  cartilage 

assist  in  supporting  the  eyes,  and  in  conjunction  with  independent,  preorbital 
cartilages  form  a  kind  of  cartilaginous  eye  socket.  A  basibrachial  cartilage  is 
found  at  the  base  of  the  anterior  arms  in  some  Decapoda.  We  have  further  to 


VII 


MOLL  USC A  —MUSCULA  TURE 


127 


mention  the  nuchal  cartilage  and  the  cartilages  for  locking  the  cleft  of  the  mantle 
cavity  (p.  55).  In  the  diaphragm,  i.e.  in  the  posterior  wall  of  the  visceral  dome, 
over  which  the  mantle  depends,  there  is  in  the  Decapoda  a  cartilage  near  the 
funnel,  the  diaphragm  cartilage.  Finally  must  be  mentioned  a  dorsal  cartilage, 
which  is  specially  strongly  developed  in  Sepia.  It  lies,  posteriorly,  on  the  anterior 
border  of  the  mantle,  where  the  latter  pro- 
jects over  the  neck ;  it  bears  the  same 
relation  to  the  nuchal  cartilage  as  does 
the  cartilaginous  projection  on  each  side  of 
the  mantle  to  the  cup-shaped  socket  at  each* 
side  of  the  base  of  the  funnel  or  siphon  (cf. 
Fig.  80). 

In  Sepia  the  dorsal  cartilage  is  continued 
in  the  shape  of  a  cartilaginous  rod  running 
up  on  each  edge  of  the  shell.  The.  inner 
edges  of  these  rods  have  a  groove  into  which 
the  edge  of  the  shell  fits,  and  thus  form  a 
kind  of  fold  round  its  lateral  edges. 

In  the  Octopoda  there  is  a  cartilaginous 
band  on  each  side  in  the  dorsal  integument 
which  may  correspond  with  the  dorsal  carti- 
laginous rods  in  Sepia.  It  is  possible  that 
the  "internal  shell"  of  the  only  Octopod 
in  which  a  shell  is  found,  viz.  Cirrho- 
teuthis,  is  not  in  reality  homologous  with 
the  shell  of  the  Decapoda,  but  corresponds 
with  the  cartilaginous  bands  of  Octopus 
fused  in  the  middle  line. 

The  (basipterygial)  cartilages,  univers- 
ally found  at  the  bases  of  the  fins  in  the 
Decapoda,  complete  the  list. 

With  regard  to  the  musculature 

Of  the  Dibranchia,  that  of  the  mantle,       ^G-  HO-Diagram  of  the  more  important 

,         „  ,      ,  ,  '    parts   of  the   Dibranchiate  musculature. 

the     tins,    and    the    arms    Cannot     be    Body  seen  from  the  left  side.    «,  Ventral ;  d, 

described  in  detail.       AVe    note,  how-  -dorsal;  a,  anterior  ;p,  posterior;  1,  depressor 

ever,   that   the    pallial    musculature   SS^'SSLTfLSSTii 

IS    principally    attached    to    the    shell    laris;  5,  adductor  infundibuli;   6,   shell;  7, 
Or    tO    the    dorsal    Cartilage,    the     fin-    dorsal cartilage ;S,nuchal cartilage;  9, cephalic 

musculature  to  the  fin-cartilages,  and   ZS^X-^-ESSS 

the    brachial    musculature    tO  the  an-    wall  of  the  visceral  dome ;  12,  corresponding 

terior   side  Of   the    Cephalic  Cartilage,    cartilaginous  knob  on  the  inner  wall  of  the 

j  ,  ,,       i-i          i  •  -i          ,-      mantle,  which  fits  into  11 ;  13,  funnel  or  siphon 

and  partly  to  the  basi-brachial  carti-  (illfundibuiuin) ;  14,  diaphragm  cartilage, 
lage  when  such  is  present. 

The  remaining  musculature  can  be  best  explained  with  the  assist- 
ance of  the  accompanying  diagram  (Fig.  110),  which  represents  the 
musculature  of  Enoploteuthis. 

The  strong  paired  depressor  infundibuli  (1)  rises  from  the  shell 
on  each  side  (or  from  the  dorsal  cartilage),  and  runs  downwards  and 
backwards  to  the  base  of  the  funnel  and  to  the  cartilaginous  socket. 
From  it  spring  most  of  the  muscles  of  the  anterior  wall  of  the  funnel. 


128  COMPARATIVE  ANATOMY  CHAP. 

The  retractor  capitis  lateralis  (2),  which  is  also  paired,  rises  from 
the  same  point  as  the  depressor  infundibuli,  runs  into  the  head,  and 
is  attached  to  the  cephalic  cartilage.  The  retractor  eapitis  medianus 
(3),  originally  paired,  but  usually  become  single  by  fusion,  arises  at 
the  posterior  (inner)  side  of  the  shell,  and  also  runs  into  the  head, 
and  is  attached  to  the  cephalic  cartilage. 

In  the  Dibranchia,  the  first  muscles  which  fuse  are  the  two  median  retractors  of 
the  head  (Onychoteuthis),  these  then  fuse  more  completely  with  the  lateral  retractors 
(Ommastrephes,  Sepioteuthis,  Loligo,  Sepiola),  so  that  finally  (Sepia)  the  whole  of  the 
musculature  running  from  the  shell  into  the  head  forms  a  muscular  sheath  open 
posteriorly.  This  sheath  encloses  the  lower  portion  of  the  visceral  cavity,  which  is 
principally  occupied  by  the  digestive  gland  or  liver,  and  thus  forms  a  kind  of 
muscular  hepatic  capsule.  The  posterior  opening  in  this  capsule  may  finally  become 
completely  closed  by  the  depressor  infundibuli,  in  that,  on  the  one  hand,  its 
anterior  edges  fuse  with  the  posterior  and  median  edges  of  the  capsule,  and,  on  the 
other,  it  sends  out  numerous  muscles  to  the  diaphragm,  forming  the  diaphragma 
musculare. 

The  muscular  hepatic  capsule,  i.e.  all  the  muscles  forming  it,  the 
retractors  of  the  head  and  the  depressors  of  the  siphon,  may  without 
doubt  be  accepted  as  the  homologue  of  the  columellar  muscle  of  other 
Molluscs.  Like  the  latter,  they  run  down  from  the  shell  or  its  vicinity 
into  the  head  and  foot  (represented  by  the  siphon). 

The  adductors  of  the  funnel  (5)  have  still  to  be  mentioned. 
They  rise  from  the  cephalic  cartilage  and  run  upwards  and  backwards 
to  the  funnel.  Finally,  the  collaris  (4)  is  a  strong  muscle  which  runs 
forwards  right  and  left  from  the  wall  of  the  funnel,  and  is  attached 
to  the  lateral  edges  of  the  nuchal  cartilage.  In  the  Octopoda  and  Sepiola, 
where  a  pallio- nuchal  concrescence  (cf.  pp.  54,  55)  has  rendered  a 
nuchal  locking  cartilage  unnecessary,  the  collaris  passes  uninterruptedly 
over  the  neck  like  a  saddle,  forming  a  closed  circle  round  the  nuchal 
portion  of  the  body. 


XIII.  The  Nervous  System. 

(As  a  general  introduction  to  this  section  the  reader  may  be  referred  to  pp.  27,  28.) 

A.  Amphineura. 

The  nervous  system  of  the  Amphineura  is  very  significant  from 
the  point  of  view  of  the  comparative  anatomist.  Its  most  important 
peculiarities  may  be  briefly  described  as  follows  :— 

1.  The  ganglionie  cells  are  either  not  at  all  or  not  exclusively 
localised  in  definite  ganglia. 

2.  Four  nerve  cords  run  through  the  body  from  before  backward. 
These  contain  not  only  nerve  fibres,  but  ganglion  cells  distributed  along 
their  whole  length.     They  might  suitably  be  called  medullary  cords, 
and  must  be  considered  as  belonging  to  the  central  nervous  system. 


vii  MOLLUSCA—THE  NERVOUS  SYSTEM  129 

One  pair  of  these  cords  run  along  the  body  laterally,  these  are  the 
lateral  or  pleurovisceral  cords ;  the  second  lie  ventrally,  and  are  the 
pedal  cords.  The  visceral  and  the  pedal  cords  of  each  side  unite 
anteriorly,  and  when  so  united  become  connected  with  those  on  the 
opposite  side  by  a  transverse  commissure,  which  runs  in  front  of  and 
over  the  oesophagus  and  contains  ganglion  cells ;  this  is  the  cerebral 
or  upper  half  of  the  cesophageal  ring.  The  pleurovisceral  cords  unite 
posteriorly  above  the  rectum,  forming  a  visceral  loop.  The  pedal 
cords  are  connected  both  inter  se  and  with  the  pleurovisceral  cords  by 
anastomoses,  so  that  the  whole  nervous  system  strikingly  recalls  the 
ladder  nervous  system  of  the  Turbellaria  and  Trematoda. 

a.  Chitonidae  (Figs.  Ill  and  51,  p.  40). — The  scheme  just  given 
is  founded  upon  the  nervous  system  of  Chiton.  The  typical  gQng|ja  ^f 
the  central  nervous  system  of  the  Mollusca  are  not  yet,  in  Chiton,  found 
as  distinct  ganglia  united  by  means  of  commissures  and  connectives, 
but  the  ganglion  cells  are  equally  distributed  along  the  commissures 
and  connectives,  an  arrangement  which  is  probably  primitive.  The 
upper  cesophageal  ring  thus  corresponds  with  the  cerebral  ganglia  and 
the  commissures  connecting  them,  and  in  the  same  way  the  pedal 
cords  contain  the  whole  central  portion  of  the  pedal  nervous  system, 
and  the  pleurovisceral  cords  the  central  portion  of  the  visceral,  pallial, 
and  branchial  nervous  systems.  Only  in  one  single  species  of  Chiton 
(C.  rubicundus)  two  distinct  (cerebral)  ganglia  occur  near  each  other 
in  the  middle  line  in  the  upper  half  of  the  cesophageal  ring. 

Looking  more  closely  at  the  nervous  system  of  the  Chitonidae,  we  have  to 
observe  :  (1)  the  arrangement  of  the  cesophageal  ring  and  the  medullary  cords  ;  (2) 
the  peripheral  ganglia ;  (3)  the  nerves  of  the  ladder-like  nervous  system  ;  (4)  the  nerves 
running  from  the  central  nervous  system  (cesophageal  ring  and  medullary  cords). 

1.  Form  and  arrangement  of  the  central  nervous  system. — The  visceral  cords 
run  back  one  on  each  side  in  the  lateral  body  wall  above  the  branchial  groove  ;  these 
two  cords  unite  above  the  anus.     The  pedal  cords  run  in  the  dorsal  part  of  the 
pedal  musculature  somewhat  near  one  another,  from  before  backward,  to  end  without 
uniting  where  the  rectum  commences.     The  cesophageal  ring  consists,  in  the  first 
place,  of  the  semicircular  portion  mentioned  above,  which,  on  account  of  the  peculiar 
shape  of  the  body  of  the  Chiton,  lies  in  the  same  plane  as  the  visceral  cords.    Poste- 
riorly, each  limb  of  this  semicircle  divides  up  into  the  pedal  and  visceral  cords.     At 
the  point  where  the  pedal  cord  rises  from  the  ring,  a  cord  with  a  thickened  base 
separates  from  it  and  runs  inwards  ;  this,  uniting  below  the  mouth  with  a  similar 
cord  from  the  other  side,  forms  the  lower  half  of  the  cesophageal  ring.     The  upper 
and  lower  halves  together  form  the  closed  cesophageal  ring. 

2.  Besides  this  central  nervous  system  there  are  peripheral  ganglia  connected 
with  it  by  nerve  cords  consisting  only  of  nerve  fibres. 

(a)  The  buccal  ganglia  together  form  a  horseshoe-shaped  ganglionic  mass  below 
the  oesophagus,  which  mass  is  connected  on  each  side  by  the  cerebrobuccal  connective 
with  the  thickened  portion  of  the  lower  cesophageal  ring.     The  buccal  ganglionic 
mass  in  C.  rubicundus  divides  into  two  paired  ganglia  and  one  unpaired  ganglion 
joined  to  one  another  by  connectives.     The  buccal  ganglia  innervate  the  oesophagus 
as  far  as  the  stomach  and  also  the  oral  aperture. 

(b)  On  each  side,  from  the  lower  half  of  the  cesophageal  ring,  somewhat  further 
VOL.  II  K 


130  COMPARATIVE  ANATOMY  CHAP. 

in  than  the  buccal  connective,  a  nerve  (the  subradular  connective)  rises  and  runs 

n  ^ — • , 

,0 


FIG.  111.— Diagram  of  the  nervous  system  of  Chiton  siculus  (after  Bela  Haller).  The  mantle 
removed  on  the  right  side.  In  the  centre  and  to  the  left  the  upper  part  of  the  foot  removed,  to 
expose  the  pedal  nervous  system.  F,  Foot ;  K,  last  gill;  A,  anus  ;  0,  upper,  U,  lower  half  of  the 
resophageal  ring  ;  1,  '2,  nerves  of  the  cesophageal  ring  ;  c,  connective  to  the  anterior  visceral  ganglia  ; 
p,  connective  to  the  ganglia  of  the  subradular  organ  n  (above  on  the  left) ;  Es,  pleurovisceral  and 
pedal  cords ;  mn,  gastric  nerve  ;  So,  point  of  attachment  of  the  sphincter  oris  ;  n  (below  on  the 
right),  ni,  tt2»  nephridial  nerves  ;  m,  pallial  nerves  ;  p  (to  the  right  below),  cardial  nerves ;  v,  a 
dorsal  nerve  of  one  of  the  pedal  cords.  The  commissures  between  the  pedal  cords  are  seen,  and 
the  nerves  running  outwards  from  the  latter.  > 

"> 
forward  and  inward  to  the  subradular  ganglion.     This  ganglion  lies  in  the  sub- 


vii  MOLLUSCA—THE  NERVOUS  SYSTEM  131 

radular  organ  which  is  situated  on  the  floor  of  the  buccal  cavity.     The  two  sub- 
radular  ganglia  are  united  by  a  short  commissure. 

(c)  Two  small  gastric  ganglia,  connected  by  a  fine  commissure,  lie  at  the  anterior 
end  of  the  stomach,  and  are  joined  on  each  side  to  the  anterior  end  of  the  visceral 
cord  by  a  long  connective. 

3.  The  nerves  of  the  ladder-like  nervous  system. — The  two  pedal  cords  are  con- 
nected by  anastomosing  commissures  along  their  whole  length,  but  no  nerves  are 
given  off  by  these  commissures  to  the  pedal  musculature.     In  Chiton  rubicundus  the 
visceral  and  pedal  cords  are  united   by  numerous  connectives,   which,   in  other 
Chitonidae,  appear  either  to  be  wanting  or  to  be  reduced  to  one  single  anterior  or 
posterior  anastomosis. 

4.  The  nerves  running  from  the  central  nervous  system : — 

(a)  Nerves  of  the  oesophageal  ring. — Numerous  nerves  rise  from  the  upper  or 
cerebral  portion  of  the  oesophageal  ring  to  innervate  the  cephalic  part  of  the  mantle, 
the  snout,  the  upper  and  lower  lips,  the  gustatory  buds  on  the  lower  wall  of  the 
oral  cavity,  and  the  musculature  of  the  buccal  mass.  The  lower  portion  of  the 
cesophageal  ring,  besides  the  connectives  to  the  buccal  and  subradular  ganglia, 
sends  off  from  its  median  portion  another  pair  of  nerves,  which  run  along  the  base 
of  the  buccal  cavity. 

(&)  Nerves  of  the  pleurovisceral  cords. — Each  of  the  pleurovisceral  cords 
gives  off  two  nerves  to  each  gill.  Besides  these  they  send  many  nerves  to  the 
mantle,  and,  posteriorly,  nerves  which  enter  the  body  cavity,  probably  running  to 
the  kidneys  and  the  heart. 

(c)  Nerves  of  the  pedal  cords. — The  pedal  cords  give  off  011  each  side  seven  or 
eight  nerves  outwards  to  the  lateral  musculature  of  the  body,  and  specially  numerous 
nerves  run  down  from  it  to  the  pedal  musculature  (inner  and  outer  pedal  nerves). 
These  pedal  nerves  are  richly  branched,  and,  anastomosing  with  one  another,  form 
a  complete  neural  network  in  the  foot. 

b.  Solenogastres. — The  central  nervous  system  of  the  Solenogastres 
differs  from  that  of  the  Chitonidce  principally  in  a  tendency  to  form 
distinct  ganglia;  the  pedal  and  pleuroviseeral  cords,  nevertheless, 
still  retain  their  outer  coating  of  ganglion  cells  along  their  whole 
length.  Fig.  112  is  a  diagrammatic  representation  of  the  structure  of 
the  nervous  system  of  Proneomenia  Sluiteri.  The  fused  cerebral  ganglia 
in  the  middle  line  are  very  large.  On  both  the  pleurovisceral  and 
the  pedal  cords  ganglionic  swellings  can  be  distinguished  :  (1)  three 
pairs  of  posterior  visceral  ganglia ;  (2)  two  anterior  pedal  ganglia. 

The  posterior  visceral  ganglia  are  connected  by  cords,  which  run 
transversely  over  the  rectum  and  correspond,  to  some  extent  at  least, 
with  the  loop  by  which  the  two  visceral  strands  in  Chiton  are  united. 

The  two  anterior  pedal  ganglia  are  connected  by  a  strong  trans- 
verse commissure,  which  may  correspond  with  the  ventral  half  of  the 
03sophageal  ring  of  Chiton. 

Further,  the  pleurovisceral  cords  are  joined  with  the  pedal  cords, 
and  the  latter  are  also  connected  inter  se  by  transverse  connections 
along  their  whole  length.  The  pleurovisceral  cords  likewise  are  con- 
nected by  arched  transverse  commissures.1 

1  These  connectives  and  commissures,  however,  do  not  seem  to  run  uninterruptedly 
from  one  cord  to  the  other. 


132  COMPARATIVE  ANATOMY  CHAP. 

On  each  side  of  the  cerebral  ganglion,  a  nerve  rises,  which  runs  to 
a  ganglion  below  the  pharynx  and  behind  the  radular  sheath,  this  is 
the  sublingual  ganglion ;  this  latter  is  united  with  the  corresponding 
ganglion  on  the  other  side  by  a  short  transverse  commissure.  These 
sublingual  ganglia  probably  correspond  with  the  buccal  ganglia  of 
Chiton. 

Dondersia  is  specially  noteworthy  because  distinct  ganglionic  swellings  occur  at 
regular  intervals  along  the  pedal  cords  ;  this  is  particularly  marked  in  the  anterior 
part  of  the  body.  The  equally  regularly  repeated  transverse  commissures  joining  the 
pedal  cords,  and  the  connectives  between  the  pedal  and  visceral  cords,  start  from 
these  distinct  ganglia. 

In  Lepidomenia  hystrix,  one  ganglion  occurs  posteriorly  and  one  anteriorly  in 
each  longitudinal  trunk  (whether  pleurovisceral  or  pedal),  and  each  is  connected 
with  a  similar  ganglion  of  the  opposite  side  by  a  transverse  commissure. 

In  Neomenia  and  Chcetoderma,  no  connectives  between  the  visceral  and  pedal 


FIG.  112.— Nervous  system  of  Proneomenia  Sluiteri  (original  drawing  by  J.  Heuscher). 
1,  Cerebral  ganglia ;  2,  pleurovisceral  cords  ;  3,  4,  5,  posterior  ganglia  of  the  pleurovisceral  cords  ; 
6,  sublingual  ganglia;  7,  anterior  pedal  ganglia;  8,  right  pedal  cord;  9,  left  pedal  cord;  10,  11, 
strong  posterior  commissures  between  the  pedal  cords ;  12,  anterior  pedal  commissure ;  13,  sub- 
lingual  commissure. 

cords  have  been  observed,  and,  so  far  as  is  at  present  known,  in  Chcetoderma,  the 
commissures  between  the  pedal  cords  are  also  wanting.  Further,  in  Cha-toderma, 
the  visceral  and  pedal  cords  of  each  side  unite  together  posteriorly  to  form  one 
single  cord,  which  becomes  connected  with  the  similar  cord  on  the  other  side  by  a 
transverse  cord  which  runs  over  the  cloaca.1 


B.  Gastropoda. 

The  nervous  system  of  the  Gastropoda  is  of  great  interest  to  the 
comparative  anatomist  on  account  of  the  crossing  of  the  pleurovisceral 
connectives  in  the  Prosobranchia,  which  will  be  further  described  in  this 
section. 

The  nervous  system  of  this  class  consists  typically  of  those  parts 
which  we  have  already  mentioned  in  our  scheme  of  the  organisation  of 
the  Mollusca,  viz.  : — 

1  For  further  details  see  Simrotli's  new  edition  of  Bronn's  Klassen  und  Ordnungen 
des  Thier-reiches,  vol.  iii. 


vii  MOLLUSCA—THE  NERVOUS  SYSTEM  133 

1.  Two  cerebral    ganglia  near  or  above  the  oesophagus,  which 
are  connected  by  a  cerebral  commissure. 

2.  Two  pedal  ganglia  below  the  oesophagus,  connected  with  each 
other  by  a  pedal  commissure,  and  with  the  cerebral  ganglia  by  two 
cerebropedal  connectives. 

The  cerebral  and  pedal  ganglia  with  the  commissures  and  con- 
nectives belonging  to  them  form  a  ring  encircling  the  oesophagus, 
which  may  be  compared  with  the  cesophageal  ring  of  the  Annulata 
and  Arthropoda. 

3.  Two  pleural  or  pallial   ganglia  (between  the  cerebral    and 
pedal  ganglia),  which  are  connected  with  the  cerebral  ganglia  by  two 
cerebropleural,  and  with  the  pedal  ganglia  by  two  pleuropedal  con- 
nectives. 

4.  A  simple  or  complex  visceral  ganglion   lying  below  the  in- 
testine, united  to  the  pleural  ganglia  by   two  pleuroviseeral  con- 
nectives. 

5.  A  ganglion,  which  may  be  called  parietal,  almost  always  occurs 
in  the  course  of  each  pleuroviseeral  connective.     The  parietal  ganglion 
divides  the  connective  into  two  parts,  an  anterior  pleuroparietal  and 
a  posterior  viseeroparietal  connective. 

The  cerebral,  pedal,  and  pleural  ganglia  are  (with  unimportant 
exceptions)  always  arranged  symmetrically  to  the  median  plane  in  all 
Gastropoda.  The  pleuroviseeral  connectives  and  their  ganglia,  how- 
ever, are  only  found  in  such  a  position  in  some  Gastropoda.  In  fact, 
only  in  the  Opisthobranchia  (including  the  Pteropoda  but  excepting  A  ctceon) 
and  the  Pulmonata  are  they  symmetrical,  in  the  sense  that  the  right 
connective  and  its  ganglion  lie  entirely  on  the  right,  and  the  left 
connective  and  its  ganglion  entirely  on  the  left  side  of  the  body. 
The  Opisthobranchia  and  Pulmonata  are  therefore  called  euthyneurous 
Gastropoda. 

In  the  Prosolyranchia  and  Actceon,  the  pleuroviseeral  connectives  are 
asymmetrical,  inasmuch  as  they  cross  one  another,  the  connective 
springing  from  the  right  pleural  ganglion  running  over  the  intestine  to 
the  left  before  joining  the  visceral  ganglion,  while  the  connective 
from  the  left  pleural  ganglion  runs  under  the  intestine  to  the  right  side 
of  the  body.  In  consequence  of  this  crossing,  the  parietal  ganglion  of 
the  connective  which  springs  from  the  right  pleural  ganglion  becomes 
the  supraintestinal  ganglion,  which  lies  on  the  left  side,  and  the 
parietal  ganglion  of  the  connective  springing  from  the  left  pleural 
ganglion  becomes  the  infra-intestinal  ganglion  which  lies  on  the  right 
side.  The Prosobmnchia  suidAdceon  are  thus  streptoneurous  Gastropoda. 

The  Areas  of  Innervation  of  the  various  Ganglia. 

1.  The  cerebral  ganglia  innervate  the  eyes,  the  auditory  organs, 
the  tentacles,  the  snout  or  proboscis,  the  lips,  the  motor  muscles  of  the 
proboscis  and  buccal  mass,  and  the  body  walls  lying  at  the  base  of  the 


134  COMPARATIVE  ANATOMY  CHAP. 

snout.  Even  when  the  auditory  organs  are  found  in  close  proximity 
to  the  pedal  ganglia,  or  in  close  contact  with  them,  they  receive  their 
nerves  from  the  cerebral  and  not  from  the  pedal  ganglia. 

2.  The  pedal  ganglia  supply  nerves  to  the  musculature  of  the  foot, 
and  occasionally  to  the  columellar  muscle  also  (Patella). 

3.  The  pleural    ganglia  send  nerves   chiefly  to    the    mantle,  the 
columellar  muscle,  and  the  body  walls  lying  behind  the  head. 

4.  The  parietal    ganglia    innervate  the  ctenidia  and  osphradium, 
and  also  send  some  nerves  to  the  mantle. 

5.  The  visceral  ganglia  supply  nerves  to  the  viscera.     The  con- 
nectives and  commissures  also  may  give  off  nerves  which  belong  to  the 
areas  innervated  by  the  neighbouring  ganglia. 

6.  The  buccal  ganglia,  which  will  be  described  below,  innervate 
the  muscles  of  the  pharynx,  the  salivary  glands,  the  oesophagus,  the 
anterior  aorta,  etc. 

A  comparison  of  the  typical  nervous  system  of  the  Gastropoda  with  that  of  the 
Amphineura  reveals  the  following  homologies  : — 

1.  The  cerebral  ganglia  of  the  Gastropoda  correspond  with  the  oesophageal  ring 
of  Chiton,  with  the  exception  of  the  central  portion  of  its  lower  half ;  and  further 
with  the  cerebral  ganglia  of  the  Solenogastres. 

2.  The  pedal  ganglia  of  the  Gastropoda  answer  to  the  pedal  cords  in  the  Am- 
phineura, concentrated  each  into  a  single  ganglion.     The  arrangement  in  the  Dioto- 
cardia,  which  are  the  more  primitive  Prosobranchia,  is  very  interesting  in  this  con- 
nection ;  in  the  Diotocardia  the  pedal  ganglia  are  continued  posteriorly  as  two 
true  pedal  cords,  which,  like  those  of  the  Amphineura,  are  connected  by  transverse 
commissures. 

It  is  more  difficult  to  compare  the  pleural,  parietal,  and  visceral  ganglia  of  the 
Gastropoda  with  nerves  found  in  the  Amphineura.  The  most  satisfactory  view 
seems  to  be  that  this  whole  complex  of  ganglia,  together  with  its  connectives,  corre- 
sponds with  the  pleurovisceral  cords  of  Chiton.  The  areas  of  innervation  coincide, 
these  being  the  mantle,  ctenidia,  osphradia  (Chiton?),  and  viscera. 

3.  If  this  last  assumption  is  correct,  the  pleural  ganglion  must  be  supposed  to 
have  arisen  by  the  concentration  into  one  ganglion  of  that  part  of  the  pleurovisceral 
cord  of  Chiton  which  contains  the  pallial  ganglionic  cells,  this  concentration  having 
taken  place  at  the  anterior  end  of  the  cord,  where  it  leaves  the  cesophageal  ring.     If, 
then,  the  two  component  portions  of  each  side  of  the  ring,  the  cerebropedal  and  the 
pleural,  move  further  apart,  and  at  the  same  time  the  cerebral  and  pedal  ganglia  of  the 
ring  become  more  individualised  as  ganglia,  a  double  cerebropedal  connective  conies 
into  existence  on  each  side.    One  of  these  connectives  shows  no  ganglion  in  its  course, 
and  is  the  true  cerebropedal  connective  of  the  Gastropoda.     The  second,  however,  has 
the  pleural  ganglion  in  its  course,  and  from  this  latter  spring  the  visceral  cords  ;  this 
second  connective  is  thus  divided  into  a  cerebropleural  and  a  pleuropedal  connective. 

4.  Chiton  has  numerous  gills  on  each  side,  each  of  which  receives  two  nerves 
from  the  pleurovisceral  cord  near  it.     The  Gastropoda  have  at  the  most  two  gills, 
one  on  the  right  and  one  on  the  left.     In  correspondence  with  this  reduction,  the 
ganglionic  cells  of  the  pleurovisceral  cords  belonging  to  the  branchial  nerves  of 
Chiton  have  become  concentrated  on  each  side  into  a  single  ganglion  belonging  to  the 
single  gill.     The  parietal  ganglion  is  thus  accounted  for.      That  portion  of  each 
pleurovisceral  cord  which  lies  between  the  pleural  and  the  parietal  ganglia  becomes 
the  pleuroparietal  connective,  which  consists  of  fibres  only  without  ganglion  cells. 


vii  MOLLUSGA—THE  NERVOUS  SYSTEM  135 

5.  There  is'no  nerve  in  Chiton  homologous  with  the  visceral  ganglion  or  ganglia  of 
the  Gastropoda ;  this  is  the  chief  difficulty  in  the  comparison  of  the  two  nervous 
systems.  In  the  Amphineura,  the  pleuro visceral  cords  unite  above  the  intestine  ; 
in  all  other  Molluscs  the  point  of  junction  (which  is  the  visceral  ganglion)  lies  below 
the  intestine. 

In  Proneomenia  the  posterior  commissures  between  the  pleurovisceral  cords  are 
merely  a  more  strongly  developed  part  of  a  general  commissural  system. 


Origin  of  the  Crossing-  of  the  Pleuroviseeral  Connective 
(Chiastoneury)  (Figs.  113-116). 

Several  attempts  have  been  made  to  explain  the  peculiar  crossing 
of  these  connectives  in  the  Prosobranchia.  The  one  here  given  is  in  a 
high  degree  probable  if  not  altogether  satisfactory. 

We  must  start  with  a  supposed  racial  form  which  was  perfectly 
symmetrical,  even  in  its  nervous  system,  and  possessed  an  organisation 
somewhat  like  that  of  our  hypothetical  primitive  Mollusc  (p.  26). 
Such  an  organisation  agrees  in  most  important  points  with  that  of  the 
extant  Chitonidce  ;  only  one  gill,  however,  was  present  on  each  side. 

Further,  the  parietal  ganglia  innervated  the  gills  and  the  osphradia, 
and  were  thus  closely  connected  with  these  organs. 

The  racial  form  of  the  Gastropoda  may  have  been  surrounded  by 
a  mantle  border  which  widened  posteriorly,  i.e.  covered  a  somewhat 
deeper  mantle  cavity  which  contained  the  pallial  complex,  viz.  the 
median  anus,  to  the  right  and  left  of  which  were  the  ctenidia  and 
osphradia,  and  between  the  ctenidium  and  anus  on  each  side  the 
nephridial  aperture. 

If  we  suppose  this  pallial  complex  to  have  changed  its  position, 
shifting  gradually  forward  along  the  right  mantle  furrow,  each  cteni- 
dium would  drag  along  with  it  its  parietal  ganglion.  The  heart  and 
its  auricles  which  are  connected  with  the  ctenidium  would  also  become 
shifted. 

As  long  as  the  pallial  complex  had  not  moved  far  forward  to  the 
right,  the  pleurovisceral  connectives  would  not  cross,  but  would  only 
be  shifted  to  the  right  (Fig.  114).  We  find  the  Tectibranchia  among 
the  Opislhobranchia  apparently  at  this  stage,  the  only  difference  being 
that  they  have  already  lost  the  original  left  ctenidium  and  also  the 
original  left  auricle  (Fig.  43,  p.  33). 

If  the  pallial  organs  are  still  further  shifted  forward  along  the 
mantle  furrow  (Figs.  115,  116)  till  they  come  to  lie  quite  ante- 
riorly, and  once  more  symmetrically,  above  and  behind  the  neck,  the 
original  left  ctenidium  comes  to  lie  on  the  right,  and  the  original  right 
ctenidium  on  the  left  in  the  anteriorly  placed  mantle  cavity.  The 
original  right  ctenidium  has,  however,  dragged  its  parietal  ganglion 
over  the  intestine  to  the  left  side,  and  the  latter  becomes  the 
supraintestinal  ganglion.  The  original  left  ctenidium,  on  the 
contrary,  has  dragged  its  ganglion  below  the  intestine  to  the  right 


136 


COMPARATIVE  ANATOMY 


CHAP. 


side,  and  this  ganglion  becomes  the  infraintestinal  ganglion.  The 
pleurovisceral  connectives,  in  which  these  ganglia  lie,  now  cross  and 
give  rise  to  the  condition  called  chiastoneury.  The  visceral 


—  uvct 


FIGS.  113,  114,  115, 116.— Diagrams  to  illustrate  the  shifting  forward  of  the  pallial  complex 
along  the  right  side  of  the  body  and  the  development  of  chiastoneury.  p,  Mouth  ;  ulc,  ulpl, 
ulp,  original  left  cerebral-,  pletiral-,  and  pedal-ganglion  ;  ulpa,  urpa,  original  left  and  original  right 
parietal  ganglion  ;  ula,  original  left  auricle  ;  M.OS,  uros,  original  left  and  original  right  osphradium  ; 
•ulct,  urct,  original  left  and  original  right  ctenidiuni ;  mb,  base  of  the  mantle  ;  mr,  edge  of  the  same ; 
m,  mantle  cavity  ;  v,  visceral  ganglion  ;  ve,  ventricle  ;  a,  anus. 

ganglion    in  which    these   connectives    terminate    posteriorly    lies    as 
before  under  the  intestine. 

It  is  unnecessary  to  show  in  detail  how  this  displacement  also  affects 
the  heart  and  its  auricles,  the  osphradia,  and  the  nephridial  apertures. 


vii  MOLLUSCA—THE  NERVOUS  SYSTEM  137 

Although  chiastoneury  may  be  satisfactorily  explained  by  this 
theory  of  displacement,  the  cause  of  the  displacement  itself  has  still  to 
be  sought  (cf.  §  xiv.  p.  149). 

Special  Remarks  on  the  Nervous  System  of  the  Gastropoda. 

I.  Prosobranchia.  («)  Diotocardia. — These  are  the  most  primitive  Gastropoda. 
The  ganglia  are  not  yet  very  distinct,  thus  recalling  the  Amphineara.  The  cerebral 
ganglia  are  connected  by  two  long  commissures,  the  cerebral  commissure  running 
forward  over  the  pharynx,  and  the  labial  commissure  running  under  the  oesophagus. 
The  indistinctly  separated  buccal"gaigiia  together  form  a  horseshoe-shaped  figure, 
and  are  united  on  each  side  by  a  connective  with  the  thickened  root  of  the  labial 
commissure. 

The  pleural  ganglia  lie  close  to  the  pedal  ganglia,  so  that  no  distinct  pleuro- 
pedal  connectives  can  be  distinguished.  The  pedal  commissure  is  very  short,  and 
contains  ganglion  cells.  From  each  pedal  ganglion,  a  long  pedal  cord  runs  back  into 
the  foot ;  these  two  pedal  cords  contain  ganglion  cells  along  their  whole  length,  and 
are  connected  by  transverse  commissures.  These  cords  and  commissures  thus  exhibit 
the  same  arrangement  as  in  the  Amphineura.  The  pedal  cords  innervate  the  mus- 
culature of  the  foot  and  the  epipodium.  There  is  only  one  indistinct  visceral 
ganglion,  which  is  joined  to  the  pleural  ganglia  by  two  pleuro visceral  connectives, 
crossed  in  the  typical  way. 

In  Fissurella  only  does  a  ganglion  occur  on  the  supraintestinal  pleuro  visceral 
connective.  In  no  other  Diotocardian  is  there  a  ganglion  at  the  point  of  departure 
of  the  strong  branchial  nerve  from  the  pleurovisceral  connective  ;  this  nerve,  how- 
ever, forms  the  branchial  ganglion  just  below  the  osphradium  at  -the  base  of  the 
gill.  Where  a  ctenidiuin,  or  merely  an  osphradium,  is  found  on  each  side,  there  is  a 
branchial  ganglion  close  to  it  ;  where  only  the  left  (ur)  gill  is  retained  (Turbinidce, 
Trochidfc),  only  the  left  branchial  ganglion  is  found.  Since,  as  a  rule,  the  parietal 
ganglia  are  wanting  in  the  Diotocardia,  and  the  branchial  ganglia  in  the  Monotocardia, 
the  branchial  ganglia  of  the  Diotocardia  have  been  considered,  with  much  prob- 
ability, as  intestinal  ganglia,  which  have  shifted  away  from  the  pleurovisceral  connec- 
tives and  towards  the  bases  of  the  gills.  As,  however,  Fissurella  possesses  both  a 
supraintestinal  and  a  left  branchial  ganglion,  it  would  be  necessary  to  assume  that 
an  originally  single  ganglion  had  here  become  divided  into  two. 

The  symmetrical  pallial  nerve  is  always  connected  by  a  pallia!  anastomosis 
with  the  asymmetrical  pallial  nerves  on  the  same  side  of  the  body.  The  symmetri- 
cal pallial  nerve  rises  out  of  the  pleural  ganglion,  the  asymmetrical  nerves  out  of 
the  parietal  ganglion,  or  the  pleuroparietal  connective. 

The  nervous  system  of  the  Neritidcf  and  Helicinidce  are  peculiar,  in  that  the  supra- 
intestinal  pleurovisceral  connective  and  its  corresponding  ganglion  are  wanting. 

Docoglossa. — The  only  essential  difference  between  the  nervous  system  of  Patella 
(Fig.  117)  and  the  typical  system  of  other  Diotocardia  lies  in  the  fact  that  the 
pleural  and  pedal  ganglia  are  joined  by  a  distinct  pleuropedal  connective. 

(6)  Monotocardia  (Fig.  118). — The  parietal  ganglia  are  always  present.  The 
cerebral  commissure  is  short,  and  lies  behind  the  pharynx.  The  labial  commissure 
is  wanting  (except  in  the  Paludinidce  and  Ampullaridce).  The  pedal  cords  and 
transverse  commissures  are  wanting  (except  in  the  Architcenioglossa:  Paludinidce, 
Cydophoridce,  Cyprccidcv}.  The  number  of  visceral  ganglia  varies  from  one  to 
three. 

The  progressive  development  of  so-called  Zygoneury  is  noteworthy.  In  the 
Diotocardia,  a  pallial  anastomosis  exists  between  the  symmetrical  and  asymmetrical 


138 


COMPARATIVE  ANATOMY 


CHAP. 


pallial  nerves  on  eacli  side.  If  this  anastomosis  were  to  shift  along  the  two  pallial 
nerves  of  one  side  to  their  places  of  origin,  i.e.  the  ganglia  from  which  they  spring, 
it  would  become  a  pallial  connective  uniting  the  pleural  and  parietal  ganglia  of 
the  same  side  of  the  body.  There  would  thus  arise  a  new  accessory  pleurointestinal 

connective,  which  would  be  'symmetrical 
and  not  twisted,  and  thus  unlike  the  asym- 
metrical twisted  connective  already  existing. 
Zygoneury  thus  depends  on  the  development 
of  such  a  pallial  connective.  In  the  large 
majority  of  cases  in  which  it  occurs  it  takes 
place  on  the  right  side  (a  few  fiostrifcra, 
viz.  some  of  the  Cerithiidce,  Ampullariidce, 
Turitellidce,  Xenophoridce,  Struthiolar  ii<l<i  , 
Chenopidce,  Strombidce,  Calyptrceidce,  and  in 
all  Probosddifera  siphonostomata  and  all 
Stenoglossa}.  Less  frequently,  zygoneury 
takes  place  on  the  left  side  (Ampullariidce, 
a  few  Crepidulidcc,  Naticidce,  Lamellariidcc 
Cyprceidcc}.  In  other  Prosobranchia  there 
is  only  a  pallial  anastomosis  on  each  side,  as 
in  the  Diotocardia  ;  the  nervous  system  is 
then  called  dialyneurous. 

The  progressive  concentration  of  the 
central  nervous  system  of  the  Monotocardia, 
which  keeps  pace  with  the  development  of 
zygoneury,  must  be  emphasised.  The  con- 
nectives uniting  the  various  ganglia  con- 
tinually shorten,  so  that  at  last  anteriorly 
on  the  oesophagus  there  is  a  collection  of 
ganglia  ;  these  are  the  cerebral,  pleural, 
pedal,  infraintestinal,  and  supraintestinal 
ganglia,  all  lying  close  together,  to  which 
must  be  added  the  small  buccal  ganglia. 
Only  the  visceral  ganglia  remain  far  back 
in  the  visceral  dome. 

In  Natica,  where  the  anterior  part  of  the 
foot  is  strongly  developed,  and  is  bent  back 
over  the  head  (Fig.  98),  a  propedal  ganglion  be- 
FIG.    117.—  Nervous   System  of  Patella    comes  differentiated  from  the  pedal  ganglion. 
(adapted    from    figures    by    Pelseneer    and          The  nervous  system  of  the  Heteropoda 
Bouvier).    1,  Cerebral  ganglion  ;  2,  cerebral    requires  fresh  investigation.     So  far  as  we 

fc  th       certainl     haye  crossed 

J  /       . 

visceral  connectives,  and  are  therefore  Proso- 

branchia,  and,  as  the  rest  of  their  organisa- 
tion    shows,    Monotocardia.      The    cerebral 

^  and  the  pedal  ganglia  (pleuropedal 
&     6 
ganglia  ?)  are  far  apart,  so  that  the  cerebro- 

pedal  connectives  are  very  long.1 
II.  Opisthobranchia.—  The  nervous  system  of  this  order,  in  which  the  typical 
Gastropodan  ganglia  are  developed,  is  further  characterised  :  (1)  by  the  absence  of 


commissure  ;  3,  labial  ganglion  ;  4,  buccal  gan- 
glion  ;  5,  cerebropleural  connective  ;  6,  cerebro- 
pedal  connective;  7,  nervus  acusticus;  8, 
auditory  vesicle  ;  9,  pleural  ganglion  ;  10,  pedal 
commissure  ;  11,  right,  12,  left  osphradium  ; 
13  visceral  ganglion  ;  14,  supraintestinal  gan- 
glion  ;  15,  pedal  cords  ;  16,  indication  of  an 
infraintestinal  ganglion. 


1  Cf.    Pelseneer's  Introduction  d  V  etude  des  Mollusques,  8vo,  Bruxelles,  1894,  pp. 
104,  105. 


VII 


MOLLUSCA—THE  NERVOUS  SYSTEM 


139 


chiastoneury,  i.e.  the  pleurovisceral  connectives  do  not  cross  (except  in  Acttcon)  ;  and 
(2)  by  a  marked  tendency  to  concentration  of  the  ganglia  around  the  posterior  end 
of  the  pharynx. 

(a)  Tectibranchia.— As  a  rule  only  the  right  parietal  ganglion  is  found  (in  Actceon 
the  left  is  also  present).  A  nerve  rises  from  it  which  innervates  the  ctenidium,  the 
osphradium,  and  the  mantle,  and  forms  a  branchial  ganglion  at  the  base  of  the  gill. 
A  delicate  lower  cerebral  commissure  is  often  found,  which  runs  along  the  pedal 


FII;.  US.— Nervous  System  of  Cyclostoma  elegans  (after  Lacaze-Duthiers).  1.  Tentacular 
nerve ;  2,  eye ;  3,  cerebral  ganglion  ;  4,  pe<lal  ganglion  ;  5,  infraintestinal  ganglion  ;  6,  visceral 
ganglion  ;  7,  osphradium  ;  8,  supraintestiiial  ganglion  ;  9,  auditory  vesicle  ;  10,  pleiiral  ganglion. 

commissure  below  the  pharynx,  and  may  be  compared  with  the  labial  commissure 
of  the  Diotocardia. 

As  types  of  the  Tectibrancltia  we  may  take  Bull  a  as  representative  of  the 
Ccphalaspida:,  and  Aplysla  as  representative  of  the  Anaspidcc  (Aplysiidce). 

Fig.  119  gives  the  nervous  system  of  Jtulla  Aydatis ;  only  three  points  concern- 
ing it  need  be  mentioned  :  (1)  The  pleural  ganglia  have  shifted  till  they  lie  close 
to  the  cerebral  ganglia,  the  cerebro pleural  connectives  becoming  correspondingly 
shortened.  (In  Action  these  ganglia  have  even  fused,  and  are  no  longer  to 
be  distinguished  externally.)  (2)  There  are  three  visceral  ganglia.  (3)  The 
commissures  are  comparatively  long.  (4)  The  parapodia  are  innervated  from 
the  pedal  ganglia. 

In  man}-  Ccpkalaspido:,  moreover,  no  distinct  right  parietal  ganglion  exists.     It 


140 


COMPARATIVE  ANATOMY 


CHAP. 


seems  to  have  moved  up  to  the  right  pleural  ganglion,  or  to  have  fused  with  it,  so 
that  the  nerve  running  to  the  branchial  ganglion  rises  direct  from  the  right  pleural 
ganglion. 

The  nervous  system  of  the  Pteropoda  ihccosomata,  which  we  derive  from  Cephala- 
spidce, bears  a  general  correspondence  to  that  of  the  latter,  especially  in  the  fact 
that  the  pleural  ganglia  shift  near  to  or  fuse  with  the  cerebral  ganglia.  The 
pleurovisceral  connectives  are  so  much  shortened  that  the  ganglia  occurring  in 
their  course  lie  close  to  the  cerebral  and  pedal  ganglia.  There  are  usually  two  such 


FIG.  119.— Nervous  System  of  Bulla  hydatis  (after  Vayssiere).        FIG.  1-20.— Nervous  System 

I,  Buccal  ganglion;  2,  cerebral  ganglion;  3,  pleural  ganglion;  4,     of  Aplysia,  diagram,  combined 
pedal  ganglion  ;  5,  part  of  the  right  pleural  ganglion  (?) ;  7,  eye  ;     from     several     sources.       1, 
8,  cerebral  commissure ;  9,  pedal  commissure ;  10,  auditory  vesicle  ;     Buccal ;  2,  cerebral ;  3,  pleural ; 

II,  right  parietal  ganglion;  12,  13,  14,  visceral;    15,  branchial     4,  pedal;  5,  right  parietal;  6, 
ganglia.  visceral  ganglion ;   7,  osphra- 

ditim  ;  8,  genital  ganglion  ;  9, 
branchial  ganglion. 

ganglia  (the  right  parietal  and  a  visceral  ganglion  ?),  less  frequently  three  (two 
intestinal  and  one  visceral  ganglion  ?).  The  pedal  ganglia  also  innervate  the  tins, 
which  correspond  with  the  parapodia  of  the  Cephalaspidce. 

Fig.  120  represents  the  nervous  system  of  Aplysia,  one  of  the  Anaspidce.  The 
two  cerebral  ganglia  have  moved  close  to  each  other  in  the  middle  line.  The  pleural 
ganglia  here,  unlike  those  of  the  Cephalaspidce.  lie  close  to  the  pedal  ganglia,  so 
that  the  pleuropedal  connectives  are  much  shortened.  The  pedal  commissure  is 
double,  the  anterior  commissure  is,  relatively  speaking,  short  and  thick,  the  posterior 
long  and  thin.  The  long  pleurovisceral  connectives  run  back  from  the  pleural 


VII 


MOLLUSCA—THE  NERVOUS  SYSTEM 


141 


FIG.  1-21.  —  Nervous 
System  of  Notarchus 
punctatus  (after  Vays- 
siere),  diagrammatic.  1, 


5,  right  parietal  ganglion  ; 

6,  visceral  ganglion. 


ganglia,  and  enter  two  ganglia  lying  side  by  side  ;  that  to  the  right  represents  the 

right  parietal  ganglion,   innervating  chiefly  the  gill   and  osphradium,   the  nerves 

running  to  these  organs  forming  a  ganglion  at  the  base  of 

each  ;  that  to  the  left  is  the  visceral  ganglion.     One  of  the 

nerves  which  run  from  the  latter  forms  a  genital  ganglion  at 

the  base  of  the  accessory  glands  connected  with  the  genital 

organs.     In  other  Anaspidce,  such  as  Notarchus  (Fig.  121), 

the  pleurovisceral  connectives  are  so  much  shortened  that 

the  parietal  and  visceral  ganglia  lie  close  to  the  periceso- 

phageal  group  of  ganglia,  which  then  consists  of  two  cerebral, 

two  pedal,  and  two  pleural  ganglia,  and  further,  the  right 

parietal  and  the  visceral  ganglia.     The  two  cerebral  ganglia 

are   further  connected   by  a   thin   l&iver  commissure.     The 

parapodia  are  always  innervated   from  the   pedal   ganglia. 

The  nervous  system  of  the  Pteropoda  gymuosomata,  which 

are  nearly  related  to  the  Anaspidce,  corresponds  in  all  essential 

points  with  the  nervous  system  of  the  latter,  being  of  the 

same  type  as  that  of  Xotarchus. 

(b)  Nudibranchia  and  Ascoglossa. — The  nervous  system 

is   here   characterised   by  very  great   concentration   of  the 

typical  Molluscan  ganglia,  and  by  a  tendency  to  the  forma-    Buccal ;   2,  cerebral ;   3, 

tion  of  numerous   accessory  ganglia   (at   the   bases  of  the    Plural ;  4,  pedal  ganglia : 

tentacles  and  rhinophores,  and  at  the  roots  of  their  nerves, 

in  the  course  of  the  genital  nerves,  etc.).     The  pleural  gan- 
glion has  moved  close  to  the  cerebral  ganglion,  and  may  fuse  with  it.     The  pedal 

ganglia  have  also  moved  towards  the  cerebral  ganglia 
so  that  now  the  whole  cesophageal  complex  of  gan- 
glia lies  almost  entirely  on  the  dorsal  side  of  the  oeso- 
phagus. The  pedal  commissure  which  runs  under 
the  gullet,  and  is  sometimes  double,  is  thus  very 
much  lengthened.  The  pleurovisceral  connectives 
are  short,  and  occasionally  enter  an  unpaired  visceral 
ganglion,  which  has  also  been  drawn  into  the  ceso- 
phageal complex.  This  single  ganglion  of  the  visceral 
connectives  ma}-  be  wanting  (Fig.  122)  ;  in  that  case 
the  two  visceral  connectives  appear  like  a  commissure 
between  the  two  pleural  ganglia  runningunder  the  oeso- 
phagus and  parallel  with  the  pedal  commissure,  some- 
times even  united  with  it.  The  fusion  of  all  the 
ganglia  belonging  to  the  peri-cesophageal  complex  is 
carried  very  far  in  such  animals  as  Tethys,  where  the 
pleural  and  pedal  ganglia  of  each  side  may  fuse  with 
the  cerebral  ganglion.  The  pleuro  -  cerebropedal 
ganglion  thus  formed  shifts  towards  the  dorsal 
1,  Buccal ;  2,  cerebral ;  3,  pleural ;  middle  line  close  to  the  similar  ganglion  of  the  other 

4,  pedal  ganglia ;  5,  commissure  be-    si(i6j  ^th  which  it  forms  a  large  supra-cesophageal 

ganglionic  mass.  Its  composition  out  of  the  six 
typical  ganglia  can,  however,  be  made  out  by  the 
grouping  of  the  ganglion  cells  and  the  arrangements 
of  the  nerve  tracts.  A  nerve  leaves  this  mass  on 
each  side,  the  two  uniting  under  the  gullet.  These 
form  the  pedal  commissure,  which  when  closely  examined  is  found  to  be  double.  A 
third  delicate  commissure  running  under  the  oesophagus  connects  the  lateral  portions. 


FIG.  12-2. —Nervous  System  of 
Janus  (after  Pelseneer  simplified). 


tween  the  two  pleural  ganglia,  which 
corresponds  with  the  two  pleuro- 
visceral connectives  of  other  Mol- 
lusca ;  6,  pedal  commissure  ;  7, 
auditory  vesicle  ;  8,  eye  ;  9,  ganglion 
of  the  rhinophore. 


142 


COMPARATIVE  ANATOMY 


CHAP. 


of  the  supra-cesophageal  mass,  and  represents  the  visceral  commissure,  in  which  is 

found  a  small  visceral  ganglion. 

Among   the  Nudibranchia  the  two  buccal   ganglia  are   always   found  on   the 

posterior  and  lower  wall  of  the  pharynx.     They  are  connected  with  each  other  by  a 

buccal  commissure,  and  with  the  brain  by  two  cerebrobuccal  connectives,  in  whose 

course  accessory  ganglia  may  be  found. 

The  whole  peri-oesophageal  complex  of  ganglia  is  in  the  Nudibranchia  enclosed  in 

a  capsule  of  connective  tissue. 

III.  Pulmonata  (Fig.  123). — The  central  nervous  system  here  possesses  all  the 
typical  ganglia  of  the  Gastropoda.  These,  grouped 
together  as  in  so  many  Opisthobranchia  and  many 
Prosobranckia,  immediately  behind  the  pharyngeal 
bulb,  form  the  peri-cesophageal  complex,  into  which 
even  the  parietal  and  visceral  ganglia  have  been 
drawn.  The  cerebral  ganglia  lie  close  to  each  other 
dorsally,  and  all  the  other  ganglia,  which  are  also 
close  together,  lie  ventrally.  The  cerebropedal  and 
cerebropleural  connectives  are  consequently  always 
easily  distinguished.  In  Testacella  they  are  even  of 
some  length,  in  adaptation,  no  doubt,  to  the  special 
shape  and  the  great  development  of  the  pharyngeal 
bulb.  All  other  connectives  and  commissures,  on  the 
contrary,  are  much  shortened,  so  that  the  ganglia 
connected  by  them  lie  close  together.  A  visceral 
ganglion  is  always  found,  and  usually  also  in  each 
pleurovisceral  connective  a  parietal  ganglion.  When 
an  osphradium  is  present  (Basommatophora)  it  is 
innervated  from  the  parietal  ganglion  of  the  same 
side.  In  Pulmonata  with  a  dextral  twist,  the  osphra- 
dium lies  on  the  right,  and  in  those  with  a  sinistral 


FIG.  123.— Central  portion  of  the 
Nervous  System  of  Helix  pomatia 


what  diagrammatic,  the  ganglia 
being  in  reality  less  distinct.  1, 
Buccal  ganglion  ;  2,  optic  nerve  with 
thickened  root  (3)  arising  from  the 
cerebral  ganglion  (4) ;  5,  pedal ;  6, 
pleural ;  7,  parietal ;  8,  visceral 
ganglion. 


(after  Bohmig  and  Leuckart),  some-  twist  on  the  left ;  in  the  former  the  right  parietal 

ganglion  is  the  larger,  and  in  the  latter  the  left. 
The  smaller  parietal  ganglion  may  also  fuse  witli 
the  neighbouring  pleural  ganglion.  Lobes  are  often 
formed  in  the  cerebral  ganglia,  in  which  certain 
groups  of  nerves  have  their  origin.  The  pedal  com- 
missure is  often  double.  Buccal  ganglia  are  always 

found.     They  lie  posteriorly  on  the  pharynx  below  the  resophagus,  and  are  joined  to 

one  another  by  the  buccal  commissure  and  to  the  cerebral  ganglia  by  cerebrobuccal 

connectives. 

C.  Scaphopoda. 

The  nervous  system  of  the  Scaphopoda  (Fig.  101,  p.  113)  is 
symmetrical ;  the  visceral  connectives  are  not  crossed.  The  two 
cerebral  ganglia  lie  very  near  one  another  in  front  of  (or,  if  the 
intestine  is  regarded  as  horizontal,  above)  the  gullet  over  the  snout ; 
the  two  pedal  ganglia,  close  to  one  another,  lie  on  the  anterior  side 
of  the  foot,  more  or  less  at  its  centre,  and  are  joined  to  the  cerebral 
ganglia  by  two  long  cerebropedal  connectives.  The  two  pleural 
ganglia  lie  close  to  and  above  the  cerebral  ganglia,  so  that  the 
cerebropleural  connective  is  very  short.  The  pleuropedal  connective 


vii  MOLLUSCA—THE  NERVOUS  SYSTEM  143 

at  once  fuses  with  the  cerebropedal,  the  two  entering  the  pedal 
ganglion  as  one  connective.  Posteriorly,  to  the  right  and  left  of  the 
rectum,  near  the  anus,  there  are  two  visceral  ganglia  of  the  pleuro- 
visceral  connectives,  joined  to  one  another  by  a  commissure  running 
behind  the  intestine.  There  are  no  special  parietal  ganglia  distinct 
from  the  visceral  or  the  pleural  ganglia. 

There  are  four  buccal  ganglia,  two  behind  the  gullet  or  below  it  (if  the  intestine 
is  supposed  to  be  horizontal),  and  two  lying  laterally  and  anteriorly  to  (or  above) 
the  muscular  mass  of  the  radula.  The  anterior  are  connected  with  the  posterior,  and 
these  to  the  cerebral  ganglia  by  connectives,  and  the  two  posterior  and  two  anterior 
i liter  sc  by  commissures  running  behind  (under)  the  cesophagus.  Xerves  run  from 
the  posterior  buccal  ganglia  to  the  small  ganglia  of  a  subradular  organ. 

D.  Lamellibranehia. 

The  nervous  system  (Fig.  124),  like  the  whole  organisation  of  the 
Lamellibranehia,  is  perfectly  symmetrical,  and  consists  typically  of 
three  pairs  of  ganglia:  (1)  the  eerebropleural ;  (2)  the  pedal;  and 
(3)  the  viseeroparietal  ganglia.  These  three  pairs  of  ganglia  lie,  as 
a  rule,  far  apart,  and  the  connectives  uniting  them  are  therefore  long. 
The  two  pedal  ganglia  lie  close  together,  while  the  two  eerebropleural 
and  the  two  viseeroparietal  ganglia  are  connected  by  distinct  com- 
missures beset  with  ganglion  cells. 

1.  The  eerebropleural  ganglia  are  the  result  of  the  fusion  of  the 
cerebral  with  the  pleural  ganglia.     In  the  Protobranchia,  however,  the 
pleural   ganglia   are   still    distinct,    and    lie   immediately   behind    the 
cerebral  ganglia  at  the  commencement  of  the  visceral  connectives.      In 
Xiicula,  the    pleuropedal  connectives   are   distinct  for  some  distance, 
and   then  unite  with   the    cerebropedal   connectives.      In    Solenomya 
they  still  have  separate  roots,  but  are  otherwise   fused  along  their 
whole  length  with  the  cerebropedal. 

The  eerebropleural  ganglia  are  supracesophageal,  and  are  in 
contact  with  the  anterior  adductor  muscle,  when  this  is  present. 
They  send  nerves  into  the  oral  lobes,  the  anterior  adductor,  and  the 
mantle. 

2.  The  pedal  ganglia  lie  at  the  base  of  the  foot. 

3.  The  third  pair  of  ganglia,  which  correspond  with  the  ganglia 
of  the  visceral  connectives  in  the  Gastropoda,  lie  posteriorly  beneath 
the   rectum,  behind   the  foot,  and  are  generally  in  contact  with  the 
posterior   adductor   muscle  ;   in   the   Protobranchia,  however,   they  lie 
much  further  forward.      Their  area  of  inner vation  corresponds  with 
that  of  the  combined  parietal  and  visceral  ganglia  of  the  Gastropoda, 
for  these  viseeroparietal  ganglia  supply  with  nerves  the  two  ctenidia, 
the  two  osphradia,  the  posterior  portion  of  the  mantle,  the  posterior 
adductor,  and  the  viscera. 

The  buccal  or  stomodaeal  nervous  system  is  much  reduced ;  this  reduction  is 
connected  with  the  absence  of  a  muscular  pharynx  and  of  all  buccal  armature.  The 


144 


COMPARATIVE  ANATOMY 


CHAP. 


anterior  portion  of  the  intestine  receives  nerves  from  the  visceral  connectives.  Since 
the  fibres  of  these  nerves  have  been  proved  to  originate  in  the  cerebral  ganglia,  we 
may  assume  that,  on  the  degeneration  of  the  pharynx,  the  buccal  connectives  united 
with  the  visceral  connectives,  so  that  the  intestinal  nerves  now  rise  from  the  latter 
and  do  not  come  direct  from  the  brain.  In  the  Pholadidce  and  Teredinidoc  the 
visceral  connectives  are  united  in  front  of  the  visceroparietal  ganglia  by  a  second 


FIG.  124.— Nervous  system  of  Cardium  edule  (after  Drost),  seen  from  the  ventral  side.  The 
left  mantle  (the  right  in  the  figure)  has  been  removed  and  the  right  bent  back  ;  the  foot  has  been 
laid  on  one  side.  1,  Oral  lobes ;  2,  3,  4,  pallial  nerves,  running  nearly  parallel  to  the  edge  ;  2,  the 
nerve  of  the  pallial  edge  ;  5,  mantle  ;  6,  gill ;  7,  point  of  junction  of  the  principal  pallial  nerves  ;  8, 
mantle  edge  of  the  respiratory  aperture  ;  9,  ditto  of  the  anal  aperture ;  10,  posterior  adductor  ;  11, 
viscero-parietal  ganglion  ;  12,  branchial  nerve  ;  13,  foot ;  14,  pedal  ganglion  ;  15,  left  cerebropleural 
ganglion  ;  16,  mouth  ;  17,  right  cerebropleural  ganglion  ;  18,  anterior  adductor. 

commissure,  which  runs  under  the  intestine,  and  may  perhaps  be  considered  as  a 
buccal  commissure  shifted  far  back. 

The  mantle  is  innervated,  as  is  clear  from  the  above,  partly  from  the  cerebro- 
pleural, and  partly  from  the  visceroparietal  ganglia. 

The  two  anterior  pallial  nerves,  which  rise  from  the  cerebropleural  ganglia,  run 
back  along  the  edges  of  the  mantle,  to  join  the  two  posterior  pallial  nerves  which 
originate  in  the  visceroparietal  ganglia.  A  nerve  thus  runs  parallel  to  the  edge  of 
the  mantle  on  each  side  (nerve  of  the  pallial  edge),  and  like  a  connective,  unites  the 
anterior  cerebropleural  ganglion  witli  the  posterior  visceroparietal  ganglion.  This 


VII 


MOLLUSCA—THE  NERVOUS  SYSTEM 


145 


pallial  nerve  gives  off  branches  to  the  organs  at  the  edge  of  the  mantle  and  to  the 
siphons,  and  is  further  connected  with  a  rich  nerve  plexus  in  the  mantle  fold,  in 
which  certain  connecting  nerves,  further  from  the  edge  of  the  mantle,  but  running 
parallel  to  it,  are  particularly  strongly  developed.  A  varying  number  of  small 
peripheral  ganglia  attain  development  in  the  pallial  plexus  and  in  the  siphonal 
nervous  system. 

E.  Cephalopoda. 

The  symmetrical  nervous  system  of  all  Cephalopoda  is  marked  by 
the  great  concentration  of  the  typical  Molluscan  ganglia,  including 
those  of  the  visceral  connective. 

In  the  following  description  of  the  nervous  system,  we  shall  consider  the  body  in 
its  physiological,  not  in  its  true  morphological  position,  i.e.  we  shall  imagine  the 
pharynx  and  oesophagus  to  be  running 

horizontally  as  in  other  Molluscs  (cf.  p.  //  \  1 

36).  The  true  morphological  position 
will  be  given  in  brackets  after  the  con- 
ventionally accepted  position. 

1.  Tetrabranehia  (Figs.  125,  126). 

In  the  complex  of  ganglia 
which  in  Xautihis  surrounds  the 
oesophagus  behind  the  great  buccal 
mass,  and  which  is  not  yet  com- 
pletely enclosed  in  the  cephalic 
cartilage,  the  ganglia  are  not  very 
distinct  from  the  commissures  and 
connectives.  The  cerebral  ganglia 
(14,  in  Figs.)  are  represented  by  a 
broad  band-like  nerve  cord  running 
over  (morphologically  in  front  of) 
the  oesophagus,  and  from  them  run 
two  ganglionic  cords,  one  anterior 
(lower)  and  one  posterior  (upper),  FIG  125._Nervous  sygtem  of  Nautilus(after 

Which  pass  JUSt  below  (behind)  the  Jhering).  1,  Buccal  ganglion;  2,  pharyngeal 
CeSOphagUS.  The  anterior  (3)  re-  g^g11* ;  %,  pedal  commissure ;  4,  infundibular 

presents  the  pedal,  and  the  posterior  ^^££^2  ta^^ffefe  2£?^ 

(15)     the     Combined     pleural     and    swells  to  form  a  ganglion  (cf.  Fig.  126);  6,  nerves 

visceral  ganglia.  for  the  other  tentacles;  7>  pedal  cord  (=pedai 

m,  ,        .  .         ganglia);  8,  auditory  organ;  9,  olfactory  nerve;  10, 

The    Cerebral     COrd     gives     rise    optic  ganglion  ;  ll,  nerve  of  the  optic  tentacles ; 

laterally  to  the    large    Optic    nerves    12,  connective  to  the  pharyngeal  ganglion ;  13, 

(each  of  which  at  once  swells  into  labia;.  neriyes';1  14'   Cereb1ral  °ord  <=cerebral 

ganglia)  ;  15,  pleurovisceral  cord. 

an  optic  ganglion),  numerous  nerves 

to  the  lips,  the  nerves  for  the  optic  tentacles,  the  auditory  and  olfactory 

nerves,  and  the  cerebrobuccal  connectives. 

From  the  pedal  cord,  nerves  run  to  the  tentacles  round  the  mouth 
and  to  the  funnel.     In  the  female,  the  nerves  for  the  inner  circle  of 
VOL.  II  L 


^ 


146 


COMPARATIVE  ANATOMY 


CHAP.  VII 


tentacles  come  from  a  braehial  ganglion,  which,  however,  does  not 

supply  all  the  tentacles  (Fig.  126,  a); 
this  is  joined  to  the  pedal  ring  by  a 
brachiopedal  connective. 

The  pleuroviseeral  cord  gives  off 
numerous  pallial  nerves  (there  is  no 
stellate  ganglion),  and  two  strong  vis- 
ceral nerves  which  run  near  the  middle 
line  accompanying  the  vena  cava,  inner- 
vate the  gills,  the  osphradia,  and  the 
FIG.  126.— Nervous  system  of  Nautilus,  blood-vessels,  and  form  a  genital  gan- 

from  the  right  side.    Numbering  the  same    ^QU  facfa  up  jn  faQ  visceral  dome, 
as  in  Fig.  125.    a,  Ganglion  for  the  ten-    c 

tacles  of  the  posterior  and  inner  lobes  in  The  sympatlletic  nervous  system  consists 

the  female.  .      *  .  .         . 

of  an  mfra-cesophageal  commissure,  which  rises 

from  the  cerebral  ganglion,  and  passes  close  under  the  oesophagus  in  the  musculature 
of  the  buccal  mass  ;  two  ganglia,  a  pharyngeal  and  a  buccal  ganglion,  are  found  on 
each  side  in  its  course. 


2.  Dibranehia  (Figs.  127,  128). 

The  peri-oesophageal  mass  of  ganglia,  comprising  the  whole  of  the 
central  nervous  system,  is  entirely  enclosed  in  the  cephalic  cartilage. 
The  large  typical  ganglia  are  so  crowded  together  that  it  is  extremely 
difficult  to  distinguish  them  one  from  another,  and  the  connectives 
and  commissures  are  not  visible  externally.  The  whole  complex  has 
a,  continuous  cortical  layer  of  ganglion  cells. 

The  more  or  less  distinct  separation  of  each  pedal  ganglion  into 
two,  one  anterior  (lower)  and  one  posterior  (upper),  is  characteristic 
of  the  Dibranehia.  The  former  of  these  is  the  braehial  ganglion,  and 
innervates  the  arms,  which  must  be  considered  as  parts  of  the  foot ;  and 
the  latter  is  the  infundibular  ganglion,  and  innervates  the  siphon, 
which  may  be  regarded  as  the  epipodium.  This  differentiation  of  the 
pedal  ganglia  can  be  traced  to  the  great  development  of  that  part  of 
the  foot  (viz.  the  arms)  which  surrounds  the  head.  In  the  same  way 
in  Natica,  where  the  anterior  part  of  the  foot  is  strongly  developed, 
and  is  bent  back  over  the  head,  a  propedal  ganglion  becomes 
differentiated  from  the  pedal  ganglion.  The  braehial  ganglia  become 
joined  in  the  Dibranehia  to  the  cerebral  ganglia  by  cerebrobrachial 
connectives.  In  Eledone  and  Octopus,  they  are  further  connected  with 
one  another  by  a  thin  supraoesophageal  commissure. 

The  pleura!  ganglia  lie  laterally  in  the  perioesophageal  mass,  while 
the  ganglia  of  the  visceral  connectives,  i.e.  the  parietal  and  visceral 
ganglia  which  lie  close  together,  their  connectives  having  shortened  as 
much  as  is  possible,  form  the  posterior  (upper)  portion  of  the  infra- 
oasophageal  mass. 

The  following  are  the  connectives  which  are  revealed  by  sections 
through  the  peri-cesophageal  mass  : — 


FIG.  127.— Anatomy  of  Octopus  (after  Leuckart  and  Milne  Edwards).  The  body  is  cut  open  posteriorly,  the  mantle  laid 
>ack  to  the  right  and  left,  and  the  liver  removed.  1,  Brachial  artery  ;  2,  brachial  nerve  ;  3,  pharynx  ;  4,  buccal ;  5,  cerebral 
anglion  ;  6,  efferent  duct  of  the  upper  salivary  glands  ;  7,  funnel ;  8,  upper  salivary  glands  ;  0,  crop  ;  10,  anus  ;  11,  afferent 
n-anchial  vessel  (branchial  artery) ;  12,  left  renal  aperture  ;  13,  efferent  branchial  vessel  (branchial  vein) ;  14,  gastric  ganglion  ; 
c>,  left  auricle;  16,  spiral  c*cum  of  the  stomach;  17,  renal  sac;  18,  water  canal;  19,  ventricle;  20,  ovary;  21,  rectum;  22. 
fferenfr  ducts  of  the  digestive  gland  (liver),  cut  through  near  its  opening  into  the  intestine  ;  23,  mantle ;  24,  stomach  ;  25, 
ight  ctenidiurn  ;  26,  aperture  of  the  right  oviduct ;  27,  stellate  ganglion  ;  28,  nerve  to  the  gastric  ganglion ;  29,  upper  salivary 
land ;  30,  aorta  ;  31,  oesophagus  ;  32,  optic  ganglion  ;  33,  lower  salivary  glands. 


148 


COMPARATIVE  ANATOMY 


CHAP. 


(1)  Two  cerebro-brachial ;  (2)  two  cerebro-infundibular ;  (3)  two 
cerebropleural ;  (4)  two  brachio-infundibular ;  (5)  two  pleuro-infundi- 
bular;  (6)  two  pleurobrachial  connectives.  The  close  proximity  of 
the  visceral  ganglia  to  the  peri-cesophageal  mass  makes  it  impossible 
any  longer  to  distinguish  the  visceral  connectives. 

The  cerebral  ganglia  give  rise  to  the  two  optic  nerves  (which  soon  swell  into  the 

enormous  optic  ganglia  at 

A  the  bases  of  the  eyes),  the 

auditory  nerves,  the  olfac- 
tory nerves  (which  for  a 
certain  distance  fuse  with 
the  optic  nerves),  and  the 
connectives  of  the  buccal 
ganglia. 

The  brachial  ganglia 
send  off  separate  nerves  to 
the  arms,  which  nerves  are 
connected  by  a  hoop-like 
commissure  round  the  base 
of  the  circle  of  arms.  Run- 
ning through  the  arms,  the 
nerves  swell  into  succes- 
sive ganglia  which  corre- 
spond with  the  transverse 
rows  of  acetabula. 

The  separation  of  the 
pedal  ganglion  into  a  bra- 
chial and  an  infundibular 
ganglion  can  be  proved  on- 
togenetically  and  anatomi- 
cally. There  is  no  such 
separation  in  the  male 
Nautilus,  the  brachial  and 
infundibular  nerves  spring- 
ing from  one  and  the  same 
ganglion.  In  Argonauta 
(Fig.  128,  F)  the  separation 
is  not  externally  visible, 
but  in  Octopus  (E)  we  see 
the  first  traces  of  it ;  in 
Sepia  (D),  Loligo  (C),  and 
Sepiola  (B),  it  becomes 
more  and  more  evident,  till 
finally  in  Ommatostreplies 

(A)  the  distinct  brachial  ganglion  has  moved  away  from  the  infundibular  ganglion, 

with  which  it  is  joined  by  a  slender  externally  visible  connective. 

In  this  same  series,  the  separation  of  the  so-called  upper  buccal  ganglion  from 

the  cerebral  ganglion  also  takes  place,  the  buccal  remaining  united  to  the  brachial 

ganglion  by  the  brachiobuccal  connective. 

The  parietal  ganglia  give  rise  to  the  two  large  pallial  nerves.    Each  of  these  runs 

backward  and  upward,  and  enters  the  stellate  ganglion  on  the  inner  surface  of  the 


FIG.  128.— Central  nervous  system  of  various  Dibranchia, 
from  thenright  side.  All  the  figures  after  Pelseneer.  A,  Ommato- 
strephes  ;  B,  Sepiola ;  C,  Loligo ;  D,  Sepia ;  E,  Octopus ;  F,  Argo- 
nauta.  1,  Cerebral ;  2,  pedal ;  3,  visceral ;  4,  brachial ;  5,  upper 
buccal  ganglion  ;  6,  infundibular  nerve;  7,  visceral  nerve;  8,  optic 
nerve  cut  through;  9,  pallial  nerve ;  10,  brachial  nerves ;  and  in 
Fig.  B  the  pharynx  (ph),  and  resophagus  (ce)  are  drawn  in  black. 


vii     MOLLU8CA—THS  ASYMMETRY  OF  THE  GASTROPODA   149 

mantle.  Numerous  nerves  radiate  into  the  mantle  from  this  ganglion,  one  of  them, 
which  runs  dorsally,  looking  like  the  direct  continuation  of  the  pallial  nerve  through 
the  ganglion.  The  pallial  nerve  often  divides  into  two  branches  sooner  or  later  after 
it  has  left  the  parietal  ganglion  ;  one  of  the  branches  running  to  and  through  the 
stellate  ganglion,  to  unite  beyond  it  with  the  other  branch  which  runs  past  the 
ganglion.  The  two  stellate  ganglia  are  often  connected  by  a  transverse  commissure. 

The  visceral  ganglia  give  off,  near  the  middle  line,  two  visceral  nerves,  which 
innervate  the  rectum,  the  ink-bag,  the  gills,  the  heart,  the  genital  apparatus,  the 
kidneys,  and  certain  parts  of  the  vascular  system.  The  two  genital  branches  of 
these  nerves  are  connected  by  a  commissure. 

The  sympathetic  nervous  system  consists  of  a  buccal  ganglion  lying  beneath 
(behind)  the  oesophagus  in  the  buccal  mass  ;  this  ganglion  is  joined  to  the  upper 
buccal  or  pharyngeal  ganglion  by  a  buccal  connective.  Two  nerves  run  up  along 
the  oesophagus  from  the  lower  buccal  ganglion  to  the  gastric  ganglion,  which  lies 
on  the  stomach,  and  innervates  the  greater  portion  of  the  intestine  and  the  digestive 
gland  (liver). 


XIV.  An  Attempt  to  explain  the  Asymmetry  of  the  Gastropoda. 

i. 

Chiastoneury,  i.e.  the  crossing  of  the  two  pleuro- visceral  connectives  in  the 
Prosobranchia,  may  be  explained  on  the  three  following  assumptions. 

1.  The  ancestors  of  the  Prosobranchia  were  symmetrical  animals  ;  the  mantle 
cavity  lay  behind  the  visceral  dome  and  in  it  the  pallial  complex,  that  is,  the  ctenidia, 
osphradia,  uephridial  apertures,  genital  apertures,  and,  in  the  centre,  the  median 
anus. 

2.  The  visceral  commissure  or  ganglion  lay  beneath  the  intestine. 

3.  The  pallial  complex  shifted  gradually  from  behind  forward,  along  the  right 
side  of  the  body  (cf.  p.  136). 

The  position  of  the  pallial  complex  in  the  Tectibranchia,  among  the  Opisthobranchia, 
on  the  right  side,  can  also  be  thus  explained.  The  pallial  complex  in  its  forward 
movement  in  these  animals  has  either  not  yet  reached  the  anterior  position  or, 
having  reached  it,  has  shifted  back  again.1  The  visceral  connectives  are  therefore 
not  crossed. 

The  above  assumptions  do  not,  however,  explain — 

1.  The  asymmetry  which  is  brought  about  in  some  Gastropoda  by  the  dis- 
appearance of  one  ctenidium,  one  osphradium,  and  one  renal  aperture. 

2.  The  coiling  of  the  visceral  dome  and  shell,  especially  the  dextral  or  sinistral 
spiral  twist. 

3.  The  relation  existing  between  the  manner  in  which  the  visceral  dome  and 
shell  are  coiled,  on  the  one  hand,  and  the  special  asymmetry  of  the  asymmetrical 
organs  (ctenidia,  osphradia,  nephridia,  anus,  genital  organs)  on  the  other. 

4.  The  cause  of  the  shifting  forward  of  the  pallial  complex. 

2. 

It  is  unnecessary  to  discuss  the  first  of  the  above  assumptions,  viz.  that  the 
ancestors  of  the  Gastropoda  were  symmetrical  animals,  since  all  Molluscs  except 
the  Gastropoda  are  symmetrical,  i.e.  the  Amphineura,  the  Lamellibranchia,  the 
Scaphopoda,  and  the  Cephalopoda. 

1  See  note  to  §  13,  p.  158. 


150  COMPARATIVE  ANATOMY  CHAP. 

The  assumption  that  the  pallial  complex  originally  lay  posteriorly  is  also  well 
founded.  In  all  symmetrical  Molluscs,  the  anus  lies  as  the  centre  of  the  complex 
posteriorly  in  the  middle  line,  and  further,  in  all  symmetrical  Molluscs,  the  nephiidial 
and  genital  apertures  lie  posteriorly  at  the  sides  of  the  anus.  When  the  ctenidia 
and  osphradia  have  been  retained  in  symmetrical  Molluscs,  they  lie  symmetrically 
on  the  posterior  side  of  the  visceral  dome.  This  is  the  case  in  the  Cephalopoda,  and 
in  the  most  primitive  Lamellibranchia,  the  Protobranchia  (Nucula,  Leda,  Solenomya), 
and  even  in  some  Chitonidce,  and  those  Solenogastrcs  which  still  have  rudiments  of 
gills. 

In  keeping  with  the  posterior  position  of  the  pallial  complex,  the  mantle  fold 
which  hangs  down  round  the  base  of  the  visceral  dome  is,  in  symmetrical  Molluscs, 
widest  posteriorly  where  it  has  to  cover  the  complex  ;  at  this  part  the  mantle 
furrow  deepens  into  a  mantle  cavity. 

In  connection  with  the  second  assumption,  it  still  remains  unexplained  why  in 
the  Amphineura  the  commissure  between  the  pleuro-visceral  cords  runs  over  the 
intestine  ;  whereas  on  the  other  hand,  in  all  other  symmetrical  Molluscs,  the 
visceral  ganglion  lies,  as  in  the  Gastropoda,  below  the  intestine. 


The  third  assumption,  that  the  pallial  complex  has  shifted  forward,  requires 
separate  discussion. 

If  the  pallial  complex  did  thus  shift  forward,  chiastoneury  must  necessarily 
have  taken  place  ;  the  original  left  half  of  the  complex  must  necessarily  have  become 
the  present  right  half,  and  vice  versa.  Further,  the  right  pleuro-visceral  connective 
would  have  to  become  the  supra-intestinal  connective  and  the  left  the  infra-intestinal 
connective  ;  the  original  right  parietal  ganglion  the  supra-intestinal  ganglion,  and 
the  original  left  parietal  the  infra-intestinal  ganglion.  But  why  did  such  a  shifting 
take  place  ?  We  shall  here  attempt  to  answer  this  question. 

Cause  of  the  shifting  forward  of  the  pallial  complex. — We  have  assumed  the 
symmetrical  racial  form  of  the  Gastropoda  (with  posterior  mantle  cavity  and  sym- 
metrical pallial  complex)  to  be  a  dorso-ventrally 
flattened  animal  with  a   broad  creeping  sole,  a 
snout-like  head  with  tentacles  and  eyes,  and  a 
somewhat   flat  cup  -  shaped   shell   covering   the 
dorsal  side  of  the  body  (Fig.  129).     It  therefore 
resembled  in  outward  appearance  a  Fissurella,  a 
Patella,  or  a  Chiton,  if  we  assume  the  imbricated 
"p1**"  shell  of  the  last  to  be  replaced  by  a  single  shell. 

The  body  of  such  a  racial  form  was  only  pro- 

FIG.    129. —Hypothetical   primitive  ,    .J      „  ,         , 

Gastropod,  from  the  side,  o,  Mouth ;  tected  doi'sally  bY  the  shell.  The  hard  surface 
fc,  head ;  sm,  shell  muscle ;  oso,  apical  along  which  the  animal  slowly  crept  served  to 
shell  aperture ;  a,  anus ;  n,  renal  aper-  protect  its  lower  side,  the  dorsal  shell  being 
*u™o;t^'mantlecavity;  c*>  ctenidium ;  pressed  firmly  against  the  substratum,  when 

necessary,  by  the  contraction  of  a  powerful  shell 

muscle  (cf.  Fig.  106,  p.  122).  When  the  shell  was  thus  pressed  down,  communica- 
tion between  the  pallial  cavity  and  the  exterior  (for  the  purpose  of  inhaling  and 
exhaling  the  respiratory  water,  and  ejecting  the  excreta,  excrement,  and  genital 
products)  was  rendered  possible  by  means  of  a  cleft  in  the  posterior  edges  of  the 
mantle  and  shell. 

Unlike  their  racial  form,  all  known  Gastropoda  (except  those  whose  body  form 
has  been  secondarily  modified,  generally  in  connection  with  the  rudimentation  of  the 
shell)  are  distinguished  by  the  fact  that  the  viscera  with  their  dorsal  integumental 


vii      MOLLUSCA—THE  ASYMMETRY  OF  THE  GASTROPODA   151 


covering  protrude  hernia-like  in  the  form  of  a  high  spire-like  visceral  dome,  with 
which  the  shell  corresponds  in  shape.  The  uncoiled  shell  of  every  snail  is  as  a 
matter  of  fact  spire-shaped. 

The  development  of  such  a  shell  and  dome  has  already  been  recognised  as  due  to 
the  increased  protection  needed  by  the  body  when  the  capacity  for  creeping  becomes 
developed.  The  whole  of  the  softer  part  of  the  body  can  be  withdrawn  into  such,  a 
shell,  and,  further  to  increase  the  protection,  an  operculum  is  often  developed  on  the 
foot  for  closing  the  aperture  of  the  shell,  when  the  animal  has  retired  into  it.  The 
shell  muscle  of  the  racial  form  no  longer  serves  for  pressing  the  shell  against  the 
surface  on  which  it  rests,  but  for  withdrawing  the  head  and  foot  into  the  shell.  It 
becomes  the  columellar  muscle  (Fig.  131,  sm}. 

Taking  in   turn   the  different   stages   in  the   development   of   the   Gastropod 


FIG.  131. 

(Lettering  in  this  and  in  the  following  three 
figures  the  same  as  in  Fig.  129.) 


FIG.  130.— Hypothetical  primitive  Gastropod, 
from  above,  o,  Mouth ;  ulc,  idpl,  ulp,  original 
left  cerebral,  pleural  and  pedal  ganglia ;  ulpa, 
iirpa,  original  left  and  right  parietal  ganglia  ; 
tila,  original  left  auricles  ;  uos,  uros,  original  left 
and  right  osphradia  (Spengel's  organs) ;  ulct,  urct, 
original  left  and  right  ctenidia  (gills) ;  mb,  base 
of  the  mantle  ;  mr,  edge  of  the  mantle  ;  m,  mantle 
cavity;  r,  visceral  ganglion;  ve,  ventricle;  o, 
anus. 

shell,  we  have  as  the  first  and  most  important  its  dorsal  spire-like  prolongation. 
In  this  way  the  cup-shaped  shell  of  the  racial  form  becomes  a  high  conical  shell  like 
that  of  Dcntalium. 

Such  a  shell  carried  vertically  by  the  animal  (Fig.  131)  would,  when  the  latter  is 
at  rest,  be  in  a  state  of  unstable  equilibrium,  which  would  be  upset  by  movement  or 
by  the  slightest  pressure  from  without.  It  is  also  evident  that  when  the  animal  is 
in  motion  a  vertically  placed  spire-like  shell  would  be  extremely  awkward. 

If  we  assume  the  shell  to  be  carried  at  some  other  angle  to  the  body,  we  have 
the  following  possible  positions  : — 

1.  The  shell  might  be  carried  inclined  forward  (Fig.  132).  Such  a  position  is 
the  most  unfavourable  imaginable  for  locomotion,  for  the  functions  of  the  mouth, 
and  for  the  sensory  organs  on  the  head. 


152 


COMPARATIVE  ANATOMY 


CHAP. 


On  the  other  hand,  such  a  position  is  the  most  favourable  imaginable  for  the 
functions  of  the  organs  belonging  to  the  posteriorly  placed  pallial  complex,  which 
now  lie  dorsally,  since  in  this  position  the  mantle  cavity  is  subjected  to  least  pressure 

m     el 


FIG.  13$ 


from  the  viscera  and  from  the  columellar  muscles.     The   downward   pressure  of 
the  visceral  mass  which  now  takes  place  would  tend  indeed  to  widen  the  cavity. 

2.  The  shell  might  be  carried  inclined  backwards  (Fig.  133).     This  position  is 
the  most  favourable  imaginable  for  locomotion  and  for  the  functions  of  the  organs 


FIG.  133. 

of  the  head,  which  would  thus  be  free  on  all  sides.  It  is,  however,  the  most 
unfavourable  imaginable  for  the  functions  of  the  organs  of  the  pallial  complex, 
which  now  lie  beneath,  the  visceral  dome.  The  mantle  cavity  has  to  bear  the  whole 
pressure  of  the  visceral  mass,  and  especially  that  of  the  columellar  muscle  ;  it  would 

be  squeezed  together,  so  that  the 
•  *•  circulation  of  the  respiratory  water 
would  be  prevented  or  at  least 
rendered  more  difficult,  as  would 
also  the  ejection  of  the  excreta,  ex- 
crement, and  sexual  products. 

3.  Finally,  the  shell  may  be 
carried  inclined  to  the  right  or  left 
(Fig.  134).  This  is  neither  the  most 
favourable  nor  the  most  unfavour- 
able position  for  locomotion,  for  the 
head,  and  for  the  pallial  complex. 
It  is  an  imaginable  intermediate 
position. 

In  this  position  there  is  no  dead  point,  as  shifting  of  the  parts  would  always  be 
possible,  and  the  shell  be  enabled  to  take  up  the  position  most  suitable  for  locomo- 
tion and  for  the  functions  of  the  cephalic  organs,  and  the  mantle  cavity  that  best 
suited  for  the  exercise  of  the  functions  of  the  pallial  complex  lying  within  it. 

.Assuming  that  the  shell  is  inclined  to  the  left  (Fig.  135),  the  pressure  brought 
to  bear  on  the  mantle  cavity  would  vary  in  amount  in  different  areas  of  that  cavity. 
It  would  be  greatest  on  the  left  side,  and  would  continually  decrease  towards  the 


FIG.  134. 


vii     MOLLUSCA—THE  ASYMMETRY  OF  THE  GASTROPODA    153 


right.  On  the  left  there  would  be  a  pressure  from  the  front  which  would,  so  to 
speak,  squeeze  out  the  pallial  complex  backwards  over  to  the  right.  It  must  further 
be  noted  that  the  point  subjected  to  least  lateral  pressure  and  to  the  greatest  down- 
ward pull  lies  on  the  right, 
which  has  become  the  upper 
side  of  the  visceral  dome.  At 
this  point  the  mantle  furrow 
will  most  easily  deepen,  and 
become  more  spacious.  Into 
such  a  deepening  the  organs  of 
the  pallial  complex  which  are 
being  pressed  from  the  left 
have  room  to  move  forward  to 
the  right.  Here  we  have  the 
first  step  in  the  shifting  for- 
ward of  the  pallial  complex 
along  the  right  mantle  furrow. 
Further,  as  soon  as  the  least 
shifting  of  this  sort  has  taken 
place,  the  shell  and  visceral 
dome  can  move  slightly  from 
their  present  position  on  the 
left,  towards  that  backward 
position  which  we  have  seen 
to  be  the  most  favourable  im- 
aginable for  locomotion  and  for  iines  indicates  the  amount  of  the  pressure,  a,  Point  of  greatest 
the  functions  of  the  cephalic  pressure ;  b,  point  of  least  pressure.  Th«  arrows  give  the 

direction  in  which  shifting  takes  place.  It  is  evident  that 
the  left  side  of  the  pallial  complex  is  subjected  to  greater 
pressure  than  the  right. 


FIG.  135.— Diagram  illustrating  the  variations  of  pres- 
sure to  which  the  shell  and  visceral  dome  are  subjected 
when  inclined  to  the  left.  The  thickness  of  the  concentric 


organs. 

If  we  suppose  this  process 
gradually  to  be  completed,  the 
shell  and  visceral  dome  finally  gain  the  most  favourable  backward  position,  and  the 
pallial  complex  is  gradually  shifted  forwards  along  the  right  mantle  furrow.  The 
pallial  complex  thus  lies  anteriorly  on  the  upper  side  of  the  visceral  dome,  which 
now  points  backwards.  This  anterior  position  is  that  of  the  least  upward  pressure, 
or  rather  of  the  greatest  downward  pull,  i.e.  it  is  the  point  at  which  the  mantle 
cavity  can  most  easily  deepen  and  widen,  and  where  the  pallial  organs  can  best 
fulfil  their  functions. 

The  position  of  the  shell  and  the  pallial  complex  characteristic  of  the  Gastropoda 
is  now  attained,  and  with  it  chiastoneury  and  the  inverse  position  of  the  organs  of 
the  pallial  complex. 

4. 

The  second  stage  in  the  development  of  the  Gastropod  shell  is  the  coiling  in 
one  plane  of  the  visceral  dome  and  shell. 

If  the  Gastropod  visceral  dome  assumes  the  most  favourable  inclined  position  above 
described,  it  will,  under  normal  conditions,  change  its  conical  shape.  The  side  which 
lies  uppermost  will  become  arched  and  the  lower  side  concave.  This  change  of  form 
is  caused  by  the  stronger  growth  of  the  integument  of  the  visceral  dome  and  mantle 
on  that  side,  which,  in  the  inclined  position  of  the  visceral  dome,  is  the  most 
stretched  or  pulled.  The  visceral  dome  also  becomes  curved  in  one  plane,  and  the 
shell  naturally  adapts  itself  to  the  changes  of  shape  of  the  dome.  Again,  the  shell 
could  not  remain  conical,  because  a  large  part  of  the  dorsal  integument  (base  of  the 
visceral  dome)  would  then  be  uncovered,  and  in  consequence  of  the  increase  of  those 


154  COMPARATIVE  ANATOMY  CHAP. 

parts  of  the  body  not  covered  by  the  shell  there  would  come  a  time  when  the  body 
could  no  longer  be  completely  withdrawn  into  it. 


Before  discussing  the  third  stage  in  the  development  of  the  Gastropod  shell,  we 
must  consider  its  growth.  This,  from  a  geometrical  point  of  view,  is  of  three  kinds  : 
growth  in  height,  peripheral  growth,  and  radial  growth  or  increased  thickness  of 
the  shell  wall.  This  last  does  not  here  concern  us. 

Supposing,  for  simplicity's  sake,  the  shell  to  be  conical,  growth  in  height  occurs 
at  the  base  (or  aperture  of  the  shell),  and  takes  place  by  means  of  continual  deposits 
of  bands  of  new  material  at  the  edge  of  the  aperture,  by  the  growing  edge  of  the 
mantle. 

Peripheral  growth  is  the  enlargement  of  the  circumference  of  the  base  or  aperture 
of  the  shell. 

If  the  height  and  the  peripheral  growth  remain  uniform  round  the  whole 
aperture  of  the  cone  (which  is  assumed  to  be  round),  the  cone  increases  without 
altering  its  shape. 

If,  however,  the  growth  in  height  is  not  uniform,  but  steadily  and  symmetrically 
increases  along  each  side  from  an  imaginary  minimum  point  to  a  diametrically 
opposite  maximum  point,  the  peripheral  growth,  however,  remaining  uniform,  a 
spirally  twisted  hollow  cone  is  produced. 

If  the  minimum  and  maximum  points  in  this  growth  continue  throughout  in 
one  and  the  same  plane,  a  symmetrical  shell  coiled  in  this  plane  of  symmetry 
results. 

If,  however,  as  growth  increases,  the  maximum  point  shifts  from  the  symmetrical 
plane,  say  to  the  left  (the  minimum  point  shifting  in  the  opposite  direction  to  the 
right),  the  maximum  and  minimum  points  no  longer  trace  on  the  spirally  coiled 
shell  straight  but  spirally  twisted  lines,  and  the  conical  shell  is  then  not  coiled 
symmetrically  in  one  plane,  but  asymmetrically  in  a  screw-like  spiral.  We  then 
have  what  conch ologists  call  a  dextrally  twisted  shell. 

The  growth  of  the  Gastropod  shell  actually  takes  place  in  this  last  manner. 

6. 

This,  the  dextral  (or  sinistral)  coiling  of  the  Gastropod  shell,  is  the  last  stage  to  be 
discussed.  If  the  visceral  dome  and  shell  which  are  twisted  in  one  plane  pass,  in  growth, 
from  an  incline  to  the  left  to  a  backward  incline,  this  is  equivalent  to  the  continual 
shifting  of  the  point  of  maximum  growth  to  the  left  and  that  of  minimum  growth 
to  the  right ;  the  necessary  consequence  being  a  dextral  screw-like  spiral  twist. 

It  must  be  borne  in  mind — 

1.  That  the  peripheral  growth  remains  constant,  i.e.  that  the  outline  of  the 
growing  edge  of  the  mantle  remaining  uniform,  the  increasing  aperture  of  the  shell 
also  retains  the  same  form. 

2.  That  the  additions  to  the  shell  by  the  mantle  edge  are  made  in  the  form  of 
bands  of  new  material,  the  already  formed  firm  shell  not  altering  in  shape. 

3.  That  the  growing  edge  of  the  mantle,  which  secretes  the  shell  substance,  does 
not,  in  the  course  of  the  gradual  change  from  the  left  to  the  backward  incline,  itself 
become  twisted,  but  retains  its  position  in  relation  to  the  rest  of  the  body.     It  is 
thus  only  the  maximum  and  minimum  points  of  growth  in  height  which  become 
shifted  along  the  edge  of  the  mantle. 

4.  It  must  be  noted  that  this  description  of  the  manner  in  which  a  dextrally 
twisted  shell  arose  only  applies  to  that  stage  in  the  ontogenetic  or  phylogenetic 


vii     MOLLUSCA—THE  ASYMMETRY  OF  THE  GASTROPODA    155 

development  of  the  shell  during  which  its  displacement  in  a  backward  direction  and  the 
shifting  forward  of  the  pallia!  complex  occur.  When  once  the  result  most  favourable 
to  the  animal,  i.e.  the  anterior  position  of  the  mantle  cavity  and  the  backward 
direction  of  the  shell,  are  attained,  further  displacement,  which  would  be  dis- 
advantageous, does  not  take  place.  It  is,  then,  not  at  first  sight  evident  why, 
when  the  need  for  displacement  ceases,  its  action  still  continues,  i.e.  why,  though 
displacement  ceases,  the  visceral  dome  and  shell  continue  to  grow  in  a  dextral 
twist  and  not  symmetrically.  This  point  will  be  explained  below. 


For  the  sake  of  clearness  we  have  treated  separately  the  three  important  factors 
in  the  development  of  the  Gastropod  shell,  viz.  (1)  the  formation  of  a  tall  conical 
shell,  (2)  the  spiral  coiling  of  the  same,  and  (3)  the  special  manner  of  coiling  in  a 
dextral  twist.  In  reality  these  three  factors  do  not  denote  special  stages,  but  all 
operate  simultaneously.  The  continually  increasing  protrusion  of  the  visceral  dome 
was  accompanied  by  the  dextral  twist,  as  a  consequence  of  the  twisting  of  the 
visceral  dome  from  its  incline  to  the  left  to  the  most  favourable  backward  incline, 
by  which  the  pallial  complex  was  shifted  forward. 

8. 

The  results  of  ontogenetic  research  favour  the  theory  here  advanced.  We 
have  first  to  note  the  fact  that  the  anus  (the  centre  of  the  pallial  complex)  and  the 
mantle  fold  originally  lie  posteriorly.  They  come  to  lie  anteriorly  in  the  embryo 
not  by  active  shifting,  but  by  the  cessation  of  growth  on  the  right  side  between  the 
mouth  and  anus,  and  its  continuation  on  the  left  side.  There  is,  however,  no 
difficulty  in  harmonising  this  ontogenetic  method  of  gaining  the  object  with  the 
phylogenetic  method. 

9. 

So  far  we  have  placed  mechanical  and  geometrical  considerations  in  the  fore- 
ground. But  these  necessarily  coincide  with  utilitarian  considerations.  Every 
alteration  in  the  direction  we  have  been  considering  means  an  improvement  in  the 
organisation  of  the  animal,  an  advantage  to  enable  it  the  better  to  maintain  the 
struggle  for  existence.  The  formation  of  a  spire-like  shell,  which  has  been  recog- 
nised as  the  starting-point  in  the  development  of  the  asymmetry  of  reptant  Gastro- 
pods, was  the  only  method  by  which  complete  protection  of  the  whole  body  could 
be  attained,  and  must  therefore  be  considered  to  have  been  advantageous  under  the 
circumstances.  We  might  further  conclude  this  from  the  fact  that  the  possession 
of  such  a  shell  actually  distinguishes  the  Gastropoda  from  the  primitive  Mollusca, 
which  the  Chitonidce  are  rightly  considered  most  nearly  to  represent. 

10. 

One  apparently  important  objection  to  the  theory  here  set  forth  must  be  mentioned. 
If  the  first  factor  in  the  asymmetry  of  the  Gastropod  body  is  the  development  of  a 
high  spire-like  shell,  and  if  the  arrangement  of  the  nervous  system  is  necessarily 
connected  with  the  coiling  of  the  shell  in  a  definite  direction,  how  can  we  account 
for  forms  such  as  Fissurella  ?  This  Diotocardian  genus  actually  belongs  to  the  most 
primitive  Gastropods,  because  the  symmetry  of  the  pallial  complex  is  still  retained. 
But  it  possesses  an  asymmetrical  nervous  system  and  the  typical  chiastoneury  of 
the  Prosobranchia,  and  nevertheless  a  flat  cup-shaped  symmetrical  shell.  We  thus 
here  have  secondary  characteristics  of  the  inner  organisation  combined  with  an 


156 


COMPARATIVE  ANATOMY 


CHAP. 


apparently  primitive  shell.  The  latter  is,  however,  only  apparently  primitive,  as  can 
be  proved  systematically  and  ontogenetically.  The  forms  most  nearly  related  to 
Fissurella,  such  as  the  primitive  genus  Pleurotomaria  (Fig.  136  A),  Polytremaria 
(Fig.  136  B),  and  Scissurella,  have  spacious  spirally  coiled  dextrally  twisted  shells. 
In  Haliotis  (Fig.  136  D)  the  shell  becomes  flat  and  the  coiling  indistinct,  as  is  also 
the  case  to  some  extent  in  Emarginula  (Fig.  136  C),  till  finally  in  Fissurella  (Fig. 


FIG.  136.— Shells  of  A,  Pleurotomaria ;  B,  Polytremaria ;  C,  E,  Emarginula ;  D,  Haliotis ; 
F,  Fissurella;  G,  H,  stages  in  the  development  of  the  shell  of  Fissurella;  I,  shell  of 
the  Gastropod  racial  form,  with  marginal  cleft ;  K,  the  same,  with  apical  perforation ; 
L,  Lamellibranch  shell ;  M,  shell  of  Dentalium,  seen  from  the  apical  cleft.  The  shell  clefts 
and  perforations  are  black,  o,  Mouth  ;  a,  anus  ;  ct,  ctenidium. 

136  F)  it  again  secondarily  becomes  flattened  or  cup-shaped  and  symmetrical.  Fis- 
surella even  passes  ontogenetically  through  an  Emarginula  stage,  in  which  the  shell 
is  distinctly  spirally  coiled  (Fig.  136  G,  H).  We  may  therefore  conclude,  with  as 
much  certainty  as  is  possible  in  morphological  questions,  that  the  outwardly  sym- 
metrical Fissurella  descends  from  forms  with  high  spirally  coiled  shells.  Its  return 
to  a  flat  symmetrical  shell  may  have  been  determined,  as  in  the  Patellidce,  Capulidce, 
etc.,  by  adaptation  to  certain  biological  conditions. 


11- 

The  explanation  given  above  seems  to  throw  new  light  on  many  as  yet  unsolved 
problems  in  the  morphology  of  the  Mollusca,  such  as  the  asymmetry  of  the  pallial 
complex  in  most  Gastropoda.  Many  ' Diotocardia,  all  Monotocardia,  all  Opistho- 
branchia,  and  all  Pulmonata  show  marked  asymmetry  in  the  pallial  complex.  The 
asymmetry  consists  principally  in  the  absence  of  one  gill,  one  osphradium,  and  one 
nephridial  aperture.  The  inner  organisation  also  shows  reflections  of  this  asym- 
metry in  the  nervous  system,  and  the  absence  of  one  kidney  and  one  auricle.  On 
closer  inspection,  it  is  found  that  it  is  the  original  left  half  of  the  pallial  complex 


vii     MOLLUSCA—THE  ASYMMETRY  OF  THE  GASTROPODA    157 

(which  in  a  Prosobmnch  would  lie  to  the  right  in  the  mantle  cavity  near  the  anus) 
which  is  wanting.  The  anus  is  no  longer  the  centre  of  the  pallial  group  of  organs,  but 
lies  outermost  on  one  side.  "While  in  the  Prosobranchia,  for  example,  the  original 
left  half  of  the  pallial  complex  (which  would  now  lie  on  the  right)  has  disappeared, 
those  organs  of  the  complex  (the  original  right)  which  are  retained,  shift  from 
the  left  to  occupy  the  empty  space.  Consequently,  we  find  the  anus  no  longer 
anteriorly  in  the  middle  line,  but  on  the  right  side,  close  to  the  extreme  right  of  the 
mantle  cavity. 

But  what  is  the  reason  of  the  disappearance  of  the  left  half  of  the  pallial  complex 
in  the  Monotocardia,  Opisthobranchia,  and  Pulmonata  ? 

In  answering  this  question  we  must  refer  back  to  paragraph  3,  where  it  was  seen 
that  if  the  spire-like  shell  assumes  the  only  possible  lateral  inclination,  the  mantle 
cavity  and  the  pallial  complex  within  it  are  subjected  to  unequal  pressure.  If  the 
shell  is  inclined  to  the  left,  the  side  of  the  posterior  mantle  cavity  subjected  to  the 
greatest  pressure  is  the  left,  and  the  pressure  continually  decreases  towards  the  right. 
These  variations  of  pressure  are  also  retained  during  the  whole  time  in  which  the 
backward  displacement  of  the  shell  and  the  forward  displacement  of  the  pallial 
complex  takes  place.  In  other  words,  i.e.  described  in  terms  of  our  theory,  from  the 
very  commencement  of  the  development  of  the  Gastropod  organisation,  the  original 
left  organs  of  the  pallial  complex  were  subjected  to  unfavourable  conditions.  In 
this  left -sided  compression  of  the  mantle  cavity  the  ctenidium  especially  would 
necessarily  be  reduced  in  size  and  become  rudimentary,  and  might  entirely  disappear. 

As  a  matter  of  fact,  the  original  left  half  of  the  pallial  complex  (which  would 
now  lie  on  the  right)  has  entirely  disappeared  in  many  Diotocardia  (the  so-called  Azy- 
gobranchia),  in  all  Monotocardia,  and  in  the  Opisthobranchia.  The  fact  that  the 
original  right  gill,  the  only  one  remaining,  has  also  disappeared  in  the  Pulmonata 
is  accounted  for  by  the  change  to  aerial  respiration.  It  is  an  interesting  fact  that 
in  the  Basommatophora  the  original  right  osphradium  is  retained. 

If,  however,  the  original  left  gill  did  not  quite  disappear,  but  only  became 
smaller,  we  should  have  to  expect  that  in  such  Diotocardia  as  still  possess  two  gills, 
the  original  left  (now  the  right)  would  be  the  smaller.  This  would  be  the  case  at 
least  in  the  more  primitive  forms  with  shells  still  twisted.  Haliotis  and  Fissurella 
are  the  only  Molluscs  to  which  this  applies.  In  Haliotis,  whose  shell  is  still 
twisted,  the  right  (originally  left)  gill  is  in  reality  the  smaller.  But  in  Fissurella 
and  Subemarginula,  where  the  asymmetry  of  the  mantle  cavity  has  been  secondarily 
lost,  the  inequality  in  the  size  of  the  gills  has  also  disappeared. 

12. 

Another  unsolved  problem  remains.  Why  does  the  shell  continue  to  grow 
asymmetrically  coiled  with  a  dextral  twist,  after  the  cause  of  this  asymmetry,  viz. 
the  change  from  the  incline  to  the  left  to  the  backward  incline  of  the  shell,  simultane- 
ously with  the  shifting  forward  of  the  mantle  cavity  and  pallial  complex,  has  ceased 
to  act,  i.e.  when  the  shell  has  definitely  assumed  the  posterior,  and  the  pallial 
complex  the  anterior,  position  ?  The  explanation  of  this  lies  in  the  asymmetry  so 
early  apparent  in  the  mantle  cavity,  which  from  the  beginning  is  more  spacious 
to  the  right  (now  left)  than  to  the  left,  the  consequence  being  that  the  left  half  of 
the  pallial  complex  atrophied.  This  asymmetry  of  the  pallial  complex  and  mantle 
cavity  remained  after  the  displacements  of  shell  and  pallial  complex  had  been 
definitely  accomplished  in  the  Prosobranchia,  i.e.  the  asymmetrical  growth,  and 
therefore  the  continuous  coiling  of  the  visceral  dome  and  shell  in  a  spiral  twist, 
continued. 

In  altogether  exceptional  conditions,  which  rendered  a  flat  cup-shaped  shell 


158 


COMPARATIVE  ANATOMY 


CHAP. 


useful,  the  return  to  symmetry  in  the  pallial  complex  and  mantle  cavity  or  fold 
would  be  advantageous,  since  then  symmetrical  growth  of  the  shell  could  take 
place.  If  the  difference  between  the  maximum  and  minimum  growth  in  height 
is  but  slight  the  shell  would  be  but  slightly  coiled,  and  if  the  peripheral  growth 
is  pronounced,  while  the  growth  in  height  is  insignificant,  a  flat  cup-shaped  shell 
would  result  (Haliotis,  Emarginula,  Fissurella,  Patella,  etc.). 


13. 

Chiastoneury  only  takes  place  Avhen  the  original  right  half  of  the  pallial  complex 
crosses  over  to  the  left  of  the  median  line  anteriorly. 

This  crossing  of  the  line  of  symmetry  has  actually  taken  place  in  the  Proso- 
branchia.  The  original  right  gill  in  them  lies  quite  to  the  left  of  the  mantle  cavity. 
In  the  Azygobranchia  and  Monotocardia  the  hind-gut  with  the  anus  has  at  the  same 
time  become  displaced  into  the  right  (original  left)  narrower  gill-less  half  of  the 
mantle  cavity,  which,  however,  is  still  spacious  enough  to  contain  the  rectum.  The 
Prosobranchia  are  streptoneurous. 

In  the  Tedibranchia  and  Opisthobranchia  the  pallial  complex  is  found  on  the  right 
side  of  the  body,  and  has  nowhere  crossed  the  median  line  anteriorly.  There  is 
therefore  no  chiastoneury  among  the  Opisthobranchia,  i.e.  their  visceral  connectives 
are  never  crossed.1 

In  the  Pulmonata  the  pallial  complex  has  shifted  far  forward,  but  it  has  not 
passed  the  middle  line  with  any  organ  which,  drawing  the  parietal  ganglion  and  the 
visceral  connective  with  it,  could  have  brought  about  chiastoneury.  For  the  left 
(original  right)  gill,  the  only  one  elsewhere  retained, 'disappeared  (apparently  very  early) 
in  the  Pulmonata.  The  osphradium,  which  is  retained  in  aquatic  Pulmonata,  is  the 
original  right,  and  still  lies  on  the  right  side.  In  considering  the  arrangement  of 
the  nervous  system,  it  is  really  immaterial  whether  we  assume  that  the  hind-gut 

has  shifted  back  to  the  right 
secondarily,  and  the  osphra- 
dium moved  to  near  the  re- 
spiratory aperture,  or  that 
the  hind-gut  never  reached 
the  median  line,  and  that 
the  osphradium  never  passed 
over  it.  The  Pulmonata  are 
thus  euthyneurous. 

14. 

We  saw,  in  paragraph 
3,  that  with  a  strongly  de- 
veloped visceral  dome  and 
posteriorly  placed  pallial 
complex,  a  shell  inclined 
forward  or  coiled  forward  is 
an  impossibility  for  a  rep- 
tant  Gastropod.  But  such 
a  shell  is  not  an  impossibility  for  an  animal  which  does  not  creep.  For  example, 
in  a  swimming  animal,  whose  shell,  partly  filled  with  air,  serves  as  a  hydrostatic 
apparatus,  there  is  no  reason  why  a  much  developed  visceral  dome  and  shell  should 

1  Except  in  Actceon,  an  exception  which  makes  it  probable  that  in  the  Opistho- 
branchia the  pallial  complex  has  secondarily  returned  from  an  anterior  position. 


FIG.  137.— Nautilus,  diagram,     do,  Dorsal ;  re,  ventral ;  vo, 
anterior ;  hi,  posterior. 


UN. 


. 


vii     MOLLUSCA—THE  ASYMMETRY  OF  THE  GASTROPODA    159 

not  become  coiled  forward,  the  original  posterior  position  of  the  pallial  complex 
being  retained  as  the  most  favourable  under  such  circumstances.  As  an  example 
of  this  we  have  the  Nautilus,  all  Nautiloidea  and  Ammonitidea,  with  their  exogas- 
trically  (anteriorly)  coiled  shells  and  posteriorly  placed  pallial  complexes  (Fig.  137). 
The  coiling  of  the  shell  of  Spirula  forms  an  exception  to  that  of  all  other  Mol- 
lusca,  being  endogastric.  With  regard  to  this  we  have  to  consider  first,  that  the 
shell  of  Spirula  is  internal  and  rudimentary,  and  that  the  backward  coiling  does  not 
in  any  way  affect  the  posteriorly  placed  mantle  cavity  ;  and  second,  that  only  the 
modern  genus  Spirula  has  such  a  shell.  The  Miocene  genus  Spirulirostra  has  its 
phragmacone  endogastrically  bent  but  not  coiled,  and  the  older  Belemnitidce  never 
have  either  curved  or  coiled  shells.  Moreover,  the  shell  of  this  whole  group,  being 
internal  and,  as  far  as  the  original  purpose  of  a  shell,  protection  of  the  body,  is  con- 
cerned, rudimentary,  does  not  come  under  consideration  in  the  present  discussion. 


15. 
In  an  animal  living  in  mud,  like  a  limicolous  bivalve,  there  appears  no  reason 


940 


FIG.  139.— Hypothetical  transition 
form  between  Dentalium  (Fig.  138) 
and  the  racial  form  of  the  Gastropoda 
(Fig.  140),  from  the  left  side. 


FIG.  138. — Dentalium,  diagram  from 
the  left  side,  g,  Genital  gland  ;  W, 
cephalic  tentacles. 


FIG.  140.— Hypothetical  racial  form 
of  the  Gastropoda,  from  the  left  side. 


why  the  shell  should  not  simply  become  elongated,  and  why  the  mantle  cavity  and 
pallial  complex  should  not  retain  the  posterior  position.     Dentcdium  (Fig.  138)  is 


160  COMPARATIVE  ANATOMY  CHAP. 

distinctly  in  this  condition,  being  the  symmetrical  primitive  Gastropod  adapted  to 
life  in  mud,  and  provided  with  a  turret-like  shell  and  posterior  pallial  complex. 
The  perforation  at  the  upper  end  of  the  shell,  which  freely  projects  from  the 
mud,  is  of  great  morphological  importance,  corresponding  physiologically  with  the 
siphons  of  the  limicolous  Lamellibranchia.  A  comparison  between  Dentalium  and 
a  Fissurella  with  its  pallial  complex  twisted  back,  and  with  a  long  and  turret- 
like  shell,  is,  from  our  point  of  view,  very  appropriate.  A  Fissurella,  so  transformed, 
would  almost  exactly  resemble  the  hypothetical  symmetrical  racial  form  of  the  Gas- 
tropoda, in  which,  however,  we  should  have  to  assume  a  mantle-  and  shell-cleft 
reaching  to  their  edges  (cf.  Fig.  136,  I). 

The  anatomy  of  the  Protobranchia,  which  has  recently  been  more  closely  studied, 
and  especially  the  posterior  position  of  the  two  gills,  the  flat  sole  for  creeping,  and 
the  presence  of  the  pleural  ganglia,  justify  us  in  deriving  the  Lamellibranchia  also 
from  the  racial  form  of  the  Gastropoda,  in  which  the  cleft  edge  of  the  mantle  would 
correspond  with  the  posterior  or  siphonal  edge  of  the  mantle  in  the  former.  This 
edge  of  the  mantle,  having  a  similar  physiological  function,  often  possesses  tentacles, 
papillae,  etc.,  in  both  groups. 

Dentalium  further  fits  in  with  our  theory,  for  the  forward  curve  and  the  position 
of  the  columellar  muscle  on  the  anterior  side  of  the  visceral  dome  which  would  be 
disadvantageous  to  a  freely  reptant,  is  not  so  to  a  limicolous,  animal. 

16. 
The  Dextral  and  Sinistral  Twists. 

Most  Gastropods  have  the  visceral  dome  and  shell  twisted  dextrally.  The  direction 
of  .the  twist  has  been  determined  by  the  fact  that  the  visceral  dome  and  shell  origin- 
ally inclined  to  the  left,  and  then  more  and  more  backward,  thus  pushing  the 
pallial  complex  along  the  right  mantle  furrow.  It  cannot  be  determined  why  the 
incline  to  the  left  was  originally  chosen.  The  shell  might  just  as  well  have  inclined  to 
the  right  at  first,  and  then  more  and  more  backward,  pushing  the  pallial  complex  along 
the  left  mantle  furrow.  The  consequent  asymmetry  would  then  have  been  exactly 
reversed.  To  take  a  concrete  example  :  in  a  Monotocardian,  with  visceral  dome  and 
shell  twisted  sinistrally,  the  original  left  parietal  ganglion  would  become  the  supra- 
intestinal  ganglion  on  the  right.  The  original  right  half  of  the  pallial  complex 
would  disappear,  and  the  left  half  which  persisted  would  lie  to  the  right  of  the  anus 
or  rectum,  which  would  take  up  its  position  to  the  left  of  the  median  line. 

Gastropoda  with  sinistrally  twisted  shells  are  actually  known,  many  of  them 
having  the  asymmetrical  organs  in  the  inverse  position  which  corresponds  with  this 
twist.  Such  are,  among  the  Prosobranchia,  Neptunea  contraria,  Triforis,  and  occa- 
sional specimens  of  Buccinum;  among  the  Pulmonata,  Physa,  Clausilia,  Helicter, 
Amphidromus,  and  occasional  specimens  of  Helix  and  Limmaea.  In  Bulimus  per- 
versus,  individual  specimens  with  either  sort  of  shell  are  found,  with  the  special 
asymmetry  of  the  organs  belonging  to  it. 

17. 

There  are,  however,  snails  whose  shells  are  dextrally  twisted,  but  which  possess 
the  organisation  of  animals  with  sinistrally  twisted  shells.  This  is  the  case  among 
the  Prosobranchia  in  the  sinistrally  twisted  sub-genus  Lanistes  of  the  genus  Ampul- 
laria;  among  the  Pulmonata,  in  Choanomphalus  Maacki  and  Pompholyx  solida; 
among  the  Opisthobranchia,  in  those  Pteropoda  which,  whether  as  adults  (Lima- 
cinidce)  or  larvae  (Cymbuliidce),  have  a  twisted  shell.  This  fact  is  entirely  against 
our  theory  in  explanation  of  the  asymmetry  of  the  Gastropoda,  for  this  theory 


vii     MOLLUSCA—THE  ASYMMETRY  OF  THE  GASTROPODA    161 


points  to  a  causal  connection  between  the  spiral  coiling  of  the  visceral  dome  and 
shell  on  the  one  hand  and  the  special  asymmetry  of  the  asymmetrical  organs  on  the 
other.  The  above-mentioned  exceptions  to  the  rule  can,  however,  be  explained  as 
follows.  The  spiral  of  a  dextrally  twisted  shell  can  by  degrees  become  flattened  in 
such  a  way  that  the  shell  may  be  simply  coiled  in  one  plane  or  may  nearly  approach 
that  condition.  In  this  case  the  spiral  might  again  assert  itself,  but  on  the  side 


B 


D 


FIG.  141.— Seven  forms  of  Ampullaria  shells  (diminished  in  various  degrees),  seen  in  the  upper 
row  from  the  aperture  of  the  shell,  in  the  lower  from  the  dorsal  side.  The  head,  foot,  and  oper- 
culum  are  arbitrarily  drawn  merely  for  the  purpose  of  facilitating  a  comparison  between  dextrally 
and  sinistrally  twisted  shells. 

opposite  to  that  on  which  the  umbilicus  originally  lay,  and   in  this  way  a  false 
spiral  might  form  on  the  umbilical  side  and  a  false  umbilicus  on  the  spiral  side. 

The  transition  from  a  dextrally  twisted  to  a  falsely  sinistrally  twisted  shell,  which 
latter  was,  however,  genetically  dextrally  twisted,  is  illustrated  in  Fig.  141  by 
means  of  the  shells  of  seven  species  of  the  genus  Ampullaria.  Ampullaria  Swain- 
soni  Ph  ?  (G)  and  A.  Geveana  Sam  (F)  are  dextrally  twisted  with  distinctly  project- 
ing spiral.  In  A.  crocastoma  Ph  (E)  the  spiral  is  flat,  in  A.  (Ceratodes)  rotula 
Mss.  (D)  and  A.  (Ceratodes)  chiquitensis  d'Orb  (C)  the  spiral  is  already  pushed 
through  or  sunk,  yet  we  find  a  true  umbilicus 
on  the  umbilical  side.  In  A.  (Lanistes)  Bol- 
teniana  Chemn.  (B),  and  still  more  in  A. 
purpurea  Jon.  (A),  the  false  spiral  appeal's  on 
the  umbilical  side,  and  on  the  spiral  side  a  false 
umbilicus  is  found. 

However   plausible   this  explanation  may 
appear,  it  can  only  be  proved  to  be  correct  if 
it   is   found   that   where   a   spiral   operculum 
occurs,  the  direction  of  its  spiral  is  opposite  to 
that  of  the  spiral  of  the  shell  (Fig.  142,  A,  B, 
C),  and  the  commencement  of  the  spiral  is 
always  turned  to  the  umbilical  side  of  the  shell, 
operculum,  but  such  occur  in  the  Pteropoda. 
VOL.  II 


FIG.  142. 


Lanistes  has  not  a  spirally  twisted 
In  those  Pteropods  which  combine  a 


162  COMPARATIVE  ANATOMY  CHAP. 

siriistrally  twisted  shell  with  the  organisation  belonging  to  a  dextrally  twisted 
Gastropod,  the  operculum  exactly  corresponds  with  that  of  a  dextrally  twisted  shell. 
In  Peradis,  in  the  larvae  of  the  Cymbuliidce  and  in  Limacina  retroversa  Flemniing, 
the  operculum  (the  free  surface  of  which  must  be  viewed)  is  sinistrally  twisted,  and 
the  starting-point  of  the  twist  faces  the  (false)  spiral,  which  in  these  falsely  sinistrally 
twisted  Gastropods  lies  in  the  place  of  the  original  umbilicus. 

This  apparent  exception  is  thus  shown  to  be  quite  in  keeping  with  the  rule  above 
established. 


XV.  The  Sensory  Organs. 
A.  Integumental  Sensory  Organs. 

In  the  integument  of  the  Mollusca  there  are  epithelial  sensory  cells 
(Flemming's  cells),  which  vary  in  number  and  arrangement,  and 
may  be  scattered  over  large  areas.  Two  kinds  of  these  cells  may  be 
distinguished  according  to  their  form.  One  kind,  which  is  found  only 
in  Lamellibranchs,  consists  of  large  epithelial  cells  with  large  terminal 
plates  which  form  part  of  the  body  surface  and  carry  tufts  of  pro- 
jecting sensory  hairs  ("paint-brush  cells,"  Pinsel-Zellen).  The  second 
kind  of  cells  are  found  in  all  classes  of  Mollusca.  They  are  long, 
filiform,  or  spindle-shaped,  swelling  at  one  point  where  the  nucleus 
lies.  They  sometimes  carry  a  tuft  of  sensory  hairs,  sometimes  none. 
Each  kind  of  cell  is  continued  at  its  base  into  a  nerve  fibre,  which 
runs  into  the  nervous  system.  A  distinct  specific  function  can  hardly 
be  attributed  to  these  epithelial  cells.  They  may  respond  to  very 
various  stimuli,  chiefly  mechanical  and  chemical,  and  thus  may  act  in 
an  indefinite  way  as  tactile,  olfactory,  and  gustatory  cells. 

They  may  become  more  specialised  in  function,  when  crowded 
together  in  certain  areas  of  the  body,  and  may  then  represent  special 
sensory  organs.  Between  the  individual  cells  composing  such  a 
sensory  organ,  however,  other  epithelial  cells  (glandular,  ciliated, 
and  supporting  cells)  are  always  found. 

1.  Tactile  Organs. 

The  tactile  function  of  the  integumental  sensory  cells  is  likely  to 
assert  itself  at  exposed  parts  of  the  body  surface,  such  as  the  ten- 
tacles, epipodial  processes,  siphons,  at  the  edge  of  the  mantle  in  the 
Lamellibranchia,  and  at  the  edge  of  the  foot,  etc.  We  cannot,  how- 
ever, assume  that  even  in  these  places  the  sensory  cells  are  sensitive 
only  to  mechanical  stimuli. 

2.  Olfactory  Organs. 

(a)  The  Osphradium. 

As  has  been  proved  to  be  the  case  in  the  Prosobranchia,  sensory  cells 
occur  scattered  among  the  other  epithelial  cells  throughout  the  whole 


vii  MOLLUSCA—THE  SENSORY  ORGANS  163 

epithelial  lining  of  the  mantle  cavity.  Here,  as  in  other  parts  of  the 
body,  three  kinds  of  epithelial  cells  can  be  distinguished :  (1)  undiffer- 
entiated  cells,  which  may  contain  pigment,  and  are  usually  ciliated  ; 
(2)  glandular  cells  ;  (3)  sensory  cells.  The  proportions  in  which 
these  three  kinds  of  cells  appear  varies  in  different  regions  of  the 
mantle.  If  glandular  cells  prevail  on  a  certain  area,  that  area  assumes 
a  glandular  character,  and  may  even  develop  into  a  sharply  localised 
epithelial  gland  (e.g.  the  hypobranchial  gland).  On  the  gills,  undiffer- 
entiated  ciliated  cells  predominate.  Where  sensory  cells  predominate 
a  sensory  character  is  given  to  the  region ;  such  a  region,  if  sharply 
circumscribed,  the  sensory  cells  continually  increasing  in  number, 
becomes  a  pallial  sensory  organ.  The  gradual  development  and  con- 
tinuous differentiation  of  such  an  organ  may  be  particularly  well 
traced  in  the  Prosobranchia,  the  sensory  organ  developed  being  the 
osphradium.  In  consequence  of  its  position  in  the  mantle  cavity,  and 
especially  on  account  of  its  proximity  to  the  gill,  it  has  been  assumed 
that  its  principal  function  is  to  test  the  condition  of  the  respiratory 
water,  or,  in  other  words,  that  it  is  an  olfactory  organ. 

The  osphradium  among  the  Prosobranchia  is  least  differentiated  in  the  Dioto- 
fj.i I'dia.  In  the  Fissurdlidcc  it  does  not  exist  as  a  sharply  localised  organ.  In  the 
Mbnotocardia  it  becomes  more  and  more  differentiated,  and  has  a  special  ganglion, 
and  finally  in  the  Toxiglossa,  it  reaches  the  maximum  of  its  development. 

A  review  of  the  position  and  number  ofj;he  osphradia  has  already  been  given  in 
another  place  (§  V.  p.  71).  As  an  example  of  the  special  form  and  structure  of  this 
organ  we  select  the  highly  developed  osphradium  of  a  Toxiglossa,  Cassidaria 
tyrrliciia. 

The  osphradium  of  Cassidaria  is  a  long  organ,  pointed  at  both  ends,  which  lies 
to  the  left  of  the  ctenidium  on  the  mantle  in  the  mantle  cavity.  As  in  other  highly 
specialised  Monotocardia  (Fig.  71,  p.  73)  it  looks  like  a  gill  feathered  on  both  sides,  and 
has  on  that  account  been  regarded  and  described  as  an  accessory  gill.  It  consists  of 
a  ridge  rising  from  the  mantle,  which  in  transverse  section  is  almost  square,  and 
carries  on  each  side  125  to  150  flat  leaflets,  which  stand  at  right  angles  to  the 
surface  of  the  mantle,  and  are  so  closely  crowded  that  their  surfaces  are  in  contact. 
The  ridge  consists  almost  exclusively  of  the  long  osphradial  ganglion.  Each  leaflet 
receives  from  this  ganglion  a  special  nerve,  which  runs  along  its  lower  projecting 
edge,  and  sends  off  four  principal  branches  into  it.  In  its  dorsal  pallial  side  each 
leaflet  contains  blood  sinuses,  which  communicate  with  a  sinus  lying  above  the 
ganglion  in  the  ridge. 

These  principal  nerves  in  the  leaflets  branch,  and  their  last  and  finest  ramifica- 
tions penetrate  the  supporting  membrane  between  the  epithelium  and  the  sub- 
epithelial  tissues.  These  become  connected  with  the  branches  of  the  interepithelial 
ganglion  cells,  each  of  which  again  is  connected  with  a  spindle-shaped  epithelial 
sensory  cell.  The  branched  interepithelial  cells  are  connected  together  by  their 
processes, 

The  sensory  epithelium  above  described  is  developed  on  the  lower  surfaces  of  the 
osphradial  leaflets,  i.  e.  those  turned  to  the  mantle  cavity,  the  indifferent,  non-ciliated 
cells  on  these  surfaces  being  filled  with  granules  of  yellow  pigment,  while  in  the  upper 
surfaces  of  the  leaflets  these  cells  are  devoid  of  pigment  and  ciliated.  Glandular 
cells  are  also  found  definitely  arranged  in  the  epithelium  of  the  osphradial  leaflets. 


164  COMPARATIVE  ANATOMY  CHAP. 

The  osphradial  nerve  usually  springs  from  the  pleuro-visceral  connective  (from 
the  parietal  ganglion  when  this  is  present) ;  in  the  Lamellibranchia  it  comes  from 
the  parieto-visceral  ganglion.  The  osphradial  nerve  is  generally  a  lateral  branch  of 
the  branchial  nerve. 

In  the  Lamellibranchia,  the  important  fact  has  been  demonstrated  that,  although 
the  osphradial  nerve  comes  from  the  parieto-visceral  ganglion,  its  fibres  do  not 
actually  rise  from  this  ganglion  ;  but  they  pass  along  the  pleuro-visceral  connective 
and  have  their  roots  in  the  cerebral  ganglion. 

(b)  Olfactory  Tentacles. 

Certain  experiments,  to  which,  however,  some  exception  might  be 
taken,  seem  to  show  that  the  large  optic  tentacles  of  terrestrial 
Pulmonata  are  also  olfactory.  It  is  also  generally  accepted,  though 
still  not  certainly  established,  that  the  posterior  or  dorsal  tentacles 
(rhinophores)  of  the  Opisthobranchia  are  olfactory  organs.  These 
rhinophores  (Fig.  93,  p.  98)  often  show  increase  of  surface,  usually 
in  the  shape  of  more  or  less  numerous  circular  lamellae  surrounding 
the  tentacle  like  a  collar.  The  rhinophores  are  also  often  ear-shaped 
or  rolled  up  conically.  Not  infrequently  they  can  be  retracted  into 
special  pits  or  sheaths.  They  are  innervated  from  the  cerebral 
ganglion  by  means  of  a  nerve  which  forms  a  ganglion  at  the  base  of 
each. 

At  the  lateral  and  lower  edge  of  the  cephalic  disc  of  the  Cephalaspidce,  an  organ 
which  is  considered  to  have  arisen  by  the  fusion  of  the  labial  and  cephalic  tentacles, 
there  are  structures  which  are  thought  to  be  olfactory,  and  which,  where  most 
developed,  take  the  form  of  several  parallel  "  olfactory  lamellae. "  standing  up  on 
the  disc. 

(c)  Olfactory  Pits  of  the  Cephalopoda. 

In  the  Dibranchia  there  is  on  each  side,  above  the  eye,  a  pit  which 
is  considered  to  be  olfactory.  Its  epithelial  base  consists  of  ciliated 
and  sensory  cells,  and  underneath  it  lies,  close  to  the  optic  ganglion, 
an  olfactory  ganglion.  The  nerves  running  to  this  ganglion  come 
from  the  ganglion  opticum,  but  really  originate  in  the  cerebral 
ganglion.  It  looks  as  if  these  olfactory  organs  were  the  remains  of 
the  posterior  tentacles  of  the  Gastropoda,  and  were  comparable  with 
the  rhinophores  of  the  Opisthobmnchia.  In  Nautilus  the  place  of  the 
olfactory  pit  is  occupied  by  the  upper  optic  tentacle.  We  have 
already  seen  that  Nautilus  still  retains  true  osphradia. 

(d)  The  Pallial  Sensory  Organs  of  the  Lamellibranehia. 

Several  Asiphoniata  have,  in  addition  to  the  osphradia,  epithelial 
sensory  organs,  which  lie  on  small  folds  or  papillae  to  the  right 
and  left  of  the  anus,  between  it  and  the  posterior  end  of  the  gill. 
These  are  innervated  by  a  branch  of  the  posterior  pallial  nerve. 

Epithelial  sensory  organs  of  various  forms  (plates  of  sensory  epithelium,  sensory 
lamellse,  or  papillae,  tufts  of  small  tentacles)  are  found  on  the  mantle  in  the 


vii  MOLLUSCA—THE  SENSORY  ORGANS  165 

Siphoniata  ;  these  lie  ou  the  retractor  muscles  of  the  siphons  aiid  at  the  base  of  the 
branchial  siphon.  These  pallial  sensory  organs  also  are  innervated  by  the  posterior 
pallial  nerves,  and  may  correspond  with  the  anal  sensory  organs  of  the  Asiphoniata. 
Their  function  is  unknown,  but  is  supposed  to  be  analogous  to  that  of  the 
osphradia. 

(e)  Olfactory  Organs  of  the  Chitonidse. 

In  the  mantle  furrow  of  the  Chitonidce  there  are  epithelial  sensory 
organs  which  are  considered  to  be  olfactory.  These  are  ridges  and 
prominences  with  extraordinarily  high  epithelium,  consisting  of 
glandular  cells  and  thread-like  sensory  cells.  In  Chiton  Icevis  and 
C.  cajetanus  there  are,  on  each  side  of  the  mantle  furrow,  two  sensory 
ridges  extending  along  the  whole  length  of  the  row  of  gills ;  one  of 
these,  the  parietal  ridge,  belongs  to  the  outer  wall  of  the  furrow, 
while  the  paraneural  ridge  runs  along  the  base  of  the  furrow,  above 
the  bases  of  the  gills  and  under  the  pleuro-visceral  cord.  The  para- 
neural  ridge  is  continued  a  short  distance  along  the  inner  surface  of 
each  gill,  so  that  each  gill  has  an  epibranchial  sensory  prominence.  In 
front  of  the  first  pair  of  gills  and  near  the  last  the  sensory  cells  in  the 
paraneural  ridge  become  far  more  numerous  in  comparison  with  the 
glandular  cells.  Chiton  sicuhis,  C.  Polii,  and  Acanthochiton  (in  which 
the  numerous  gills  reach  far  forward)  have  no  parietal  and  paraneural 
ridges.  The  sensory  epithelium  in  these  animals  is  confined  to  two 
prominences,  paraneural  in  position,  behind  the  last  pair  of  gills, 
and  connected  with  a  high  epithelium  covering  the  pallial  wall  of 
the  most  posterior  part  of  the  furrow. 

All  these  sensory  epithelia  seem  to  be  innervated  from  the 
pleuro-visceral  cords. 

The  question  as  to  the  relation  of  these  sensory  epithelia  in  the  Chitonidce  to  the 
osphradia  of  other  Molluscs,  which  here  presents  itself,  is  difficult  to  answer.  In 
position  the  osphradia  best  correspond  with  the  epibranchial  prolongations  of  the 
paraueural  ridges  in  Chiton  Icevis  and  C.  cajetanus. 

3.  The  "  Lateral  Organs  "  of  the  Diotoeardia. 

At  the  bases  of  the  epipodial  tentacles  of  Fissurella  and  the 
Trochidce,  and  at  the  base  of  the  lower  tentacles  of  the  epipodial  ruff 
of  Haliotis,  and  also  in  other  parts  near  the  ruff,  sensory  organs  are 
found  which  have  been  compared  with  the  lateral  organs  of  Annelids. 
They  consist  of  patches  of  sensory  epithelium,  which  may  form 
either  spherical  projections  or  pit-like  depressions.  The  epithelium 
of  these  sensory  organs  which  lie  at  the  lower  side  of  the  bases  of  the 
epipodial  tentacles,  consists  of  sensory  cells,  each  of  which  is  provided 
with  a  sensory  seta,  and  pigmented  supporting  cells.  Each  of  these 
sensory  organs  is  innervated  by  the  nerve  of  the  tentacle  near  it, 
which  nerve  originates  in  the  pedal  cord  and  forms  a  ganglion  in  the 
base  of  each  epipodial  tentacle. 


166  COMPARATIVE  ANATOMY  CHAP. 


4.  Gustatory  Organs. 

Folds  and  prominences  found  in  the  mouth  in  some  divisions  of 
the  Mollusca  have  been  taken  for  gustatory  organs,  although  there  are 
no  physiological  and  hardly  any  histological  grounds  for  this  opinion. 
The  existence  of  so-called  gustatory  pits  on  a  prominence  in  the  buccal 
cavity  has  been  proved  only  in  a  few  Chitonidce  and  Diotocardia  (Haliotis, 
Fismrella,  Trochus,  Turbo,  and  Patella).  This  "  gustatory  prominence  " 
(which  has  been  best  examined  in  Chiton)  lies  on  the  floor  of  the 
buccal  cavit}7",  close  behind  the  lip.  A  few  gustatory  pits  are  found 
in  its  epithelium,  sunk  somewhat  below  the  surrounding  epithelium. 
They  consist  of  sensory  cells  with  freely  projecting  sensory  cones,  and 
of  supporting  cells. 

On  each  side  of  the  mouth  in  the  Pulmonata  lies  an  oral  lobe, 
and  under  its  deep  epithelium,  which  is  covered  by  a  thick  cuticle, 
lies  a  ganglion.  Smaller  ganglia  are  found  in  the  small  lobes  at 
the  upper  edge  of  the  mouth.  All  these  ganglia  receive  nerves  which 
radiate  from  a  branch  of  the  anterior  tentacle  nerve.  These  oral 
lobes  (Semper's  organ)  are  considered  to  be  gustatory  organs. 

5.  Subradular  Sensory  Organ  of  Chiton. 

In  the  buccal  cavity  of  Chiton  a  subradular  organ  of  unknown 
physiological  significance  has  been  found.  It  is  described  as  "  a  pro- 
minence lying  below  and  in  front  of  the  radula,"  and  in  shape  re- 
sembles two  beans  with  their  concave  edges  turned  to  one  another, 
the  ends  touching ;  the  space  between  them  forms  a  channel  into 
which  a  small  gland  opens.  Below  this  organ  lie  two  ganglia,  the 
subradular  or  lingual  ganglia  (cf.  section  on  the  nervous  system).  The 
epithelium  of  the  subradular  organ  consists  of  green  pigmented 
ciliated  cells  and  two  kinds  of  sensory  cells.  A  similar  organ  occurs 
in  Patella,  but  has  not  been  thoroughly  examined,  and  at  the  same 
part  in  various  Diotocardia  there  is  a  prominence,  which,  however,  has 
no  sensory  cells.  The  Scapliopoda  also  possess  a  subradular  organ. 

6.  The  Sensory  Organs  on  the  Shell  of  Chiton. 

There  are  numerous  organs  definitely  arranged  on  the  shell  of 
the  Chitonidce  which  have,  no  doubt  correctly,  been  considered  as 
sensory,  i.e.  tactile  organs  (Fig.  143).  They  are  called  aesthetes,  and 
lie  in  pores  on  the  tegmentum  (rf.  p.  39) ;  they  are  club-shaped  or  cylin- 
drical, and  each  carries  a  deep  cup-like  chitinous  cap.  Each  megal- 
sesthete  gives  oif  all  round  numerous  fine  branches  or  miersesthetes, 
each  of  which  ends  in  a  swelling  which  carries  a  small  chitinous 
cap.  The  body  of  the  {esthetes  consists  principally  of  long  cells  like 
glandular  cells ;  it  is  produced  into  a  fibre  which  runs  along  the 
base  of  the  tegmentum,  and  from  here  passes  together  with  the 


VII 


MOLLUSCA—THE  SENSORY  ORGANS 


167 


fibres  of  the  other  aesthetes  of  the  shell-plate,  between  the  tegmentum 
and  articulamentum  to  the  surrounding  pallial  tissue,  or  else  pene- 
trates the  articulamentum. 

The  significance  of  the  separate  constituent  parts  of  the  {esthetes  and  their 
fibrous  strands  is  not  yet  certainly  known.  It  is  probable  that  they  are  innervated 
from  the  dorsal  lateral  branches  of  the  plenro-visceral  cords.  It  is  even  not  known 
whether  the  fibrous  strands  are  their  nerves,  or  whether  the  clear  fibres  running 
through  them  are  long  sensory  cells  whose  nuclei  may  lie  between  the  glandular 
cells,  and  in  connection  with  nerve  fibres. 

We  are  perhaps  justified  in  assuming  that  the  {esthetes  are  merely  modifications 


ttik 


FIG.  143.— Section  of  the  tegmentum  of  Chiton  laevis  showing  an  aesthete  (after  Blumrich). 
ink,  Micnesthete ;  j>cr,  periostracum  ;  sk,  principal  aesthete  ;  t,  tegmentum ;  dz,  cells  resembling 
glandular  cells  ;  hf,  clear  fibres  ;  fs,  fibrous  strand  ;  c,  chitinous  cap. 

of  the  spines  with  their  papillae  and  formative  cells,  which  are  so  common  in  the 
integument  of  the  Chitonidcc.  The  chitinous  cap  would  then  represent  part  of  the 
chitinogenous  base  of  the  spine. 

The  sensory  nature  of  the  aesthetes  is  rendered  highly  probable 
by  the  circumstance  that  in  a  few  species  of  Chiton  individual  megal- 
aesthetes  are  transformed  into  eyes. 

Each  eye  is  furnished  with  a  pigmented  envelope,  which  is  pene- 
trated by  the  micraesthetes,  and  outwardly  covered  by  an  arched 
layer  of  the  tegmentum  which  forms  the  cornea.  Under  this  is  a 
lens,  and  under  this  again  a  cell  layer,  which  is  regarded  as  a  retina, 
and  to  which  is  attached  a  fibrous  strand  (optic  nerve  ?)  corresponding 
with  the  fibrous  strands  of  the  ordinary  aesthetes. 


B.  Auditory  Organs. 

All  Mollusca  except  the  Amphineura  possess  auditory  organs,  which 
appear  very  rarely  in  the  embryo.     They  take  the  form  of  two  almost 


168 


COMPARATIVE  ANATOMY 


CHAP. 


closed  auditory  vesicles  (otoeysts),  whose  epithelial  walls  usually  con- 
sist of  ciliated  and  sensory  cells.  The  interior  of  the  otocyst  is  filled 
with  fluid  and  contains  a  varying  number  of  otoliths  (1  to  over  100). 
These  vary  in  size,  form,  and  chemical  constitution,  and  in  the  living 
animal  oscillate  in  the  fluid  in  which  they  are  suspended. 

The  otoeysts  are  usually  found  on  or  near  the  pedal  ganglia,  rarely 
far  from  it.  It  is,  however,  well  established  that  the  auditory  nerve 
does  not  originate  in  this  ganglion  but  in  the  cerebral  ganglion,  though 
it  often  runs  along  close  to  and  even  in  contact  with  the  fibres  of 
the  cerebropedal  connective. 

In  most  cases  the  otoeysts  arise  as  invaginations  of  the  outer 
epithelium.  An  interesting  discovery  has  recently  been  made,  that  in 

primitive  Lamellibranchs 
(Nucula,  Leda,  Yoldia)  each 
of  the  otoeysts  even  in  the 
adult  still  opens  by  means  of 
a  long  canal  on  the  surface 
of  the  foot.  In  such  cases 
the  otoliths  are  particles  of 
sand  or  other  foreign  matter 
taken  in  from  outside.  In 
Cephalopoda,  the  remains  of 
the  canal  of  invagination  is 
retained  (Kolliker's  canal), 
but  it  ends  blindly. 

The  auditory  organs  are 
most  highly  developed  in 
those  Molluscs  which  are 
good  swimmers,  especially  in 
the  Cephalopoda  and  Hetero- 
poda.  Among  these,  maeulse 
and  eristse  aeustiese  are 
developed. 

Heteropoda. — The  struc- 
ture of  the  auditory  organ 
of  Pterotrachea  (Fig.  144), 
which  has  been  thoroughly 
examined,  is  as  follows  : — 

The  wall  of  the  otocyst  consists  in  the  first  place  of  a  structure- 
less membrane  surrounded  by  muscle  and  connective  tissue.  Inside 
the  vesicle,  which  is  filled  with  fluid,  a  calcareous  otolith,  built  up  of 
concentric  layers,  is  suspended.  The  inner  surface  of  the  vesicle  is 
lined  by  an  epithelium,  containing  three  different  sorts  of  cells : 
auditory,  ciliated,  and  supporting  cells.  The  auditory  cells,  which 
carry  immobile  sensory  hairs,  are  found  on  the  wall  of  the  otocyst  at 
a  point  (macula  acustica)  diametrically  opposite  to  the  place  where  the 
auditory  nerve  enters.  At  this  spot  there  is  a  patch  formed  of 


FIG.  144.  —  Auditory  organ  of  Pterotrachea  (after 
Glaus).  1,  Auditory  nerve  ;  2,  structureless  membrane  ; 
3  and  4,  ciliated  cells ;  5,  otolith ;  6,  auditory  cells ;  7, 
supporting  or  isolating  cells ;  8,  large  central  auditory 


VII 


MOLLUSCA—THE  SENSORY  ORGANS 


169 


numerous  auditory  cells,  and  in  their  midst,  separated  from  the  rest  by 
four  supporting  or  isolating  cells,  one  large  central  auditory  cell. 
On  the  larger  remaining  surface  of  the  wall  of  the  otocyst,  separated 
by  undifferentiated  cells,  are  found  flatter  ciliated  cells,  which  carry 
very  long  cilia  or  setae,  exhibiting  peculiar  movements.  They  some- 
times lie  flat  along  the  inner  wall  of  the  vesicle,  and  at  other  times  (it 
is  said  in  response  to  strong  auditory  stimuli)  stand  upright,  projecting 
towards  the  centre  of  the  vesicle,  and  supporting  the  otolith. 

The  auditory  nerve,  which  enters  the  otocyst  at  a  point  exactly 
opposite  the  central  cell,  at  once  radiates  in  the  form  of  fibres  over  the 
whole  wall  of  the  vesicle  "  as  meridians  radiate  from  the  pole  on  a 
globe,"  finally  innervating  the  bases  of  the  auditory  cells. 

The  two  otocysts  of  the  Cephalopoda  are  still  more  complicated ; 
they  lie  in  two  spacious  cavities  of  the  cephalic  cartilage.  The  sensory 
epithelium  is  here  found  on  a  macula  acustica  and  on  a  kind  of  ridge, 
the  crista  acustica,  which  projects  inwards.  Otoliths  are  only  found 
on  the  macula  acustica.  The  auditory  nerve  divides  into  two  branches, 
one  going  to  the  macula,  and  the  other  to  the  crista  acustica.  Kolliker's 
canal,  above  mentioned,  which  is  internally  ciliated  and  ends  blindly, 
runs  out  of  the  otocyst  as  the  remains  of  the  aperture  of  the  original 
invagination. 

Experiments  made  on  Cephalopods  have  shown  that  one  of  the 
functions  of  the  otocysts  is  to  regulate  the  position  of  the  animal 
while  swimming. 


They  are  cup-shaped 


C.  Visual  Organs. 

1.  Optic  Pits. 

These  are  the  simplest  form  of  visual  organ, 
depressions  of  the  body 
epithelium,  which  at  the 
base  of  the  cup  forms  the 
retina.  The  depression  is 
sometimes  very  shallow,  at 
other  times  deep,  and  like 
a  wide  bottle  with  a  short 
narrow  neck.  The  optic 
nerve  enters  at  the  base  of 
the  depression  and  spreads 
out  over  it.  The  epithelial 
wall  or  retina  consists,  ap- 
parently in  all  Gastropoda, 
Of  two  kinds  Of  long  thread-  FlG  145._Eye  of  Nautilus  (after  Hensen).  1,  Optic 
like  Cells  :  ( 1 )  Clear  Cells  ravity  (pit) ;  2,  layers  of  rods  ;  3,  pigment  layer ;  4,  layer  of 
Without  pigment  and  (2}  visual  cells;  5>  la>'er  of  ganglion  cells  ;  6,  branches  of  the 

pigmented  cells.    Whether 

either  or  possibly  both  of  these  kinds  can  be  considered  as  retinal  cells 


170 


COMPARATIVE  ANATOMY 


CHAP. 


is  still  a  disputed  question.  In  certain  cases  it  has  been  proved  that 
the  pigment  in  the  second  kind  lies  peripherally ;  the  axis  is  free 
from  pigment,  and  may  perhaps  be  considered  as  the  sensitive  portion 
of  the  cell.  In  this  case,  the  clear  cells  would  be  undifferentiated 
supporting  cells,  or  secreting  cells.  The  retina  is  covered,  on  that 
side  of  it  which  faces  the  cavity,  by  a  thick  gelatinous  cuticle,  or  the 
whole  cavity  is  filled  by  a  gelatinous  body  often  called  a  lens.  The 
clear  or  secreting  cells  have  been  thought  to  yield  this  gelatinous  mass, 
but  there  is  a  tendency  to  regard  them  now  rather  as  retinal  cells. 

Optic  pits  are,  among  the  Gastropoda,  only  found  in  such  Diotocardia  as  show 
primitive  characteristics,  e.g.Haliotidce,  Patellidce,  7rochidce,  Delphinulidce,  and  Stoma- 
tiidcc. 

In  connection  with  the  claim  that  Nautilus  (Fig.  145)  is  the  most  primitive  form 
among  extant  Cephalopoda,  it  is  interesting  to  find  that  both  its  eyes  are  optic  pits. 
Each  sensory  cell  of  the  retina,  i.e.  of  the  epithelial  wall  of  the  depression,  possesses 
a  cuticular  rod  projecting  towards  the  cavity,  and  a  layer  of  ganglion  cells  is 
intercalated  between  the  ramifications  of  the  optic  nerve  and  the  retina. 


2.  Optic  Vesicles  or  Vesicular  Eyes. 

Optic  vesicles  are  developed  from  optic  pits  both  ontogenetically 
and  phylogenetically  by  the  approximation  of  the  edges  of  the  pit, 

which  finally  fuse.  A  vesicle  is  thus 
formed,  over  which  there  is  a  continuous 
layer  of  epithelium  (Fig.  146).  The 
outer  epithelium  is  free  from  pigment 
over  the  eye,  and  is  called  the  outer 
cornea,  while  the  immediately  subjacent, 
and  also  unpigmented,  epithelial  wall  of 
the  vesicle  forms  the  inner  cornea.  The 
epithelial  base  of  the  original  depression 
here  again  forms  the  retina ;  its  cells 
contain  distinct  rods  projecting  towards 
the  cavity  of  the  vesicle,  which  is  filled 
with  a  gelatinous  mass.  The  optic 
nerve  usually  swells  into  a  peripheral 
ganglion  opticum  before  reaching  the 
retina. 
FIG.  i46.-Eye  of  a  Puimonate.  i,  The  tentacular  eyes  of  most  Gastro- 

Outer,  2,  inner  cornea ;  3,  body  epithe-  podd,  except  those  DiotoCCirdia  which 
Hum;  4,  vitreous  body;  5,  retina;  6,  have  CUp-Hke  eyCS,  are  of  this  simple 
ganglion  opticum  ;  7,  optic  nerve.  •, 

character. 


3.  The  Eye  of  the  Dibranehiate  Cephalopoda. 

This  is  one  of  the  most  highly-developed  eyes  in  the  whole  animal 
kingdom.     It  is  a  further  development  of  the  cup-shaped  and  vesicular 


VII 


MOLLUSCA—THE  SENSORY  ORGANS 


171 


eyes.     In  the  Tetrabranchiate  ^'auf-ilii*,  as  we  have  seen,  the  cup-shaped 
eye  persists  throughout  life. 

These  lower  stages  (i.e.  the  cup-shaped  and  vesicular  stages)  of  the 
eye  are  passed  through  ontogenetically.  First  a  cup-like  depression  is 
formed  (primary  optic  pit),  then  this  becomes  constricted  to  form  a 
vesicle  (primary  optic  vesicle),  the  inner  wall  of  which  becomes  the 
retina,  while  the  outer  (which  corresponds  with  the  inner  cornea  of 
the  vesicular  eye)  becomes  the  inner  corpus  epitheliale.  This  em- 
bryonic optic  vesicle  then  becomes  further  complicated  \  the  integu- 
ment over  it  (the  outer  cornea  of  the  vesicular  eye)  rises  in  the  form 


FIG.  14'.— Development  of  the  eye  of  the  dibranchiate  Cephalopoda.  1,  Body  epithelium, 
which  becomes  the  outer  corpus  epitheliale  ;  2,  inner  wall  of  the  optic  depression,  which  becomes 
the  retina  ;  3,  outer  wall  of  the  optic  vesicle,  which  becomes  the  inner  corpus  epitheliale  ;  4,  fold 
which  forms  the  iris  ;  5,  fold  which  forms  the  secondary  cornea ;  6,  portion  of  the  lens  formed  by 
the  outer  corpus  epitheliale  ;  7,  portion  of  the  same  formed  by  the  inner  corpus  epitheliale  ;  S,  rod 
layer  of  the  retina. 

of  a  circular  rampart,  and  then  grows  forward  towards  the  axis  of 
the  eye  like  a  diaphragm,  which  forms  the  iris,  the  aperture  left  in 
the  same  being  the  pupil.  The  integument  which  spreads  out  over 
the  circular  base  of  the  iris  is  in  close  contact  with  the  inner  corpus 
epitheliale,  and  becomes  the  outer  corpus  epitheliale. 

The  inner  corpus  epitheliale  forms  towards  the  cavity  of  the  primary 
vesicle  an  almost  hemispherical  lens,  the  outer  corpus  epitheliale  form- 
ing a  similar  lens  outwards  towards  the  pupil.  The  two  hemispheres 
lie  in  such  a  way  as  to  form  something  like  a  complete  sphere  ;  its 


172 


COMPARATIVE  ANATOMY 


CHAP. 


two-fold  origin,  however,  always  remains  evident,  its  equatorial  plane 
being  traversed  by  the  double  lamella  of  the  corpus  epitheliale. 

A  new  circular  fold  grows  over  the  eye,  forming  a  fresh  cavity 
over  it ;  this  is  the  secondary  cornea  of  the  dibranchiate  eye,  which 
must  not  be  confounded  with  the  primary  cornea  of  the  optic  vesicle 
here  represented  by  the  corpus  epitheliale.  In  most  forms  the  circular 
fold  (cornea)  does  not  altogether  close  over  the  eye ;  an  aperture 
remains  through  which  the  water  can  enter  the  anterior  chamber  of 


H  — 


FIG.  148. — Section  of  the  eye  of  Sepia  officinalis,  somewhat  diagrammatic  (after  Hensen). 
1-8,  As  in  Fig.  147  ;  1+3,  corpus  epitheliale  ;  9,  anterior  chamber  of  the  eye  opening  outward  at 
10;  11,  cartilaginous  capsule;  12,  ganglion  opticum= retinal  ganglion;  13,  nervus  opticus  ;  -2c., 
pigment  layer  of  the  retina. 

the  eye.  In  some  animals,  however,  the  secondary  cornea  closes  com- 
pletely. 

We  thus  obtain,  ontogenetically,  some  idea  of  the  general  structure 
of  the  dibranchiate  eye.  A  few  details  of  the  structure  of  the  adult 
eye  are  given  below  (Figs.  148  and  149). 

1.  The  retina  (Fig.  149)  consists  of  two  kinds  of  cells — (1)  pig- 
mented  visual  or  rod  cells,  and  (2)  limiting  cells.  Since  the  nuclei 
of  the  visual  cells  form,  with  relation  to  the  centre  of  the  vesicle,  an 
outer,  and  the  nuclei  of  the  limiting  cells  an  inner  layer,  and  since, 
between  these  two  layers,  a  limiting  membrane  traverses  the  inter- 
stices between  the  retinal  cells,  the  retina  appears  to  be  laminated, 
whereas  it  in  reality  consists  of  one  layer  of  cells.  The  rods  of  the 


VII 


MOLLUSCA—THE  SENSORY  ORGANS 


173 


5 


retinal  cells  lie   on   the  inner  side  of  the  limiting  membrane,  and  are 

thus  turned  to  the  source  of  light  and  at  the  same     

time  to  the  cavity  of   the  primary  vesicle.     The     ^:;^  ;;-.^-^  •    :^     t 
retina  is  covered  on  its  inner  side  by  a  somewhat     !]&&£> 
thick  membrana  limitans. 

2.  The  eye  is  surrounded,  except  on  the  side 
turned  to  the  surface  of  the  body,  by  a  cartila- 
ginous capsule,  which  resembles  the  sclerotica  in 
the  vertebrate  eye  ;  this  cartilage,  where  it  covers 
the  retina,  is  perforated  like  a  sieve,  so  that  the 
optic  nerves  can  pass  through  it. 

3.  Immediately    underneath    the    cartilaginous 
floor  of  the  retina  lies  a  very  large  ganglion  opticum, 
in  the  form  of  a  massive  cerebral  lobe.     From  this 
rise  the  nerves  which  run  to  the  retina  through  the 
perforations  of  the  cartilaginous  capsule. 

4.  The  two  halves  of  the  lens,  which  are  unequal 
in   size   (the   outer   being  the  smaller),  consist  of 
homogeneous  concentric  laminae. 

5.  The  cavity  of  the  primary  vesicle  (between 
the   retina  and   the  lens)  is   filled    with  perfectly 
transparent  fluid. 

It  has  been  proved  that,  as  in  the  Arthropoda 
and  Vertebrata,  the  pigment  granules  of  the  rod 
cells,  which  in  the  dark  lie  at  the  base  of  the  cell, 
under  the  influence  of  light  travel  towards  its  free 
end. 

4.  The  Dorsal    Eyes  of  Oneidium  and  the  Eyes     FIG.  uy.-Two  retinal 
at  the  edge  of  the  Mantle  in  Peeten  (Fig.   ceUs  of  a  cephaiopod. 

much    magnified    (after 

150)  and  SpOndylUS.  Grenacher).      1,    Mem- 

brana  limitans ;   2,  pig- 

These  eyes  have  been  said  to  resemble  vertebrate   ment ;    3,    secreted 
eyes  in  structure,  because  in  them  the  visual  rods   lhreads'>  4>  «erve  fibre; 

*^  5,   rod  \   6,  pi^nncnt  *    * , 

are  turned  away  from  the  light,  being  directed  limiting  ceii ;  s,  limiting 

inwards  tOWardS  the  body.  membrane ;     9,    retinal 

They  are  vesicular  eyes,  but  in  them  it  is  the  c 
outer  wall  of  the  vesicle,  that  turned  to  the  light,  which  becomes  the 
retina,  while  the  inner  wall  (which  in  other  Molluscs  forms  the  retina) 
is  a  pigmented  epithelium.  At  the  same  time  the  outer  or  retinal  wall 
is  invaginated  towards  the  inner  pigmented  wall,  as  is  the  endoderm 
towards  the  ectoderm  in  the  formation  of  the  gastrula.  The  conse- 
quence of  this  is,  that  the  cavity  which  in  other  Mollusca  is  filled  by  the 
gelatinous  mass  (lens)  disappears,  and  the  vesicle  becomes  a  flattened 
thick-walled  plate  (Pedeii)  or  cup  (Oneidium),  consisting  of  a  pigment 
layer  and  a  retina.  The  body  epithelium  which  passes  over  the  eye  is 
unpigmented  and  transparent,  and  here  becomes  the  cornea.  Beneath 


174 


COMPARATIVE  ANATOMY 


CHAP. 


the  cornea,  within  the  optic  cup  or  on  the  plate,  lies  a  cellular  lens, 
which  in  the  dorsal  eyes  of  Oncidium  consists  of  a  few  (5)  large  cells, 
but  in  the  pallial  eyes  of  Pecten  and  Spondylus  of  very  numerous  cells. 


FIG.  150.— Section  through  the  eye  of  Pecten  (after  Patten),  c,  Cornea  ;  I,  lens  ;  ep,  pig- 
mented  body  epithelium  ;  g,  layer  of  ganglion  cells  ;  r,  retina  ;  st,  rod  layer  of  the  retina  ;  d,  tap- 
etuin  ;  e,  pigmented  epithelium ;  /,  sclerotica ;  n,  optic  nerve  ;  n\  and  n-2,  its  two  branches. 

The  development  of  this  lens  is  unknown  ;  it  is  perhaps  formed  by  a 
thickening  or  invagination  of  the  embryonic  ectoderm  which  covers 
the  eye. 

In  Oncidium,  the  optic  nerve  penetrates  the  wall  of  the  optic  cup,  as  in  the 
vertebrate  eye,  to  spread  out  on  the  inner  surface  (with  regard  to  the  centre  of  the 
vesicle)  of  the  retina,  and  to  innervate  the  retinal  cells. 


VII 


MOLLUSCA—THE  SENSORY  ORGANS 


175 


In  Pectcn,  the  optic  nerve  which  runs  to  each  eye  from  the  nerve  for  the  pallial 
edge,  divides,  close  to  the  eye,  into  two  branches.  One  of  these  runs  to  the  base  of 
the  optic  plate,  and  there  breaks  up  into  fibres,  which  radiate  on  all  sides  to  the  edge 
of  the  plate,  then  bend  over  towards  the  retina  to  innervate  some  of  its  cells.  The 
other  branch  runs  direct  to  the  edge  of  the  plate,  there  bends  round  at  a  right  angle 
and  supplies  nerves  to  the  rest  of  the  nerve  cells.  The  fibres  of  this  branch  are  not, 
however,  directly  connected  with  the  retinal  or  rod  cells,  as  there  is  a  layer  of  anas- 
tomosing ganglion  cells  interposed  between  the  two.  Between  the  pigmented  epi- 
thelium and  the  rod  layer  of  the  retina,  a,  tapetum  lucidum  is  found,  which  gives  the 
eye  of  the  Pectcn  its  metallic  lustre. 

Dorsal  eyes  are  found  in  many  species  of  Oncidium.  They  lie  at  the  tips  of  the 
contractile  papilla  found  on  the  dorsal  integument  of  this  curious  Pulmonate  ;  on  each 
papilla  three  or  four  such  eyes  occur.  Besides  these,  Oncidium  has  the  two  normal 
cephalic  eyes  usually  found  in  Gastropods. 

The  pallial  eyes  of  the  Lamellibranchiatcs,  Pectcn  and  Spondyhis,  are  found  in 
large  numbers  on  the  edge  of  the  mantle,  between  the  longer  tentacles,  and  on  the 
tips  of  shorter  tentacles.  The  rods  of  the  retina  in  Pecten,  when  fresh,  are  of  a  very 
evanescent  red  coloiir  (visual  purple  ?). 

5.  The  Eyes  on  the  Shell  of  Chiton. 

These  have  already  been  described  (p.  167).  Their  morphological  significance 
cannot  be  determined  as  long  as  their  development  is  unknown  and  their  histological 
structure  imperfectly  investigated. 


6.  The  Compound  Eyes  of  Area  (Fig.  151)  and  Peetunculus. 

These  are  found  in  great  numbers  at  the  edge  of  the  mantle,  and 
are  epithelial  organs  which  do  not  in  any  way  agree  in  structure  with 
the  other  visual  organs  found  in 
Mollusca,  but  rather  resemble 
certain  simple  Arthropodan 
eyes. 

In  form  they  resemble  an 
externally  convex  shell.  The 
unilaminar  epithelial  wall  of 
the  shell  passes,  at  its  edge, 
into  the  surrounding  pallial 
epithelium.  In  section,  its  com- 
ponent elements  appear  to  be 
arranged  like  a  fan  ("Facher- 
auge  ").  These  elements  are  of 
three  kinds  :  (1)  conical  visual 
cells,  with  their  bases  turned 
outwards;  (2)  a  sheath  of  six 
cylindrical  pigment  shells  surrounding  each  visual  cell.  Each  group, 
consisting  of  one  visual  cell  and  its  surrounding  pigment  cells,  may  be 
considered  as  a  single  eye  or  ommatidium  of  the  simplest  structure,  in 
which  the  retinula  is  represented  by  one  single  visual  cell.  (3)  Slender, 
almost  thread-like  interstitial  cells  which  stand  between  the  ommatidia. 


FIG.  101.— Section  of  the  eye  of  Area  toarbata 
(adapted  from  Rawitz).  1,  Retinal  cells  with  rod-like 
bodies  (2) ;  3,  pigment  cells  ;  4,  slender  interstitial 
cells. 


176  COMPARATIVE  ANATOMY  CHAP. 


7.  Degeneration  of  the  Cephalic  Eyes. 

It  is  becoming  more  and  more  probable  that  the  cephalic  eyes  of  the  various 
Mollusca  are  homologous  structures,  and  that  they  primitively  occurred  in  all  forms. 
They  may,  however,  under  certain  biological  conditions  become  rudimentary,  and 
even  disappear,  as  in  boring  animals  and  those  living  in  mud  or  in  the  deep  sea 
and  in  parasitic  Molluscs.  The  Lamellibranchia  and  CMtonidce  (?)  even  have 
cephalic  eyes  appearing  temporarily  during  development ;  they  disappear  later, 
when,  covered  by  the  shell,  they  are  useless.  They  may  be  replaced  by  secondarily 
acquired  visual  organs  arising  at  more  suitable  parts  of  the  body,  and  thus  we 
have  eyes  on  the  mantle  edge  in  some  bivalves  and  on  the  shell  of  some  Chitonidce. 


XVI.  The  Alimentary  Canal. 

The  alimentary  canal  is  well  developed  in  all  Molluscs,  and  is 
composed  of  (1)  the  bueeal  cavity ;  (2)  the  pharynx  or  cesophageal 
bulb;  (3)  the  oesophagus  or  fore-gut;  (4)  the  mid-gut  with  the 
stomach ;  (5)  the  rectum  or  hind-gut  with  the  anal  aperture.  The 
mouth  originally  lies  at  the  anterior,  and  the  anus  at  the  posterior 
end  or  side  of  the  body,  the  latter  in  the  mantle  furrow  or  cavity. 
The  former  always  retains  its  original  position,  but  the  latter,  as 
central  organ  in  the  pallial  complex,  becomes  shifted  more  or  less  far 
forward  along  the  right  (less  frequently  the  left)  side,  in  the  mantle 
furrow. 

When  the  visceral  dome  grows  out  dorsally  in  such  a  way  that 
the  longitudinal  axis  becomes  shorter  than  the  dorso-ventral  axis,  as 
is  the  case  in  many  Gastropods  and  Cephalopods  and  in  Dentalium,  the 
mid-gut  at  least,  with  its  accessory  gland,  the  so-called  liver,  runs  up 
into  this  dome,  filling  the  greater  part  of  it.  The  intestine  then  forms 
a  dorsal  loop,  consisting  of  an  ascending  portion  running  up  from  the 
fore-gut  and  a  descending  portion  running  down  to  the  anus.  In  the 
Gastropoda,  where  the  anus  is  shifted  more  or  less  forward,  the 
descending  portion  bends  forward  to  the  right  (rarely  to  the  left)  to 
reach  it. 

Besides  this  principal  visceral  loop,  which  is  caused  by  the 
development  of  the  visceral  dome  and  modified  by  the  displacement  of 
the  pallial  complex,  the  intestine,  in  nearly  all  Molluscs,  forms  secondary 
loops  or  coils  which  add  to  its  length.  These  loops  are  found 
principally  in  the  tubular  portion  of  the  mid-gut  which  follows  the 
stomach.  They  are  as  a  rule  most  pronounced  in  herbivorous 
animals,  which  thus  have  longer  alimentary  canals  than  carnivorous 
forms. 

The  large  digestive  gland,  usually  called  the  liver,  enters  the 
stomachal  division  of  the  mid-gut.  Functionally,  this  organ  only 
very  slightly  corresponds  with  the  vertebrate  liver,  if  indeed  it  may 
be  said  to  correspond  at  all  with  that  organ.  It  agrees  more  nearly 


.viz  MOLLUSCA—THE  ALIMENTARY  CANAL  177 

with  the  pancreas,  and  perhaps  combines  the  functions  of  the  different 
specialised  digestive  glands  of  Vertebrates. 

There  is  a  radical  difference  between  Lamellibranchs  and  other 
Molluscs,1  in  the  fact  that  in  the  latter  the  anterior  portion  of  the  fore- 
gut  which  follows  the  buccal  cavity  is  developed  as  a  muscular  pharynx 
(cesophageal  bulb,  buccal  mass),  and  carries  at  its  base  on  a  movable 
lingual  cushion  a  file -like  organ,  the  radula,  which  is  beset  with 
numerous  hard  teeth  composed  of  conchyolin  or  chitin.  The  radula 
serves  chiefly  for  mastication,  but  is  sometimes  used  in  seizing,  holding, 
and  swallowing  prey. 

None  of  the  Lamellibranchs  have  a  pharynx  provided  with  a  radula, 
they  are  therefore  called  Aglossa  as  opposed  to  all  other  Molluscs,  which 
are  Glossoplwra. 

Hard  jaws,  composed  of  conchyolin,  are  almost  always  found  in 
varying  number  and  arrangement  in  the  buccal  cavity  of  the  Glosso- 
phora, but  are  wanting  in  all  Lamellibranchs. 

One  or  two  pairs  of  glands  open  into  the  pharynx  in  the  Glossophora; 
these  are  usually  called  salivary  or  buccal  glands,  although  they  very 
slightly  if  at  all  correspond  physiologically  with  the  glands  so  named 
in  the  Vertebrata.  Glands  may  also  open  into  the  buccal  cavity.  The 
Lamellibranchs  have  no  salivary  glands. 

The  absence  of  the  pharynx,  tongue,  jaws,  and  salivary  glands  in 
the  Lamellibranchia  is  accounted  for  by  their  manner  of  life.  They  do 
not  have  to  seek  their  food.  Some  of  them  are  attached  and  others 
feed  in  the  same  way  as  attached  animals  on  small  particles  suspended 
in  the  respiratory  water  (animalculse,  microscopic  algae,  and  particles  of 
detritus)  ^yhicll  are  brought  to  the  mouth  by  means  of  ciliary  move- 
ment. These  fine  particles  require  no  mastication  before  being 
swallowed. 

This  method  of  feeding  also  affects  the  outer  organisation  of  the 
Lamellibranchia,  which  have  lost  the  cephalic  portion  of  the  body  with 
the  tentacles  and  eyes :  Aglossa  =  Aeephala  =  Lipoeephala,  and 
Glossophora  =  Cephalophora. 

In  some  Gastropoda  (Murex,  Purpura)  and  in  Dentalium  there  is  in 
connection  with  the  last  part  of  the  hind-gut  an  anal  gland,  and  in 
the  Cephalopoda  (excepting  Nautilus)  a  gland  known  as  the  ink-bag. 

The  alimentary  canal  of  the  Mollusca  runs  through  the  primary 
and  often  also  through  the  secondary  body  cavity,  attached  in  various 
ways  by  fibres  or  bands  of  connective  tissue.  Its  walls  consist  of  an 
inner  epithelium  usually  to  a  great  extent  ciliated,  an  outer  muscular 
layer  in  which  longitudinal  and  circular  fibres  occur,  not  always  in 
regular  layers,  and,  where  it  passes  through  the  primary  body  cavity, 
an  outer  envelope  of  connective  tissue. 

The  pharynx  and  perhaps  sometimes  part  of  the  oesophagus,  and  a 
part,  in  all  cases  very  short,  of  the  hind-gut,  arise  ontogenetically  out  of 
the  ectodermal  stomodaeum  and  proctodseum.  But  the  exact  limits 

1  For  the  rare  exceptions  to  this  rule,  see  p.  183. 
VOL.  II  N 


178  COMPARATIVE  ANATOMY  CHAP.- 

of  the  ectodermal  and  the  endodermal  portions  of  the  intestine  are 
difficult  to  determine. 


A.  Buccal  Cavity,  Snout,  Proboscis. 

The  alimentary  canal  has  an  oral  aperture  bordered  by  variously  -  shaped  lips, 
and  in  many  Glossophora  (in  nearly  all  Gastropoda)  leads  into  a  vestibule  or 
anterior  cavity  roofed  over  by  the  lips  and  lined  by  a  continuation  of  the  outer  wall 
of  the  head.  The  dermal  glands  are  not  unfrequently  (many  Opisthobranchia  and 
a  few  Prosobranchia)  more  strongly  developed  on  the  lips  as  labial  glands.  In 
many  Gastropods,  when  the  lips  open,  the  mouth  is  able  to  seize  and  hold  prey  like 
a  sucker. 

"Where  the  snout  is  short  it  is  simply  contractile  (the  Chitonidce,  the  Dioto- 
cardia,  most  herbivorous  Tcenioglossa,  and  many  Pulmonata  and  Nudibranchia). 
In  this  case  the  parts  immediately  surrounding  the  mouth  are  so  strongly  con- 
tractile that  when  contraction  takes  place  the  mouth  is  drawn  in  somewhat  so  as 
to  lie  at  the  base  of  a  depression.  An  exaggeration  of  this  arrangement,  combined 
with  the  prolongation  of  the  snout,  leads  to  the  formation  of  the  retractile  or 
proboscidal  snout.  The  snout  can  in  such  cases  be  invaginated  from  its  tip,  i.e. 
from  the  oral  aperture  into  the  cephalic  cavity,  the  mouth  then  lying  at  the  base  of 
the  invagination  (many  Tectibranchia,  Capulidce,  Strombidce,  Chenopidce,  Calyptrceidce, 
Cypraeidoc,  Lamellariidce,  Naticidce,  Scalaridce,  Solariidce). 

Finally,  in  many  carnivorous  Prosobranchia  (Tritoniidce,  Doliidce,  Cassididce, 
Rachiglossa,  and  a  few  Toxoglossa]  a  proboscis,  often  very  long  and  enclosed  in  a 
special  proboscidal  sheath,  is  developed  (Figs.  71  and  152) ;  this  sheath  lies  in  the 
cavity  of  the  head,  which  is'  often  prolonged  like  a  snout,  and  may  even  stretch  back 
into  the  body  cavity.  The  oral  aperture  lies  at  the  free  anterior  end  of  the 
cylindrical  proboscis,  and  we  have  to  regard  the  proboscis  with  its  sheath  as  a  very 
long  snout,  the  base  of  which,  however,  is  permanently  invaginated  into  itself.  In 
this  way  the  proximal  portion  of  the  snout  forms  the  permanent  proboscidal  sheath, 
while  the  distal  portion  with  its  terminal  oral  aperture  forms  the  proboscis.  Neither 
of  these  portions  can  be  invaginated  or  evaginated  ;  it  is  merely  a  zone  lying 
between  them  which  takes  part  in  the  retraction  of  the  proboscis  into  the  body 
cavity.  This  zone,  when  so  invaginated,  forms  a  temporary  backward  prolongation 
of  the  proboscidal  sheath,  but  when  the  proboscis  is  protruded  forms  the  basal 
portion  of  the  latter.  The  permanent  portion  of  the  proboscidal  sheath  is  connected 
with  the  wall  of  the  head  by  bands  which  make  its  evagination  impossible,  and  the 
inner  wall  of  the  permanent  proboscis  is  connected  by  muscles  or  bands  with  the 
(esophagus  lying  within  it,  so  that  this  portion  of  the  organ  cannot  be  invaginated  ; 
the  oral  aperture  can  thus  never  lie  at  the  base  of  the  proboscidal  sheath. 

When  the  proboscis  is  retracted,  there  is  therefore  an  aperture  at  the  anterior 
end  of  the  snout  or  the  head,  which  is  not  the  oral  aperture,  but  that  of  the 
proboscidal  sheath.  When  the  proboscis  is  protruded,  it  projects  beyond  the 
aperture  of  the  sheath  and  carries  at  its  point  the  oral  aperture. 

The  proboscis  is  retracted  by  means  of  muscles  attached  at  the  one  end  to  the 
body  wall  and  at  the  other  to  its  (invaginable)  base.  In  its  protrusion,  a  flow  of 
blood  towards  the  snout  probably  plays  the  chief  part,  accompanied  by  contraction 
of  the  circular  muscles  of  the  head  and  proboscis. 

The  (carnivorous)  Pteropoda  gymnosomata  also  have  a  protrusible  proboscis  (Fig. 
17,  p.  11)  provided  with  so-called  buccal  appendages.  The  same  is  present  in  the 
allied  Aplysiidcr,,  but  is  weakly  developed.  The  Thecosomata  have  no  proboscis. 

The  buccal  cavity   of  Dentalium  is  noteworthy.     It   extends  throughout  the 


VII 


MOLLUSCA—THE  ALIMENTARY  CAXAL 


179 


whole  length  of  the  freely-projecting  egg-shaped  snout,  which  carries  leaf-like  labial 
appendages.  On  each  side  of  the  buccal  cavity  there  is  a  pouch,  the  so-called  cheek 
pouch,  which  is  lined  with  glandular  epithelium  and  opens  into  the  cavity  anteriorly. 


FIG.  152.— Diagram  of  the  proboscidal  apparatus  of  the  Prosobranchia.  A,  proboscis 
retracted.  B,  The  same  protruded,  a-c,  Cephalic  integument ;  c,  edge  of  the  aperture  of  the 
proboscidal  sheath  ;  c-d,  immovable  wall  of  the  proboscidal  sheath ;  d-e,  movable  (evaginable  and 
invagiuable)  wall  of  the  same  ;  e-f,  immovable  wall  of  the  proboscis  ;  /,  edge  of  the  oral  aperture,  at 
the  anterior  end  of  the  proboscis ;  g,  pharynx ;  h,  oesophagus  ;  i,  retractor  muscle ;  k,  salivary 
glands  ;  Z,  cephalic  cavity. 

An  exact  comparative  investigation  of  the  mechanism  of  the  proboscidal 
apparatus,  the  contractile  snout,  etc.  of  the  Prosobranchia  is  still  a  desideratum. 

There  are  other  forms  of  proboscis,  differing  greatly  from  the  one  just  described 
(e.g.  that  in  the  Terebridce). 

In  the  Hctcropoda,  the  head  forms  a  long  snout  which  is  often  described  as  a 
proboscis.  The  name  is  inappropriate,  as  this  snout  is  not  retractile  and  the  mouth 
is  always  found  at  its  anterior  end. 


180  COMPARATIVE  ANATOMY  CHAP. 


B.  The  Pharynx  and  Jaws,  the  Tongue  and  Salivary  Glands. 

The  mouth  or  buccal  cavity  is  followed  in  all  Molluscs  except  the 
Lamellibranchia  by  the  pharynx  or  oesophageal  bulb  (buccal  mass). 
The  pharyngeal  cavity  opens  anteriorly  into  the  buccal  cavity,  and 
posteriorly  into  the  oesophagus.  The  pharynx  is  characterised  by  the 
possession  of  (1)  jaws,  which  lie  anteriorly  at  the  boundary  between 
the  buccal  and  pharyngeal  cavities ;  (2)  a  lingual  apparatus  at  its 
base,  and  (3)  salivary  glands,  which  usually  open  laterally  near  its 
posterior  boundary. 

1.  Jaws   are   almost  universal,   and   are  sometimes,   especially  in 
carnivorous  animals,  very  highly  developed ;  less  frequently  they  are 
rudimentary  or  wanting.     They  are  hard  cuticular  formations  of  the 
epithelium  of  the  anterior  pharyngeal  region,  and  no  doubt  composed 
of  conchyolin  or  some  related  substance,  in  a  few  cases  hardened  by 
calcareous  deposits  (e.g.  Nautilus}. 

The  jaws  serve  for  seizing  prey  or  particles  of  food.  The  great  variations  in 
number,  form,  and  arrangement  of  the  jaws  can  best  be  understood  by  assuming 
that  they  originally  extended  completely  round  the  entrance  to  the  pharynx  ;  and 
that  of  this  ring  sometimes  only  upper  and  lower  or  sometimes  only  lateral  portions 
have  been  retained. 

Such  a  complete  circle  of  jaws  is  found  at  the  entrance  to  the  pharynx  in  some 
forms,  such  as  Umbrella  and  Tylodina  (Opisthobranchia}. 

The  fresh-water  Pulmonates  have  an  upper  and  two  lateral  jaws. 

Most  Prosobranchia  and  Opisthobranchia  have  two  lateral  jaws.  These  may 
approach  so  near  one  another  as  almost  to  touch  (Haliotis,  Fissurella).  Terrestrial 
Pulmonata  have  an  upper  jaw  and  occasionally  a  weak  lower  jaw  as  well. 

The  jaws  are  particularly  strongly  developed  in  the  Cephalopoda,  which  have  an 
upper  and  a  lower  jaw,  the  two  together  resembling  in  shape  the  beak  of  a  parrot. 

In  the  Opisthobranchiate  family  Aplysiid-ce,  Notarchus,  Accra,  Dolabella,  and 
Aplysiella  have,  besides  the  lateral  jaws,  numerous  hooks'  or  small  teeth  on  the  roof  of 
the  pharyngeal  cavity.  The  hook  sacs  (Fig.  17,  p.  11)  of  the  Pteropoda  gymnoso- 
mata,  which  are  wanting  only  in  Halopsychc,  are  perhaps  to  be  derived  from  these 
pharyngeal  teeth. 

The  hook  sacs  are  paired  dorsal  outgrowths  of  the  pharyngeal  cavity,  which  vary 
in  length  and  lie  in  front  of  the  radula.  The  walls  of  the  sacs  carry  hooks  project- 
ing inward.  When  the  proboscis  of  these  carnivorous  animals  is  protruded,  the  sacs 
are  completely  evaginated,  so  that  the  hooks  come  to  lie  outside  (Fig.  17,  p.  11). 

Jaws  are  wanting  or  rudimentary  in  the  Amphincura  and  the  Scaphopoda ; 
among  the  Prosobranchia,  in  the  Toxoglossa,  Pyramidcllidcc,  Eulimidoc,  many 
Trochidce,  the  Heteropoda,  and  in  many  Nudibranchia  (Tethys,  Melibe,  Doridopsis, 
Phyllidia] ;  in  the  Ascoglossa,  and  in  certain  Tcctibranchia  (Actceon,  Doridium, 
Philine,  Utriculus,  Scaphander,  Lobiger}.  Among  the  Pulmonata  they  disappear 
in  a  series  of  Testacellidce,  being  present  in  Daudebardia  rufa,  rudimentary  in 
D.  Saulcyi,  and  wanting  in  Tcstacclla. 

2.  The  lingual  apparatus  (Figs.  153,  154)  is  highly  characteristic 
of  all  Molluscs  except  the  Lamellibranchia,  i.e.  of  all  Glossopliwa.     It 
may  be  said  that  every  animal  with  a  radula  is  a  Mollusc. 


VII 


MOLLUSCA—THE  ALIMENTARY  CANAL 


181 


The  ventral  and  lateral  walls  of  the  pharynx  are  thickened  and 
very  muscular.  On  the  floor  of  the  cavity  rises  a  tough  longitudinal 
muscular  cushion,  the  tongue.  Its  surface,  which  projects  into  the 


FIG.  153.— Longitudinal  section  (not  quite  median)  through  the  snout  of  a  Prosobranchiate, 
to  illustrate  the  pharyngeal  apparatus.  1,  Dorsal  wall  of  the  head;  2,  mouth;  3,  jaw;  4, 
raclula  ;  5.  lingual  cartilage ;  6,  muscular  wall  of  the  pharynx  ;  7,  muscles  attached  at  one  end  to  the 
pharynx  and  at  the  other  to  the  ventral  wall  of  the  head  (S) ;  9,  cavity  of  the  head ;  10,  radular 
sheath  ;  11,  oesophagus  ;  12,  aperture  of  the  salivary  gland  ;'13,  infolding  behind  the  radular  sheath. 

pharyngeal  cavity,  is  covered  by  a  rough  cuticle  consisting  of  chitin 
(or  conchyolin  ?) ;  on  this  basal  membrane  are  found  very  numerous 
hard  chitinous  teeth,  often  many  thousands,  arranged  in  close  transverse 


sin 

FIG.  154.— Median  longitudinal  section  through  the  anterior  part  of  the  body  of  Helix 
(after  Howes),  a-,  (Esophagus ;  rd,  radular  sheath ;  nc,  cerebral  ganglion ;  sl-2,  aperture  of  the 
salivary  gland  ;  oc,  muscle  mass  in  the  ventral  pharyngeal  wall ;  rd,  radula  ;  hj,  upper  jaw  ;  Zls  12, 
lips  of  the  oral  aperture;  im,  pharyngeal  muscles;  rwio,  retractors  of  the  pharynx;  pgl,  pedal 
gland. 

and  longitudinal  rows.     The  basal  membrane  and  the  teeth  together 
form  the  radula  of  the  tongue. 

The  anterior  end  of  the  tongue  projects  freely  into  the  pharyngeal 
cavity,  the  radula  bending  down  over  this  end  so  as  to  cover  for  a 


182  COMPARATIVE  ANATOMY  CHAP. 

certain  distance  its  lower  surface.  Immediately  in  front  of  the  tongue 
there  is  always  a  depression  in  the  ventral  pharyngeal  wall,  forming 
a  sort  of  pocket.  The  radula,  at  the  posterior  extremity  of  the 
tongue,  sinks  into  a  narrow  more  or  less  long  tube,  the  radular  sheath, 
which  is  an  outgrowth  of  the  pharyngeal  cavity  running  downward 
and  backward.  The  radula,  always  lying  upon  the  anterior  or 
ventral  wall  of  this  sheath,  which  is  anteriorly  thickened  to  form 
the  tongue,  extends  to  the  base  of  the  sheath,  which  is  the  place 
of  its  formation. 

The  tongue  with  the  radula  on  it  is  movable,  and  in  most  cases  its  movements  can 
be  compared  with  those  of  the  cat's  tongue  when  licking,  but  are  usually  slower. 
This  action  helps  to  rasp  the  food  which  has  been  seized,  and  often  also  broken  up,  by 
the  jaws.  The  tongue  can  either  move  inside  the  pharyngeal  or  buccal  cavities,  or 
can  be  extended  to  the  oral  aperture  or  even  protruded  more  or  less  far  beyond  it. 

In  or  under  the  fleshy  tongue,  a  lingual  cartilage  is  very  commonly  found, 
consisting  of  two  or  four  or  even  more  pieces.  This  cartilage  forms  a  support  for 

the  radula,  and  affords  firm  points  of  attachment 
for  certain  muscles  belonging  to  the  lingual 
apparatus. 

The  musculature  of  the  pharynx,  which  can 
be  separated  into  bundles  or  strands,  and  is  often 
very  complicated,  consists  first  of  the  muscles 
which  form  the  wall  of  the  pharynx,  and  which, 
being  principally  developed  ventrally  and  laterally 
round  the  radula,  determine  the  special  licking 
movements  of  the  tongue  ;  secondly,  of  muscles 

wufch  rao™ the  whole  ?"»7" or  the  1'ole  °,f  the 

lingual  apparatus,  evagmating  or  protruding  them. 
The  second  group  consists,  speaking  generally,  of  protractors  and  retractors,  attached 
at  the  one  end  to  the  pharynx  and  at  the  other  to  the  body  wrall  after  running 
through  the  cephalic  or  body  cavity.  Pressure  of  blood  may  also  take  some  part  in 
the  protrusion  of  the  pharynx. 

The  tongue  and  radula  further  often  serve  for  seizing  prey  (e.g.  in  the  carnivorous 
Heteropoda,  in  Testacclla,  etc.). 

The  radula  is  of  great  importance  in  classification.  Further  details  concerning 
it  must  be  sought  in  special  works  and  in  text-books  of  conchology.  The  points  to 
be  specially  noticed  are  (1)  the  size  and  form  of  the  whole  radula,  (2)  the  number  of 
longitudinal  and  transverse  rows  of  teeth,  and  (3)  the  form  of  the  teeth  in  each  of 
these  rows.  As  a  rule  the  transverse  rows  resemble  one  another,  but  exceptional 
rows  differently  constituted  from  those  immediately  preceding  or  following  them 
recur  at  intervals. 

Three  kinds  of  teeth  have  been,  as  a  rule,  distinguished.  First,  there  is  usually 
a  single  median  longitudinal  row  of  central  or  rachial  teeth.  On  each  side  of  this 
row  are  several  rows  of  more  or  less  similar  lateral  teeth  or  pleurae.  Finally,  at  the 
lateral  edges  of  the  radula,  there  are  single  or  very  numerous  longitudinal  rows  of 
marginal  teeth  or  uncini. 

Dental  formulae  are  used  for  the  radular  teeth,  in  the  same  way  as  for  the  teeth 
of  mammals  ;  in  these  the  number  of  central,  lateral,  and  marginal  teeth  in  a 
transverse  row  are  given. 

The  reader  will  find  the  dental  formula  of  some  of  the  Molluscs  in  the  Systematic 
Review. 


vii  MOLLUSCA—THE  ALIMENTARY  CANAL  183 

The  total  number  of  radular  teeth  varies  very  greatly,  from  16  in  Eolis  Drum- 
mondi  to  39,596  in  Helix  Ghietsbrcghti. 

As  a  rule,  the  teeth  are  most  numerous  and  finest  in  herbivorous  animals.  In 
carnivorous  Molluscs  we  have  two  extremes  :  (1)  great  development  of  the  proboscis, 
with  weak  development  of  the  pharynx  and  radula,  and  a  comparatively  small 
number  of  teeth  (carnivorous  Prosobranchia}  ;  (2)  absence  of  a  protrusible  proboscis, 
with  great  development  of  the  pharyngeal  apparatus  and  the  radula,  and  numerous, 
often  large,  teeth  (ffeteropoda,  carnivorous  Pulmonata  and  Cephalopoda). 

The  muscular  pharynx  is  most  developed  in  carnivorous  Piilmonates.  In  these 
it  may  be  half  (Daudebardid)  or  even  more  than  half  as  long  (Testacella)  as  the 
whole  body,  and  may  occupy  a  very  large  part  of  the  body  cavity.  It  is  protruded 
in  such  a  way  that  the  tongue  with  the  radula  occupy  the  anterior  end  of  the 
originated  pharynx  (Fig.  54,  A,  p.  44). 

In  very  rare  cases  (apart  from  the  Lamellibranchia]  the  radula  completely 
atrophies  ;  this  is  the  case  in  parasitic  Gastropoda  (Stilifer,  Eulima,  Thyca,  Ento- 
concha],  in  the  ComUiophilidce  (Coralliophila,  Leptoconchus,  Magilus,  Rhizochilus) , 
among  the  Nudibranchia  in  Tethys  and  Melibe,  among  the  Amphineura  in  Neomenia, 
and  certain  species  of  the  genera  Dondersia  and  Proneomenia.  In  Chcetoderma,  a 
single  tooth  of  the  radula  is  retained. 

Even  in  certain  carnivorous  Prosobranchia  which  are  furnished  with  a  proboscis, 
the  above-mentioned  reduction  of  the  whole  pharyngeal  apparatus  goes  so  far  that 
the  radula  disappears  (certain  species  of  Terebra). 

Formation  of  the  Radula. 

The  teeth  of  the  anterior  transverse  rows  of  the  radula  become  worn  out  by  use, 
and  are  continually  being  replaced  by  new  teeth  which  are  pushed  forward.  The 
formation  of  new  transverse  rows  of  teeth 
is  constantly  taking  place  at  the  posterior 
blind  end  of  the  radular  sheath.  In  Pul- 
monata and  Opisthobranchia  they  appear  as 
cuticular  formations  secreted  by  several 
transverse  rows  of  large  epithelial  cells — 
the  odontoblasts  (Fig.  156)  ;  the  basal 
membrane  which  carries  the  teeth  is  secreted 
by  the  anterior  row  or  rows,  the  teeth  them- 
selves by  the  posterior  rows. 

Each  group  of  odontoblasts  which  has  FlG  156._Longltudil,al  section  through 
formed  a  tooth  is  not  replaced  by  another,  the  posterior  end  of  the  radular  sheath  of 
but  continues  to  produce  new  teeth  behind  a  Pulmonate  (after  Rossler),  diagram.  1,  2, 
those  already  formed,  so  that  for  each  longi-  3>  4'  Formative  cells  of  the  radular  teeth ;  5, 
,1.1  c  .  , ,  , ,  .  ,  ,  •,  ,  f  formative  cells  of  the  basal  membrane ;  6.  7, 

tudmal  row  of  teeth  there  is  at  the  base  of   ^  of  ^  ^^  .  g>  ^  membrane; 

the  radular  sheath  a  group  of  odontoblasts 

which  has  produced  all  the  teeth  belonging  to  that  row.     A  layer  of  ' '  enamel "  is 

deposited  on  the  teeth  so  formed  by  the  epithelial  roof  of  the  radular  sheath. 

In  the  Chiton  idee,  Prosobranchia,  and  Cephalopoda,  the  odontoblasts  are  very 
numerous  narrow  cells,  which  form,  at  the  base  of  the  sheath,  a  cushion  divided  into 
as  many  parts  as  there  are  teeth  in  a  transverse  row  of  the  radula. 

The  radular  sheath  in  the  Pulmonata,  Scaphopoda,  Opisthobranchia,  and  Cephalo- 
poda is  short,  and  is  contained  in  the  ventral  and  posterior  muscular  wall  of  the 
pharynx,  very  seldom  projecting  posteriorly  beyond  it ;  but  in  many  Prosobranchia 
it  is  long  and  narrow,  and  reaches  back  into  the  cephalic  cavity  or  even  right  into 
the  body  cavity.  This  latter  is  especially  the  case  in  the  Diotocardia ;  in  the 


184  COMPARATIVE  ANATOMY  CHAP. 

Docoglossa  (Patella}  the  sheath,  which  lies  above  the  foot  on  the  floor  of  the  body 
cavity,  is  even  longer  than  the  body  (Fig.  158). 

3.  Salivary  glands  (buccal  glands,  pharyngeal  glands)  are  universally 
found  in  Glossophora,  i.e.  in  Molluscs  which  have  a  pharynx  and  lingual 
apparatus.  They  are  universally  absent  in  Lamellibranchs.  They  may 
occur  in  one  or  two  pairs.  The  posterior  or  in  other  cases  the  only 
pair  often  lies  on  the  wall  of  the  oesophagus,  and  sends  forward  two 
ducts  which  enter  the  pharynx  laterally,  usually  somewhat  behind  the 
point  where  the  radular  sheath  opens  into  the  pharyngeal  cavity. 
Very  little  is  known  of  the  function  of  these  glands ;  an  exact 
morphological  comparison  of  the  various  pharyngeal  glands  of  the 
Gastropoda  is  at  present  hardly  possible. 

Amphineura.  (a)  Chiton. — Two  small  delicate  buccal  glands  lie 
on  the  roof  of  the  buccal  cavity  and  open  into  the  mouth.  They  can 
therefore  hardly  be  regarded  as  pharyngeal  or  salivary  glands. 

(b)  Solenogastres. — Salivary  glands  are  here  found  in  all  genera 
except  Neomenia,  and  in  Chcetoderma.  They  are  present  in  some  species 
but  appear  to  be  absent  in  others.  A  pair  of  long  glandular  tubes 
with  high  glandular  cells 1  and  strong  muscular  walls  lie  anteriorly  under 
the  intestine  and  are  produced  in  the  form  of  two  narrow  ducts,  which 
enter  the  pharyngeal  cavity  on  the  tongue  either  separately  or  through 
a  common  terminal  portion.  Besides  these  there  is  another  pair  in 
some  species  (Paramenia  impexa,  Param.  palifera,  Proneomenia  vagans, 
Dondersia  flavens) ;  the  ducts  of  these  open  together  through  an  un- 
paired terminal  portion  on  the  dorsal  wall  of  the  pharyngeal  cavity, 
at  the  point  of  a  papilla  which  rises  from  the  base  of  a  pit-like 
depression. 

Gastropoda,  (a)  Prosobranehia. — In  most  cases  there  is  only 
one  pair  of  salivary  glands.  These  are  usually  lobed  or  branched 
glandular  masses,  which  lie,  in  the  Diotocardia,  at  the  sides  of  the 
pharynx,  in  the  Monotocardia,  at  the  sides  of  the  oesophagus.  In  the 
former  case,  the  ducts  are  short  and  do  not  pass  through  the  oesophageal 
ring  formed  by  the  nerve  centres  and  their  connectives  and  commis- 
sures, which  in  these  forms  surrounds  the  anterior  end  of  the  pharynx. 
In  the  Monotocardia,  the  ducts  are  long,  and  generally  accompany  the 
oesophagus  through  the  oesophageal  ring  (which  lies  behind  the  pharynx), 
and  open  on  the  posterior  lateral  wall  of  the  latter. 

Two  pairs  of  salivary  glands  are  found  in  certain  Diotocardia  (e.g. 
Haliotis,  Fissurella),  and  further  in  Patella,  the  Scalariidce,  lanthinidte, 
certain  Purpuridce,  Muricidce,  and  in  the  Cancellariidce. 

One  of  the  two  pairs  of  glands  in  Haliotis  is  developed  in  the  form 
of  large  lateral  glandular  sacs  covering  the  pharynx  on  the  right  and 
left  (Fig.  105,  p.  121). 

In  the  Ampullariidce,  the  ducts  of  the  salivary  glands  do  not  pass 

1  This  differs  somawhat  from  the  description  found  in   Simroth  (Bronn's  Klasien 
und  Ordnungen,  vol.  iii.  pp.  183-185). 


vii  MOLLU8CA—THE  ALIMEXTARY  CAXAL  185 

through  the  oesophageal  ring,  which  here,  as  in  the  Diotocardia,  sur- 
rounds the  anterior  end  of  the  pharynx. 

Whereas  the  salivary  glands  are,  as  a  rule,  branched  tubes  or 
acinose,  they  are  sometimes  (Scalariidce,  lanthinidce,  Cancellanidce) 
simply  tubular  or  (Doliidce,  Xemphoridce,  etc.)  sac-like. 

The  passage  of  the  ducts  of  the  salivary  glands  through  the 
cesophageal  ring  in  the  Monotocardia  may  have  come  about  by  the 
shifting  back  of  the  ring  along  the  pharynx  from  its  former  position 
in  the  Diotocardia,  where  it  encircles  the  anterior  end  of  the  pharynx 
in  front  of  the  apertures  of  these  ducts.  The  salivary  ducts  would 
thus  necessarily  become  surrounded  by  the  ring. 

The  ducts  in  the  Monotocardia  become  the  longer  the  further  the 
nerve  ring  shifts  back  from  the  mouth  and  pharynx.  They  are  very  long 
in  animals  provided  with  a  protrusible  proboscis,  where  the  ring  lies 
far  back  on  the  oesophagus,  behind  the  non-evaginable  portion  of  the 
proboscis.  The  ducts  here  run  along  the  whole  length  of  the  latter. 
But  in  those  cases  in  which  the  cesophageal  ring  has  shifted  back  more 
quickly  than  the  ducts  have  lengthened,  the  glands  lie  in  front  of  the 
ring.  In  the  event  of  the  subsequent  lengthening  of  their  ducts,  the 
glands  might  stretch  back  outside  the  ring.  The  arrangement  of 
the  glands  in  the  Toxoglossa  and  Rachiglossa  would  thus  be  explained ; 
here  the  greater  part  of  the  glands  lies  behind  the  ring,  although 
the  ducts  are  said  not  to  pass  through  it. 

The  acid  secretion  of  the  salivary  glands  of  certain  Prosobranehia 
(species  of  Dolium,  Cassis,  Cassidaria,  Tritonium,  Murex)  and  Opistho- 
Iranchw.  (Pleuro^ranclius,  Pleurobrdnchidium)  contains  2'18-4'25  percent 
of  free  sulphuric  acid.  These  carnivorous  animals  are  able,  by  means 
of  their  proboscides,  to  bore  into  other  Molluscs  and  Eckinoderms  which 
are  protected  by  calcareous  skeletons.  The  sulphuric  acid  in  their 
glands  probably  serves  for  transmuting  the  carbonate  of  lime  into 
sulphate  of  lime,  which  can  then  easily  be  worked  through  by  the 
radula. 

(6)  Pulmonata. — Two  salivary  glands  (Fig.  157,  10)  are  always 
found,  their  ducts  entering  the  pharynx  to  the  right  and  left  of  the 
boundary  between  it  and  the  oesophagus.  The  glands  lie  on  the 
oesophagus  and  the  anterior  part  of  the  stomach  in  the  shape  of  long, 
lobate,  jagged  leaves.  In  some  cases  they  are  acinose  or  round  and 
compact. 

(c)  Opisthobranehia. — The  salivary  glands,  of  which  only  one  pair 
is  almost  always  found,  here  vary  in  size  and  shape  still  more  than  in 
the  Pulmonata.  These  glands,  which  enter  the  pharynx,  must  not  be 
confounded  with  other  glands  which  in  many  Opisthobranehia  enter 
the  buccal  cavity,  and  are  sometimes  more  strongly  developed  than 
the  salivary  glands. 

Dentalium  has  no  salivary  glands  opening  into  the  pharynx,  for 
the  glandular  "cheek  pouches"  enter  the  buccal  cavity,  and  two 
diverticula  which  lie  further  back  belong  to  the  oesophagus. 


186 


COMPARATIVE  ANATOMY 


CHAP. 


The  Cephalopoda  have  a  posterior  and  an  anterior  pair  of  salivary 
glands.  Were  the  fore-gut,  which  here  rises  vertically  in  the  visceral 
dome,  to  occupy  the  horizontal  position  it  has  in  the  Gastropoda^  the 
anterior  pair  would  lie  dorsally  and  the  posterior  ventrally  with  regard 
to  it.  The  two  posterior  glands  (Fig.  127,  29,  p.  147)  are  always 
present  (except  in  Cirrhotcuthis  and  Loligopsis,  in  wrhich  they  are  said  to 
be  wanting),  and  lie  on  the  oesophagus.  Each  gland  has  a  duct,  which 
soon  unites  with  that  from  the  other  gland,  forming  a  terminal  portion 
which  accompanies  the  oesophagus  through  the  cephalic  cartilage,  and 
opens  above  the  radula  into  the  pharyngeal  cavity.  The  posterior 


FIG.  157.— Alimentary  canal  of  Helix,  dissected  out  and  seen  from  the  right  side  (after 
Howes).  1,  3,  Tentacles ;  2,  constrictor  pharyngis  ;  4,  levator  pharyngis ;  5,  depressor ;  6,  pro- 
tractor pharyngis ;  7,  pharyngeal  bulb ;  8,  radular  sheath  ;  9,  columellar  muscle,  divided  into  a 
retractor  pedis  and  retractor  pharyngis  ;  10,  salivary  glands;  11,  digestive  gland  (liver);  12,  ducts 
of  the  same  (gall  ducts)  partly  cut  open  ;  13,  hermaphrodite  gland  ;  14,  stomach  cut  open,  in  its 
base  are  seen  the  apertures  of  the  gall  ducts  15  ;  16,  mid-gut ;  17,  hind-gut ;  18,  anus. 

glands  occasionally  (e.g.  in  Oegopsidce)  fuse  behind  the  gullet,  in  which 
case  the  duct  is  single  throughout  its  whole  length. 

The  anterior  salivary  glands  are  specially  well  developed  in  the 
Odopoda  (Fig.  127,  33,  p.  147),  and  lie  on  the  pharynx,  into  which 
they  empty  their  secretions  by  a  duct,  which  seems  always  to  be 
unpaired.  In  the  Decapoda  the  anterior  glands  are  much  smaller  or 
rudimentary ;  they  are  generally  represented  by  a  single  gland  hidden 
within  the  muscular  wall  of  the  pharynx. 

Nautilus  has  no  posterior  salivary  glands,  but  there  are  glandular 
outgrowths  of  the  pharyngeal  cavity  on  each  side  of  the  tongue, 
which  perhaps  correspond  with  the  anterior  salivary  glands  of  other 
Cephalopods. 

The  Cephalopoda  ( ?  without  exception)  have  an  additional  acinose 


viz  MOLLUSGA—THE  ALIMENTARY  CANAL  187 

lingual  gland,  opening  into  that  part  of  the  pharyngeal  cavity  which 
lies  between  the  tongue  and  jaws. 

The  Lajnellibranchia,  as  already  mentioned,  have  neither  pharynx, 
jaws,  tongue,  nor  salivary  glands.  In  the  Nuculidce,  however,  which 
are  rightly  considered  to  be  primitive  forms,  the  mouth  leads  into  a 
widening  of  the  intestine,  on  each  side  of  which  a  glandular  pouch 
opens.  These  pouches  perhaps  correspond  with  the  oesophageal 
pouches  of  the  Chitonidce  and  Bhipidoglossa,  which  will  be  described 
later. 

Natica,  which  bores  through  the  shells  of  living  Lamellibranchs  and 
feeds  on  their  bodies,  has  a  sucker-like  organ  on  its  proboscis  (Fig.  98, 
p.  107).  The  epithelium  of  the  concave  side  of  this  organ,  which  is 
applied  to  the  shell  attacked,  forms  a  gland  for  secreting  acid — prob- 
ably sulphuric  acid — which  serves  for  dissolving  the  carbonate  of 
lime  of  the  bivalve  shell,  which  is  then  at  once  thrown  out  in  the  form 
of  powdered  sulphate  of  lime. 


C.  The  (Esophagus. 

That  portion  of  the  intestine  which  lies  between  the  pharynx  (or 
the  mouth  in  LameUilranchs)  and  the  stomach  is  called  the  oesophagus, 
the  stomach  being  here  used  as  the  name  of  that  widening  of  the 
intestine  into  which  the  gland  of  the  mid-gut  opens.  It  is  always 
easy  to  detect  the  anterior  boundary  of  the  oesophagus.  In  LameUi- 
lranchs it  lies  at  the  mouth,  but  in  the  Glossophora  at  the  posterior  and 
upper  end  of  the  pharynx.  The  posterior  boundary,  however,  can 
often  only  arbitrarily  be  defined,  as  the  oesophagus,  which  is  usually 
narrow  and  tubular,  often  widens  very  gradually  into  the  stomach, 
the  structure  of  its  walls  at  the  same  time  gradually  changing.  In 
other  cases,  widenings  of  the  alimentary  canal  occur  before  the 
stomach,  and  it  is  difficult  to  decide  whether  these  are  anterior 
divisions  of  the  stomach  or  posterior  widenings  of  the  oesophagus. 

In  LameUibranchia,  terrestrial  Pulmonata,  most  Opisthobranchia,  and 
the  Cephalopoda,  Decapoda  the  oesophagus  is  a  simple  ciliated  tube 
running  to  the  stomach,  being  often  provided  with  longitudinal  folds, 
and  therefore  extensible  ;  in  other  divisions,  however,  complications 
occur,  which  are  caused  by  glandular  outgrowths  or  muscular  en- 
largements. 

In  a  few  Solcnogastres  (e.g.  Proneonunm),  on  the  boundary  between  the  short 
oesophagus  and  the  mid-gut,  a  more  or  less  long  blind  diverticuliun  occurs  ;  this  is 
single,  and  runs  forward  dorsally  to  the  pharynx,  and  may  extend  over  the  cerebral 
ganglion  to  the  end  of  the  head. 

In  Chiton  there  are  two  lateral  glandular  sacs  (sugar  glands)  connected  with  the 
short  oesophagus  ;  their  inner  glandular  walls  project  into  the  lumen  in  the  form  of 
villi,  and  their  secretion  changes  boiled  starch  into  sugar. 


188 


COMPARATIVE  ANATOMY 


CHAP.  VII 


Similar  glands,  which  communicate  with  the  anterior  part  of  the  oesophagus,  are 
found  in  the  Rhipidoglossa  (e.g.  Haliotis,  Fissurella,  Turbo).  The  glandular  epithe- 
lium in  these  also  projects  in  the  form  of  villi  or  folds  into  the  lumen. 

The  so-called  crop  of  the  Docoglossa  (Patella)  no  doubt  corresponds  with  the  two 
lateral  cesophageal  sacs  in  the  Chitonidce  and  Rhipidoglossa.  This  is  a  saccular 
widening  of  the  oesophagus  (Fig.  158,  m),  which,  on  account  of  the  constitution  of 
its  walls,  has  been  compared  with  the  psalterium  of  a  Ruminant.  A  similar  widen- 
ing of  the  oesophagus  is  found  in  Cyprceidoe  and  Naticidce,  which  must  be  counted 
among  the  most  primitive  of  the  Monotocardia. 

In  those  Monotocardia  which  are  provided  with  a  proboscis,  the  length  of  the 
thin  oesophagus  is  in  proportion  to  that  of  the  proboscis. 

The  mouth  lies  at  the  tip  of  the  proboscis,  then  follows  a  short  and  often  rudi- 
mentary pharynx,  and  then  the  long  oesophagus,  which  runs  through  the  whole 
length  of  the  non-protrusible  portion  of  the  proboscis,  passes  through  the  cesophageal 


d  6, 


•dm 


til 


FIG.  158.— Median  longitudinal  section  through  Patella  (after  Ray  Lankester).  brv,  Efferent 
branchial  vessel ;  bra,  afferent  ditto  ;  asd,  duct  of  salivary  gland  sd  ;  go,  anus  ;  no,  right  nephridial 
aperture ;  sd,  salivary  gland  ;  cor,  heart ;  pe,  pericardium ;  np,  kidney  ;  d,  intestine  ;  Tip,  hepatic 
gland  (liver) ;  v,  blood  vessel ;  m  (to  the  right),  border  of  mantle  covering  the  gills ;  r,  radular 
sheath  ;  g,  gonads  ;  m,  crop  ;  ph,  pharynx ;  rd,  radula  ;  odm,  masses  of  muscle  and  cartilage  of  the 
lingual  apparatus  ;  o,  mouth ;  k,  head  or  snout. 

ring,  and  may  be  even  further  prolonged  posteriorly.  When  the  proboscis  is  re- 
tracted, the  posterior  portion  of  the  oesophagus  becomes  coiled  ;  when  the  proboscis 
is  extended,  it  lies  in  the  protruded  or  evaginated  basal  portion. 

Not  infrequently  in  carnivorous  Monotocardia  there  is  a  glandular  widening  in 
that  section  of  the  oesophagus  which  follows  the  long  proboscidal  portion.  The 
oesophagus  is  most  complicated  in  the  Rachiglossa  and  many  Toxoglossa,  where  this 
widening,  in  the  form  of  a  large  compact  accessory  gland,  can  become  separated  from 
the  intestine  (Leiblein's  gland,  poison  gland),  and  where  other  glands  and  widen- 
ings  may  occur  (Fig.  159).  It  seems  probable  that  in  certain  Prosobranchia  diges- 
tion and  resorption  takes  place  even  in  the  fore-gut. 

In  the  Pulmonata  and  Opisthobranchia,  there  is  sometimes  a  widening  (crop,  fore- 
stomach)  anteriorly  to  the  stomach,  and  in  the  same  way  the  short  oesophagus  of 
the  Scaphopoda  has  a  glandular  widening,  or  two  lateral  glandular  diverticula. 

Among  the  Cephalopoda,  the  Decapoda  have  a  simple  thin  tubular  oesophagus  ; 


Fio.  161. 


'J 


FIG.  159.— Alimentary  canal  of  Murex  trun- 
culus  (after  Bela  HaUer).  1,  Pharynx  ;  2,  ducts  of 
the  salivary  glands  (5) ;  3,  oesophagus  ;  4,  6,  and  7, 
glands  of  the  fore-gut  (8) ;  9,  digestive  gland  (liver) ; 
10,  stomach  ;  11,  hind-gut;  12,  gland  of  the  hind- 
gut  ;  13,  anus. 

FIG.  160.— Alimentary  canal  of  Sepia.  1,  Jaw; 
2,  pharynx  ;  3,  posterior  buccal  ganglion ;  4,  duct 
of  the  salivary  gland  (5) ;  6,  digestive  gland  (liver) ; 

7,  anus;  8,  rectum;  9,  efferent  duct  of  the  pigment  gland  (ink-bag),  10;  11,  stomachal  ccecum  ;  12, 
stomach ;  13,  ganglion  gastricum ;  14,  "  pancreatic  appendages"  of  the  gall  ducts  of  the  digestive  gland. 

FIG.  161.— Sketch  of  the  anatomy  of  Limacina  helicina,  from  the  right  side,  after  removal  of  the 
mantle,  heart,  and  kidney  (after  Pelseneer).  1,  Fin  (parapodium) ;  2,  foot ;  3,  central  nervous  system 
(oesophageal  ring) ;  4,  oesophagus  ;  5,  anus  ;  6,  columellar  muscle  ;  7,  duct  of  the  hermaphrodite  gland, 
7a  ;  8,  intestine  ;  9  and  10,  dental  plates  of  the  stomach  ;  11,  accessory  glands  of  the  genital  apparatus  ; 
12,  mantle  cavity ;  13,  seminal  groove  or  furrow. 


190 


COMPARATIVE  ANATOMY 


CHAP. 


the  oesophagus  of  the  Octopoda,  however,  is  provided  with  a  lateral  pouch,  the  crop 
(Fig.  127,  p.  147),  whose  walls  are  not  glandular.  This  may 
serve  as  a  reservoir  of  food  when  the  stomach  is  already 
full.  In  Nautilus,  the  crop  is  a  very  large  saccular  widening 
of  the  oesophagus,  larger  than  the  stomach  itself. 


5— 


9—  \- 


—13 


FIG.  162.— Diagram  of  the 
anatomy  of  Clio  striata, 
from  the  right  side ;  the 
heart,  kidney,  and  mantle  of 
this  side  removed  (after 
Pelseneer).  1,  Fin  (parapo- 
dium);  2,  aperture  of  the 
penis  ;  3,  right  tentacle  ;  4, 
genital  aperture  ;  5,  penis  ;. 
6,  oesophagus ;  7,  dental 
plates  of  the  stomach ;  8, 
ducts  of  the  gonad ;  9,  gonad ; 
10,  intestine ;  11,  digestive 
gland  ;  12,  ducts  of  the  same 
(cut  off) ;  13,  accessory 
glands  of  the  genital  appar- 
atus ;  14,  mantle  cavity  ;  15, 
terminal  portion  of  the  geni- 
tal ducts ;  16,  central  ner- 
vous system  (ganglion  ring); 
17,  foot ;  18,  pharynx. 


D.  The  Mid-gut  with  the  Stomach  and 
Digestive  Gland  (Liver). 

The  oesophagus  leads  into  a  wider  portion  of 
the  alimentary  canal,  the  stomach.  Into  this  the 
ducts  of  a  gland  open ;  this  gland  is  strongly 
developed  in  nearly  all  Molluscs,  and  is  usually 
called  the  liver,  but  may  be  more  appropriately 
named  the  digestive  gland,  since  it  in  no  way 
fulfils  the  functions  of  the  vertebrate  liver.  As 
far  as  is  at  present  known,  it  functions  rather  as 
a  pancreas,  or  it  combines  the  functions  of  the 
various  digestive  glands  of  the  vertebrate  intes- 
tine, no  such  thorough  division  of  labour  as  is 
found  in  the  Vertebrates  having  taken  place.  The 
digestive  gland  is,  in  most  cases,  a  richly-branched 
tubular  or  acinose  gland,  which  to  the  naked  eye 
appears  a  compact  lobate  body  of  a  brown, 
brownish-yellow,  or  reddish  colour.  Its  glandular 
epithelium  consists  of  three  sorts  of  cells — 
hepatic,  ferment,  and  calcareous  cells.  In 
many  Nudibranchia  the  gland  breaks  up  into 
branching  intestinal  diverticula,  which  spread 
through  the  body  almost  like  the  gastro-canals  or 
intestinal  branches  in  the  Turbellaria,  and  run  up 
into  the  dorsal  appendages  of  the  body  (clado- 
hepatic  Nudibrancliia). 

Choetoderma,  among  the  Solenogastres,  has  a 
simple  midgut  diverticulum,  which  may  corre- 
spond morphologically  with  the  digestive  gland 
of  other  Molluscs ;  but  in  Proneomenia,  Neomenia, 
etc.,  the  straight  mid-gut  is  provided  throughout 
its  whole  length  with  narrow  lateral  glandular 
sacs  arranged  closely  one  behind  the  other  at 
right  angles  to  it. 

A  part  of  the  mid  -  gut  gland  (the  part 
nearest  to  the  point  where  the  duct  leaves  it) 
and  the  glandular  epithelium  of  the  duct  may  be 
specially  differentiated  in  Cephalopoda,  and  may, 
finally,  form  a  distinct  system  of  glands  called  the 
pancreas  (Fig.  160). 


VII 


MOLLUSCA—THE  ALIMENTARY  CANAL 


191 


The  stomach  is  not  infrequently  a  lateral  outgrowth  of  the  mesen- 
teric  wall,  so  that  the  aperture  (cardia)  leading  into  it  from  the  oeso- 
phagus and  that  leading  out  of  it  into  the  small  intestine  (pylorus) 
are  more  or  less  near  one  another.  A  sort  of  connection  between 
these  apertures  may  arise,  a  ciliated  furrow  or  channel  bounded  by 
longitudinal  folds  running  between  them,  and  in  some  cases  continued 
into  the  adjoining  sections  of  the  alimentary  canal. 

In  the  CepJmlopoda,  the  duct  of  the  digestive  gland  (the  so-called 
hepatic  or  gall  duct)  does  not  open  direct  into  the  stomach,  but  into  a 
coecal  outgrowth  of  the  stomach,  the  spiral  eceeum. 

In  very  many  Lamellibranchia  there  is  a  diverticulum  of  the 
stomach  which  contains  within  its  lumen  a  rod-shaped  gelatinous  cuti- 
cular  formation,  called  the  crystalline  stylet.  Similar  structures  occur 
in  the  Prosobranchia,  and  especially  in  the  PJiipidoglossa  and  Toxoglossa. 

In  many  Opisthofa'anchia,  the  inner  wall  of  the  stomach  carries 
variously-arranged  cuticular  teeth,  dental  plates,  jaw  plates,  etc.,  which 
serve  for  triturating  the  food.  In  such  cases  the  muscular  wall  of  the 
stomach  is  strongly  developed. 

The  stomach  is  succeeded  by  a  narrower  tubular  section  of  the 
mid-gut,  called  the  small  intestine  (intestinum),  which  usually  forms 
coils  or  loops  ;  these  are  more  numerous  in  herbivorous  or  detri- 
tivorous  than  in  carnivorous  Molluscs. 

The  stomach,  small  intestine,  and  digestive  gland,  together  with 
part  of  the  sexual  organs,  compose  the  whole  or  by  far  the  largest 
portion  of  the  visceral  dome,  where  this  is  present. 

1.  Amphineura. 

The  Chitonidffi  show  the  typical  division  of  the  mid-gut  into  stomach,  digestive 
gland,  and  small  intestine.  The  stomach 
lies  far  forward,  and  has  a  wide  outgrowth 
on  one  side,  which  is,  functionally,  a  reser- 
voir of  secreted  matter.  The  cardia  and 
the  pylorus  lie  near  one  another.  The 
digestive  gland  is  paired  ;  the  larger  liver 
to  the  right  has  four  apertures,  while  the 
smaller  one  to  the  left  has  only  one  prin- 
cipal aperture  into  the  stomach.  The 
small  intestine  is  more  than  four  times  as 
long  as  the  body,  and  it  forms  many  loops 
which  are  constant  in  their  arrangement. 
Chiton  feeds  on  small  or  even  microscopic 
algse. 

Unlike  the  Chitonidce,  the  Solenogastres 
show  no  separation  of  the  mid  -  gut  into 
stomach  and  small  intestine.  The  mid-gut 
runs  straight  through  the  body,  the  greater 
part  of  which  it  fills.  The  glandular  lateral 
cceca  found  in  Neomenia,  Proneomc/na, 
etc.,  and  called  hepatic  diverticula,  are  caused  by  the  projection  into  the  lumen  from 


FIG.  163.— Part  of  a  horizontal  median 
section  through  Proneomenia  Sluiteri. 
Septa  of  the  first,  second,  third,  and  fourth 
order  are  seen  projecting  from  the  right  and 
left  into  the  lumen  of  the  mid-gut.  In  the 
background  is  the  dorsal  wall  of  the  gut,  with 
the  groove  which  cuts  into  the  hermaphrodite 
gland  (cf.  Fig.  53,  p.  42). 


192 


COMPARATIVE  ANATOMY 


CHAP. 


2  — 


each  side  of  narrow  septa  arranged  at  right  angles  to  the  gut,  or  transversely  (Fig.  163); 
in  these  septa,  muscle  fibres  run  down  to  the  rudimentary  foot,  and  blood  lacunre 
abound.  In  Proneomenia  Sluiteri,  septa  of  the  first,  second,  third,  or  fourth  order  can 
be  distinguished,  as  seen  in  the  figure.  The  septa  on  the  right  alternate  with  those 
on  the  left  side  of  the  body.  In  the  dorsal  middle  line  the  mid-gut  forms  a  narrow 
ciliated  longitudinal  groove  which  cuts  deep  into  the  gonad,  cilia  are  also  found  011  its 
medio-ventral  surface. 

2.  Gastropoda. 

The  digestive  gland  of  the  Gastropoda  falls  into  two  or  more  lobes,  between 
which  the  stomach  and  the  coils  of  the  small  intestine  lie  embedded.  One,  two,  or 

more  ducts  of  the  gland  may  open  into  the 
stomach.  The  walls  of  the  digestive  gland 
show  the  same  division  into  layers  as  the  wall 
of  the  alimentary  canal.  For  details  as  to  the 
ferment,  hepatic,  and  calcareous  cells  forming 
the  epithelium  of  the  gland,  and  their  physio- 
logical constitution,  the  reader  must  be  re- 
ferred to  special  histological  and  physiological 
treatises. 

In  the  Nudibranchia,  as  already  mentioned, 
the  digestive  gland  breaks  up  into,  a  system  of 
glandular  diverticula  (the  so  -  called  ' '  diffuse 
liver").  The  Aeolidiadce  (e.g.  T*ergipcs]  afford 
an  instructive  instance  of  this.  Three  diver- 
ticula rise  from  the  stomach,  two  anterior  and 
lateral,  and  one  posterior  and  unpaired.  These 
ramify  in  the  body  cavity,  and  finally  send  up 
their  last  ramifications  or  lobes  into  the  dorsal 
appendages.  The  contents  of  the  intestine  can 
penetrate  into  these  last  ramifications  of  the 
"  diffuse  liver  "  (Fig.  164). 

Further,  within  the  Nudibranchia  the  break- 
ing up  of  the  compact  digestive  gland  to  form  a 
"diffuse  liver,"  i.e.  the  loosening  from  one 
another,  and  the  spreading  out  of  the  glandular  tubes  which  are  in  close  contact  in 
the  compact  gland,  can  be  followed  almost  step  by  step.  In  the  Tritonidce  the  gland 
is  a  great  compact  mass.  In  other  families,  such  as  the  Tcthymelibidce,  Lomanotidce, 
Dendronotidcv,  Bornellidce,  Scyllceidce,  it  divides  into  two  anterior  accessory  livers 
and  a  posteiior  principal  liver,  from  which  diverticula  run  up  into  the  dorsal  append- 
ages. Finally,  the  accessory  and  principal  livers  break  up  into  separate  ' '  hepatic 
branches"  (AeolidcK),  which  in  some  cases  anastomose.  The  posterior  principal 
branch  of  the  "diffuse  liver  "  gives  off  specially  numerous  lateral  branches  ;  it  often 
widens  out  to  a  pouch,  and  may  then  be  compared  to  an  extended  gall  bag,  or  a 
posterior  diverticulum  of  the  stomach.  In  Phyllirhoe,  a  pelagic  form,  without 
dorsal  appendages,  the  "diffuse  liver"  is  simplified,  consisting  of  four  unbranched 
blind  tubes,  the  two  anterior  opening  into  the  stomach  separately,  the  two  posterior 
entering  it  together  (Fig.  19,  p.  12). 

The  stomach  of  many  Opisthobranchia  consists  of  two  divisions  separated  by  a  con- 
striction. In  some  forms,  such  as  the  Bullidce  among  the  Tcctibranchia,  the  Ptero- 
poda  thccosomata,  and  the  Tcthymelibidce,  Borndlidce,  Scyllceidce,  among  the  Nudi- 
branchia, it  is  armed  with  hard  chitinous  plates,  spines,  teeth,  etc.,  occurring  in 
varying  number  and  arrangement  on  its  inner  wall  (Figs.  161  and  162) 


FIG.  164.— Alimentary  canal  of  Aeolis 
(after  Souleyet).  1,  Pharynx ;  2,  stomach  ; 
3,  branched  digestive  gland  (liver);  4, 
anus  ;  5,  rectum. 


VII 


MOLLUSCA—THE  ALIMENTARY  CANAL 


193 


3.  Scaphopoda. 

The  mid-gut  of  Dentalium  (Fig.  165)  consists  of  a  looped  stomachal  tube  bent 


FIG.  165.— Alimentary  canal,  kidney,  and  sexual  organs  of  Dentalium,  from  behind  (after 
Lacaze-Duthiers  and  Leuckart  combined),  a,  Mouth  ;  6,  leaf-like  oral  tentacles ;  c,  snout ;  d, 
entrance  to  pharynx ;  e,  pharynx  with  radula,  /,-  g,  hind-gut;  ft,  right  kidney  ;  i,  anus;  k,  right 
nephridial  aperture  ;  I  and  q,  ducts  of  the  digestive  gland,  n  :  m  and  o,  gonad  ;  n  and  p,  digestive 
gland  (liver)  ;  r,  left  nephridial  aperture  ;  s,  left  kidney  ;  t,  stomach  ;  11,  pharynx ;  v,  lobes  or  sails 
on  which  the  filamentous  tentacles  are  placed. 

back  on  itself,  and  of  a  small  intestine  lying  in  a  tangled  coil  behind  the  esophagus. 
VOL.  II  O 


194  COMPARATIVE  ANATOMY  CHAP. 

Two  digestive  glands,  lying  in  the  upper  part  of  the  body,  open   through  wide 
apertures  into  the  stomach.     Their  form  can  be  gathered  from  Fig.  165. 

4.  Lamellibranchia. 

In  the  Lamellibranchia  the  oesophagus,  which  lies  under  the  anterior  adductor, 
widens  at  the  anterior  base  of  the  foot  to  form  the  stomach.  This  descends  some- 
what into  the  foot.  At  the  posterior  base  of  the  stomach  lie  two  apertures  ;  one  of 
these  is  the  pylorus,  and  leads  into  the  small  intestine  which  runs  more  or  less 
coiled  within  the  base  of  the  foot ;  the  other  leads  into  a  tubular  diverticulum,  the 
sheath  of  the  crystalline  stylet.  The  large  richly-branched  acinose  digestive  gland 
(liver)  opens  through  several  apertures  into  the  stomach,  with  which  it  lies  in 
the  anterior  part  of  the  pedal  cavity.  In  Pholas,  Jouannetia,  and  Teredo,  the 
stomach  has  another  ccecum  besides  the  sheath  of  the  crystalline  stylet.  In  all 
bivalves  there  is,  on  the  inner  wall  of  the  stomach,  a  gelatinous  cuticular  structure 
(dreizackiger  Korper,  fleche  tricuspide),  which  varies  in  thickness,  and  is  continued 
into  the  gelatinous  crystalline  stylet.  This  latter  is  secreted  in  concentric  layers  as 
a  cuticular  structure  by  the  epithelium  of  the  sac  in  which  it  lies.  A  plausible 
suggestion  has  recently  been  made  as  to  the  use  of  these  gelatinous  structures,  viz. 
that  they  serve  for  surrounding  with  a  slimy  envelope  foreign  particles,  such  as 
sharply-pointed  grains  of  sand,  which  enter  the  alimentary  canal  with  the  food  ;  in- 
jury to  the  delicate  walls  of  the  intestine  is  thus  avoided,  and  the  travelling  of  such 
particles  along  the  digestive  tract  is  facilitated.  The  point  of  the  crystalline  stylet 
projects  freely  into  the  lumen  of  the  intestine.  In  some  forms  it  does  not  lie  in  a 
separate  sac,  but  in  a  groove  (Najada,  Cardium,  Mytilus,  Peden,  etc.).  The  tricuspid 
body  and  the  crystalline  stylet  appear  temporarily,  and  are  renewed  periodically. 
Similar  structures  have  been  observed  in  the  stomachs  of  various  Gastropods. 
Haliotis  has  a  stomachal  ccecum  which  can  be  compared  with  the  sheath  of  the 
crystalline  stylet. 

In  the  lower  Lamellibranchs,  the  Nuculidce  and  Solenomyidce,  the  stylet  is  either 
very  slightly  developed  or  wanting.  In  the  Arcidce  also,  it  is  only  slightly 
developed. 

The  Septibranchia  (Poromya,  Cuspidarid)  are  distinguished  from  all  other 
Lamellibranchia  by  the  absence  of  coils,  and  the  consequent  shortening  of  the  small 
intestine  (cf.  on  the  intestine  of  the  Lamellibranchia,  Figs.  24,  25,  26,  27,  28, 
pp.  16,  17,  18,  19). 

5.  Cephalopoda. 

The  stomach  in  the  Cephalopoda  always  lies  in  the  dorsal  portion  of  the 
visceral  dome  in  the  shape  of  a  sac  with  a  strong  muscular  wall.  It  always  has  a 
ccecal  appendage  (stomachal  or  spiral  ccecum,  Figs.  166,  160),  which  varies  in  shape 
and  size  ;  into  this  the  digestive  gland  (liver)  opens.  This  ccecum  is  a  reservoir  for 
the  secretion  of  the  digestive  gland.  Food  never  enters  it,  there  are  even  valves 
at  the  point  of  entrance  into  the  stomach,  which  allow  the  secretion  collected  in 
the  ccecum  to  pass  into  the  stomach,  but  prevent  the  entrance  of  the  contents  of  the 
latter  into  the  ccecum. 

In  Nautilus,  the  ccecum  does  not  open  into  the  stomach,  but  into  the  commence- 
ment of  the  small  intestine,  and  is  in  the  form  of  a  small  round  vesicle  with  lamella? 
projecting  into  its  lumen.  In  Sepia  and  Sepiola  also,  the  ccecum  is  more  or  less 
round  ;  in  Rossia,  it  is  slightly  developed  ;  in  Loligo  and  Sepioteuthis,  very  long  and 
ending  in  a  point ;  in  all  Oegopsidce  and  Octopoda,  more  or  less  spirally  coiled  at  the 
blind  end. 

The  well-developed  digestive  gland  seems  to  arise  as  a  paired  organ,  even  when 


VII 


MOLLUSCA—THE  ALIMENTARY  CANAL 


195 


unpaired  in  the  adult.     The  whole  of  the  much  branched  gland  is  surrounded  by  a 
common  integument,  and  it  thus  outwardly  appears  to  be  compact. 

The  digestive  gland  of  Nautilus  consists  of  five  lobes  (four  paired  -and  one 
unpaired),  which  lie  around  the  crop.  They  have  two  ducts,  which  enter  the  ccecum 
through  a  short  common  terminal  portion. 

In  the  Dibranchia  also,  the  digestive  gland  always  lies  on  the  ventral  side  of  the 
stomach,  close  to  the  ascending  oesophagus. 
It  is  undivided,  and  round  or  oviform  in  the 
Octopoda,  Oegopsidce,  and  Sepiola.  In  Loliyo 
and  Sepioteuthis,  it  is  traversed  by  the  oeso- 
phagus and  the  aorta  ;  in  Enoploteuthis,  its 
dorsal  half  is  cut  into  two  points  by  these 
organs  ;  and  the  same  is  the  case  in  Rossia. 
In  Sepia  and  Spirula,  the  gland  forms  two 
lateral  lobes  which  are  distinct  in  Sepia,  but 
connected  along  the  middle  line  in  Spirula. 

There  are  always  two  ducts  (gall  ducts) 
which  rise  near  the  median  plane  from  the 
upper  part  of  the  gland,  and  open  into  the 
stomachal  ccecum  separately  or  through  a 
common  terminal  portion. 

The  following  facts  have  been  ascertained 
as  to  the  function  of  the  so-called  pancreas 
of  the  Cephalopoda.  It  is  originally  a 
specially  differentiated  portion  of  the  diges- 
tive gland,  and  is  easily  distinguishable  in 
the  Octopoda  by  its  different  colour  ;  it  lies 
near  that  part  of  the  gland  from  which  the 
ducts  spring.  In  Loligo.  the  pancreas  is 
found  in  the  much  thickened  wall  of  the 
ducts  themselves.  In  this  case  it  consists 
of  numerous  glandular  anastomosing  out- 
growths of  the  epithelium  of  the  ducts  into 
their  wall.  In  other  Decapoda,  these  gland- 
ular outgrowths  pass  from  the  wall  of  the 
ducts  into  the  surrounding  body  cavity,  and 


4-3 


FIG.  itiO.  —  Alimentary  canal  of  Loligo 
saglttata  (without    pharynx    and    salivary 


then  each  duct  appears  throughout  its  whole    glands)  ^^  cut  opeu  (after  Gegenbauer). 


length  to  be  covered  with  acinose  or  ramified    1,  (Esophagus  ;  -2,  probe,  inserted  into  the 
"pancreatic  appendages."     The   pancreatic    pylorus;  3,  stomach;  4,  stomachal  coacum 

secretion  contains  diastase,  and  appears  to    ,with  s*)iral  ccficum  5'  «•  *ind-gut;  8,  i"k- 
L    <•   i      /•       j-          f  ..I        ba§  ;  ">  aperture  of  the  same  into  the  hind- 
carry  out  only  one  part  of  the  functions  of  the    gut> 

digestive  gland,  viz.  that  part  which  corre- 

sponds with  the  digestive  functions  of  the  salivary  glands  in  the  higher  Vertebrates. 
The  small  intestine,  in  which  among  all  Molluscs  the  resorption  of  the  digested 
food  chiefly  (if  not  exclusively)  takes  place,  is  short  in  the  carnivorous  Cephalopoda, 
and  forms  several  coils  only  in  Trcrnoctopus  violaceus. 


E.  Hind-gut  (Rectum). 

This  is  generally  short  in  Molluscs.  Where  it  is  sharply  marked 
oft'  from  the  small  intestine,  it  usually  differs  from  the  latter  in  being 
thicker  and  more  muscular. 


196  COMPARATIVE  ANATOMY  CHAP. 

In  the  majority  of  Lamellibranchs,  and  in  nearly  all  Diotocardia,  the 
rectum  traverses  the  ventricle  ;  this  fact,  with  many  others,  supports 
the  relationship  of  these  two  groups. 

In  certain  Molluscs,  viz.  the  Scaphopoda,  a  few  Prosobranchia 
(Muriddw,  Purpuridce},  and  the  Cephalopoda,  the  hind -gut  has  an 
accessory  (anal)  gland,  which  is  well  known  in  the  Cephalopoda  as  the 
ink-bag. 

The  rectal  gland  in  Dentalium  is  a  branched  acinose  gland  opening  into  the  hind- 
gut,  according  to  one  account  through  six  separate  ducts,  and  according  to  another 
through  one  single  duct.  Eggs  and  spermatozoa  have  been  met  with  in  the  lumen 
of  this  gland,  and  it  has  been  supposed  that  they  have  been  accidentally  drawn  out 
of  the  mantle  cavity  by  the  swallowing-like  action  of  the  hind-gut,  which  has  been 
observed  in  Dentalium. 

-  The  anal  gland  found  in  some  Rachiglossa  (Monoceros,  Purpura,  Murex)  is  always 
dark  in  colour  (brown  or  violet),  and  is  either  tubular  with  many  bulgings  of  its 
wall,  or  acinose  with  an  axial  duct.  It  always  enters  the  hind -gut  near  the 
anus. 

A  gland  has  been  found  near  the  rectum  in  the  Pteropoda  thecosomata  (Clio, 
Cavolinia)  and  the  Bulloida,  and  has  been  described  as  an  anal  gland,  but  this 
requires  further  investigation. 

The  ink-bag  of  the  Cephalopoda  (Fig.  167),  which  is  wanting  only 
in  Nautilus,  is  a  much  developed  anal  gland.  It  enters  the  hind-gut 
near  the  anus.  The  ink  or  sepia  pigment  secreted  by  it  consists  of 
extremely  minute  particles  which  are  ejected  with  vehemence  from 
the  bag  and  discharged  through  the  funnel.  The  pigment  quickly 
mixes  with  the  water,  and  envelops  the  animal  in  a  pigment  cloud, 
thus  screening  it  from  its  enemies. 

Form  and  position  of  the  ink-bag  (cf.  Figs.  160,  p.  189  ;  177,  p.  213  ;  178, 
p.  214). — The  typical  position  of  the  ink-bag  is  in  front  of  the  rectum,  i.e.  in  the 
loop  formed  by  the  intestine  in  ascending  from  the  mouth  and  then  descending  to 
the  anus.  In  Spirula,  JEnoploteuthis,  and  Sepioteuthis,  the  ink-bag  is  very  small ; 
it  progressively .  increases  in  size  in  series  both  of  Decapoda  and  of  Octopoda,  its 
division  into  a  saccular  portion  and  a  duct  opening  into  the  hind-gut  in  front  of  the 
anus  becoming  more  and  more  distinct.  In  the  Octopoda,  it  lies  embedded  in  the 
upper  part  of  the  liver  within  the  muscular  hepatic  capsule  (cf.  p.  128).  It  is  still 
found  in  this  position  (between  the  liver  and  the  rectum)  in  Sepiola.  In  other 
Decapoda,  however,  the  ink-bag  is  found  shifting  higher  and  higher  in  the  visceral 
dome,  its  duct  at  the  same  time  increasing  in  length.  Finally,  in  Sepia  (and  the 
fossil  Dibranchia),  it  is  found  at  the  top  of  the  visceral  dome,  behind  the  goiiad.  Its 
duct  runs  along  the  right  side  of  the  hind-gut,  bending  round  somewhat  before 
reaching  the  anal  section  of  the  rectum  so  as  to  enter  the  latter  anteriorly.  Onto- 
genetically,  however,  even  in  Sepia,  the  ink-bag  arises  as  an  anterior  outgrowth  of 
the  rectum. 

Structure  of  the  ink-bag  in  Sepia  (Fig.  167  A). — The  ink-bag  in  this  instance 
consists  of  three  parts:  (1)  the  pigment  gland  which  secretes  the  "ink"  ;  (2)  the 
pigment  reservoir  and  the  duct,  which  forms  (3)  an  ampulla  with  a  glandular  wall  near 
its  aperture.  The  pigment  gland  is  a  sac  at  the  base  of  the  ink-bag  on  its  anterior 
wall  (that  turned  towards  the  gonad).  It  projects  into  the  cavity  of  the  ink-bag, 


VII 


MOLLUSCA—THE  ALIMENTARY  CANAL 


197 


which  serves  as  reservoir  and  duct  for  the  pigment.  The  latter,  after  being  formed 
in  the  gland,  passes  through  an 
aperture  in  its  wall  into  this 
reservoir.  The  cavity  of  the 
gland  is  traversed  by  numerous 
perforated  and  richly  vascular-  //3 

ised     lamella?     of     connective  5- 

tissue,  which  are  inter  -  con- 
nected in  such  a  way  as  to  form 
a  kind  of  sponge-like  structure. 
Xew  lamella?  are  continually 
being  put  forth  by  the  formative 
zone  of  the  gland,  which  is  a 
narrowed  portion  bent  back 
downwards,  while  the  oldest 
lamella?,  which  lie  nearest  the 
aperture  of  the  gland,  become 
detached  and  degenerate.  All 
the  lamella?  are  covered  by  a 
glandular  epithelium  and  the 
formation  of  the  pigment  can 
be  traced  in  all  its  stages  from 
its  appearance  in  the  epithelial 
cells  of  the  formative  zone  to 
its  condition  in  those  of  the 
oldest  lamella?.  In  the  forma- 
tive zone,  the  young  glandular 
cells  are  at  first  colourless.  In 
the  succeeding  lamella?,  how- 
ever, pigment  granules  increase 
in  number  and  from  the  older 
lamella?  are  emptied  into  the 
cavity  of  the  gland,  the  epi-  FIG.  167.— Morphology  of  the  pigment  gland  (ink-bag)  of 
thelial  cells  then  becoming  the  Cephalopoda  (after  P.  Girod).  A,  Median  longitudinal 
j  i  1  •  section  through  the  ink -bag  of  an  adult,  n,  Anus  ;  1,  terminal 

detached  and  breaking  up.  ^^  common  to  the  rectum  (-2)  and  the  duct  of  the  ink- 

Both  the  gland  and  the  reser-  bag  ;  3,  ampulla ;  4  and  5,  sphincter  muscles  of  the  ampulla  ; 
voir  are  surrounded  by  a  vascul-  0,  duct  of  the  ink-bag ;  7,  pigment  reservoir  ;  8,  opening  of 
arised  integument  of  connective  the  Pigment  gland  into  the  reservoir ;  9,  portion  of  the  gland 
. .  , ,  .  ,  ,  traversed  bv  lamellae  ;  10,  formative  zone  of  the  lamellae, 

tissue  ;    the   same    integument  B  Q  Varioug  stageg  in  the  development  of  tne  pigment 

forms    the    framework    of   con-  gland  ;  B,  anal  papilla  ;  0,  invagination  in  the  same  ;  D,  ap- 

nective  tissue  running  through  pearance  of  two  new  depressions  at  the  base  of  C;  these 

the  lamella?  or  trabecula?  within  increase  in  depth,  the  one  becoming  the  pigment  gland  b,  the 

,1        ••       -I  other  the  rectum  2.     In  F,  the  formative  zone  has  appeared 

in  the  gland,  in  G,  the  first  lamellae  and  the  duct.     H,  I,  K, 

The  ink-bag  is  further  envel-  ehanges  iu  the  relative  positions  of  the  rectum  and  gland  in 

oped  as  a  whole  in  a  tough  integu-  the  course  of  development,  seen  from  the  posterior  (mantle) 

ment  consisting  of  three  layers  •  si(le-    In  H>  the  rectum  lies  behind  the  ink-bag.    In  I,  the 

(1)  an  inner  glittering  silvery  latter  has  *hifted'  and  in  K  lies  behind  the  rectuin  (on  the 

,  J    mantle  side), 
layer  (argentea),  similar  to  the 

corresponding  layer  in  the  outer  integument ;  (2)  a  central  muscle  layer  (inner 
longitudinal  and  outer  circular  muscles  ;  and  (3)  an  external  layer  of  connective 
tissue. 

The  terminal  ampulla  has,  at  its  two  narrow  ends,  folds  projecting  inward  and 
functioning  as  valves  ;  it  can  be  closed  at  these  parts  by  sphincter  muscles.  The 


198  COMPARATIVE  ANATOMY  CHAP. 

ampulla  itself  also  forms  longitudinal  folds  on  its  inner  surface,  between  which 
glandular  tubes  open. 

The  anus,  in  the  Cephalopoda,  always  carries  two  lateral  projecting  appendages, 
which  are  often  lancet-shaped. 

The  short  and  narrow  hind-gut  of  the  Solenogastres  opens  into  the 
dorsal  portion  of  a  cavity,  the  cloaca,  which  lies  at  the  posterior  end 
of  the  body  ;  this,  again,  communicates  with  the  exterior  by  means  of  a 
ventral  and  very  extensible  longitudinal  slit.  Into  this  cloaca  the 
ducts  of  the  genital  organs,  which  are  morphologically  to  be  regarded 
as  nephridia,  also  open. 

In  the  Lamellibranchia,  after  the  hind-gut  has  traversed  the  heart, 
it  runs  straight  backward  over  the  posterior  adductor,  to  open  through 
the  anus  into  the  posterior  and  upper  portion  of  the  mantle  cavity 
(anal  chamber). 

On  the  position  of  the  anus,  cf.  Section  V.  on  the  arrangement  of 
the  organs  in  the  mantle  cavity. 


XVII.  The  Circulatory  System. 
A.  General. 

All  Mollusca  have  a  circulatory  system ;  in  some  divisions, 
especially  in  the  Cephalopoda  and  some  Prosobranchia,  this  may  attain  a 
high  level  of  complication  by  the  development  of  a  closed  arterial  and 
venous  vascular  system.  The  heart,  as  the  central  organ  of  propulsion, 
is  never  wanting.  It  lies  enclosed  in  the  pericardium,  a  division  of 
the  secondary  body  cavity ;  its  primitive  position  is  median,  above 
the  hind-gut.  In  the  Lamellibramhia  and  Diotocardia,  it  is  traversed 
by  the  hind-gut,  in  other  Gastropoda  it  lies  near  it.  It  is  always 
arterial,  i.e.  it  pumps  the  blood  flowing  from  the  respiratory  organs 
back  into  the  body. 

In  those  symmetrical  Molluscs  in  which  the  dorsal  portion  of  the 
body  rises  as  a  high  visceral  dome,  the  intestine  first  ascending  into 
the  dome  and  then  descending  to  the  anus,  the  heart  comes  to  lie 
behind  the  hind-gut  (Dentalium,  Cephalopoda). 

In  asymmetrical  Gastropoda,  its  position  depends  upon  that  of  the 
pallial  complex.  Where  the  hind-gut  and  anus  have  shifted  with  the 
pallial  organs  to  the  anterior  side  of  the  visceral  dome,  the  heart  also 
lies  anteriorly  (Prosobranchia,  Pulmonata,  a  few  Tectibranchia). 

The  heart  gives  rise,  as  a  rule,  to  two  large  arteries  (aorta),  one 
of  which  runs  to  the  head,  the  other  to  the  visceral  dome,  to  supply 
blood  to  the  viscera.  Not  infrequently  they  leave  the  heart  as  one 
large  vessel.  Where  the  circulatory  system  is  not  closed,  the  arteries 
sooner  or  later  convey  the  blood  to  the  primary  body  cavity  or  coelom, 
i.e.  into  the  lacunar  system.  The  venous  blood  is  sometimes  conveyed 
along  distinct  vessels,  sometimes  in  channels  without  proper  walls  into 


VII 


MOLLUSCA—THE  CIRCULATORY  SYSTEM 


199 


the  gills,  where  it  becomes  arterial  and  flows  back  through  the  auricles 
(atria)  into  the  heart. 

There  is,  typically,  one  pair  of  auricles,  one  on  each  side  of  the 
ventricle.  This  is  the  case  in  all  Molluscs  provided  with  two  sym- 
metrical gills.  The  arterial  blood  flows  out  of  the  left  gill  into  the  left 
auricle  and  thence  into  the  ventricle,  and  out  of  the  right  gill  into  the 
right  auricle  and  thence  into  the  ventricle  (Diotocardia,  Zeugobranchia, 
Lamcllibranchia,  Cephalopoda  Dibranchia).  Again,  where  a  longitudinal 


7  — 


FIG.  168.— A-H,  Diagrams  illustrating  the  relation  between  the  ctenidia,  the  heart,  and 
the  aorta.  A,  Chiton ;  B,  Lamellibranchia ;  C,  Dibranchiate  Cephalopoda ;  D,  Tetra- 
branchiate  Cephalopoda ;  E,  Prosobranchia  Diotocardia  Zeugobranchia ;  F,  Prosobranchia 
Diotocardia  Azygobranchia ;  G,  Prosobranchia  Monotocardia ;  H,  Opisthobranchia  Tecti- 
branchia.  1,  Ventricle  ;  -2,  3,  -la.  2b,  3d,  Sb,  auricles  ;  4,  vena  branchialis  =  efferent  branchial  vessel ; 
5,  aorta  ;  oa,  aorta  cephalica  ;  5?>,  aorta  visceralis  ;  6,  aorta  posterior  vel  superior  ;  7,  ctenida. 


row  of  numerous  gills  is  found  on  each  side  in  the  mantle  furrow 
(Chitonidce),  the  heart  lies  posteriorly  above  the  hind-gut,  and  has  one 
auricle  on  each  side  of  the  ventricle.  This  fact  appears  quite  as  much 
to  support  the  view  that  one  pair  of  gills  and  one  pair  of  auricles  were 
present  in  primitive  Molluscs,  as  does  the  arrangement  in  Nautilus 
(Cephalopoda  Tetrabmnchia)  the  other  view,  that  there  were  two  pairs 
of  gills  and  also  two  pairs  of  auricles. 

In  the  majority  of  Gastropoda,  where  one  of  the  two  original  gills 
has  disappeared,  the  auricle  belonging  to  it  has  usually  also  disappeared. 


200  COMPARATIVE  ANATOMY  CHAP. 

The  original  right  gill  and  right  auricle  are  usually  retained  in  Gastro- 
pods with  shells  dextrally  twisted.  In  Gastropods  with  a  true  sinistrally 
twisted  shell,  the  left  gill  and  left  auricle  are  retained. 

There  is,  however,  a  whole  division  of  the  Prosobranchia,  the 
Diotocardia,  in  which  both  auricles  are  retained.  It  is  evident  that  the 
gills  are  more  liable  to  disappear  than  the  auricles,  since  in  some 
groups  both  auricles  remain  when  one  gill  has  disappeared  (for 
details  see  opposite  page). 

When,  in  Gastropoda  with  only  one  auricle,  the  pallial  complex  has 
shifted  to  the  anterior  side  of  the  visceral  dome>  the  respiratory  organs 
lie  in  front  of  the  heart,  and  the  single  auricle  in  front  of  the  ventricle 
(Prosobranchia,  Monotocardia,  most  Pulmonata,  a  few  Opisthobranchia). 
In  those  Gastropoda,  however,  in  which  the  pallial  complex  lies  on  one 
(usually  the  right)  side  of  the  body,  the  gill  is  placed  behind  the  heart 
and  the  auricle  behind  the  ventricle.  This  is  the  case  in  nearly  all 
the  Opisthobranchia.  In  a  few  Pulmonates  also,  such  as  Testacella, 
Oncidium,  etc.,  the  auricle  lies  behind  the  ventricle,  as  a  consequence 
of  special  organic  modifications. 

The  blood,  or  rather  the  hsemolymph,  is  a  fluid  rich  in  dissolved 
albumen  (hsemocyanine),  which  assists  in  nourishing  the  body  and  in 
respiration.  Amoeboid  cells,  the  lymph  cells  or  amrebocytes,  are 
suspended  in  the  hsemolymph.  Haemoglobin  is  occasionally  found 
dissolved  in  the  hsemolymph  or  combined  with  special  blood  corpuscles. 
The  lymph  cells  either  become  detached  from  the  wall  of  localised 
blood-making  glands,  which  may  vary  in  position,  or,  in  a  more 
diffused  manner,  from  large  vascular  areas.  They  seem,  from  their 
origin,  to  be  cells  of  connective  tissue. 

The  walls  of  the  heart  and  of  the  walled  vessels  consist  of  smooth 
muscle  fibres  thickly  felted,  and  (on  the  heart)  of  an  external  endo- 
thelium  which  belongs  to  the  pericardium.  An  inner  endothelium  is 
wanting,  so  that  the  muscle  fibres  are  directly  bathed  by  the  blood. 

The  wall  of  the  ventricle  is  always  more  muscular  than  those  of 
the  auricles.  At  the  point  where  the  auricles  open  into  the  ventricle, 
valves  projecting  into  the  lumen  are  always  found,  which,  when  the 
latter  contracts,  prevent  the  return  of  blood  into  the  auricle.  Besides 
these  atrio-ventricular  valves,  there  are  occasionally  other  valves 
between  the  ventricle  and  the  aorta.  Valves  may  also  occur  in  the 
peripheral  blood  channels,  when  these  form  contractile  enlargements 
(e.g.  the  valve  between  the  branchial  heart  and  the  afferent  branchial 
vessels  of  the  Cephalopoda). 

In  various  Gastropods  and  in  Chiton  a  network  of  ganglion  cells 
and  nerve  fibres  has  been  found  in  the  wall  of  the  heart,  innervated 
by  two  nerves  of  different  origin.  The  nerve  which  runs  to  the 
ventricular  plexus  originates,  in  the  Prosobranchia,  in  the  left  parietal 
ganglion,  that  running  to  the  auricle  from  the  left  parieto-visceral 
connective.  Where  there  are  two  auricles,  they  are  innervated  from 
the  branchial  ganglia. 


vii  MOLLUSCA—THE  CIRCULATORY  SYSTEM  201 

B.  Special. 

1.  Amphineura. 

a.  Chitonidae  (Polyplacophora). — The  heart  is  symmetrical,  with  two  lateral 
auricles. 

The  ventricle  and  the  two  auricles  are  long  tubes.  The  auricles  are  in  open 
communication  with  the  ventricle  about  the  middle  of  their  length.  Besides  this, 
the  two  auricles  pass  into  one  another  posteriorly,  the  posterior  end  of  the  ventricle 
also  opening  into  them  at  this  point. 

The  ventricle  lies  against  the  dorsal  wall  of  the  pericardium,  to  which  it  is 
attached  by  a  median  band  of  endothelium.  The  ventricle  passes  into  an  aorta 
which  allows  the  blood  to  flow  into  the  coslom  through  apertures  in  its  wall.  With 
the  exception  of  the  pedal  arteries,  the  rest  of  the  circulatory  system  is  lacunar  ; 
there  are  no  vessels  with  walls  of  their  own. 

The  venous  blood  is  collected  from  the  lacunar  system  of  the  body  (primary 
ccelom)  into  longitudinal  channels  which  run  on  each  side  under  the  pleurovisceral 
cords.  From  these  channels  it  flows  into  the  gills,  where  it  becomes  arterial,  and 
returns  through  other  longitudinal  channels  which  run  above  the  pleurovisceral 
cords.  Two  transverse  channels  in  the  region  of  the  heart  (cf.  Fig.  51,  p.  40) 
convey  the  arterial  blood  into  the  auricles. 

The  two  pedal  arteries  lie  laterally  and  ventrally  with  regard  to  the  pedal  cords  ; 
they  probably  draw  their  blood  from  the  aorta  and  pass  it  on  to  the  lacunar  system 
of  the  foot. 

b.  Solenogastres. — The  heart  lies  above  the  hind-gut  on  the  dorsal  side  of  the 
pericardium.     It   does   not  lie   freely  in   the   latter,  nor   is   it   suspended    by   an 
endothelial  band,  but  simply  projects  into  the  pericardium  from  above,  so  that  only 
its  under  surface  is  covered  by  the  pericardia!  endothelium.     The  presence  of  two 
auricles  has  not  been  proved.     The  rest  of  the  circulatory  system  is  purely  lacunar. 
Specially  large  blood  channels  lie  in  the  depths  of  the  principal  septa  which  project 
into  the  mid-gut,  and  bulge  these  out.     Large  blood  sacs  are  also  occasionally  found 
in  folds  which  project  into  the  pharyngeal  cavity  from  its  wall,  and  there  are  more 
or  less  large  sinuses  in  the  folds,  which,  in   Neoinenia  and    Chcetoderma,  project 
into  the  cloaca  and  may  be  regarded  as  gills.      In  all    these  parts  the  intestinal 
epithelium  separating  the  sinus  from  the  intestine  is  ciliated,  and  respiration  no 
doubt  takes  place. 

2.  Gastropoda. 

Relation  of  the  auricles  to  the  ventricle. — The  lowest  Gastropods,  i.e.  the 
Diotocardia  among  the  Prosobranchia,  have  a  heart  with  two  auricles.  This  is  not 
only  the  case  in  the  Zeugdbraiichia  (Fissurella,  Haliotis,  etc.),  which  have  two  gills, 
but  also  in  the  Azygobranchia  (Turbi/iidce,  Trochidce,  Neritidce],  in  which  only  the 
left  (originally  the  right)  gill  has  been  retained.  No  branchial  vein  then  enters  the 
smaller  (rudimentary)  auricle  on  the  right,  the  veins  having  atrophied  with  the  gill. 
In  the  Zeugobranckia,  the  long  ventricle  lies  in  a  line  with  the  hind-gut,  which  runs 
length- wise  through  it.  In  the  Azygobranchia,  the  ventricle  lies  transversely  with 
respect  to  the  hind-gut  which  runs  through  it,  the  left  auricle  lying  in  front  of  the 
ventricle,  and  the  right  auricle  behind  it.  The  left  branchial  vein  enters  the 
anterior  (left)  auricle.  If  we  suppose  the  posterior  (right)  auricle  to  have  disappeared 
altogether,  as  is  the  case  in  all  other  Gastropoda,  the  heart  consists  of  a  ventricle 
and  one  auricle  lying  in  front  of  it,  which  receives  the  branchial  or  pulmonary  vein 
from  the  gill  or  lung  in  front  of  it. 


202  COMPARATIVE  ANATOMY  CHAP. 

This  serial  order  of  the  ventricle,  auricle,  branchial  or  pulmonary  vein  and 
respiratory  organs  is  characteristic  of  the  Azygobranchia,  Monotocardia,  and  most 
Pulmonata. 

The  Docoglossa  (Patella  and  allied  forms)  have  only  one  auricle  ;  the  ventricle 
in  Patella  (not  in  Acmcea),  however,  is  divided  into  two  parts. 

Among  the  Monotocardia,  only  Cyproea  (as  far  as  is  at  present  known)  has  a 
rudimentary  right  auricle,  closed  on  all  sides  except  at  its  aperture  into  the 
ventricle. 

Among  the  Pulmonata  there  are  forms  in  which  the  auricle  lies  behind  the 
ventricle.  This  must  be  regarded  as  a  secondarily  acquired  position,  determined  by 
the  shifting  back  of  the  anus  and  the  mantle  cavitj^  to  the  posterior  end  of  the  body 
( Testacella,  Oncidium).  In  Daudebardia,  the  auricle  still  lies  in  front  of  the  ventricle; 
nevertheless  this  genus,  like  several  other  shell-less  Pulmonates,  is  opisthopneumonic, 
i.e.  its  respiratory  network  lies  chiefly  behind  the  heart.  In  Testacella,  the  auricle 
also  lies  behind  the  heart  (cf.  p.  77). 

In  the  Opisthobranchia,  the  auricle  lies  behind  the  ventricle  ;  this  is  connected 
with  the  position  of  the  gill  at  the  posterior  end  of  the  body,  or  where  no  true 
ctenidium  is  found,  but  where  respiration  takes  place  by  means  of  anal  gills,  or 
dorsal  appendages,  or  through  the  integument,  with  the  point  of  entrance  of  the 
branchial  vein  into  the  heart  from  behind. 

In  a  few  Tectibranchia,  e.g.  Actceon,  Accra,  Gastropteron,  the  gill  lies  some- 
what far  forward,  and  the  auricle  is  then  placed  laterally,  to  the  right  of  the 
ventricle  rather  than  behind  it. 

It  is  of  great  importance,  with  regard  to  the  position  of  these 
organs  in  the  Lamellibranchw,  to  note  the  fact  that,  in  many  Diotocardia 
(e.g.  Fissurella,  Haliotes,  Turbinidce,  Trochidce,  Neritidce,  Neritopsidce,  etc.) 
the  ventricle  is  traversed  by  the  hind-gut,  while  in  all  other  Gastropods 
the  intestine  merely  runs  past  it. 

Circulation,  (a)  Prosobranchia.—  A  large  vessel,  the  aorta,  springs  from  the 
ventricle.  This  soon  divides  into  two  branches  :  (1)  the  anterior  or  cephalic  aorta 
(A.  cephalica),  and  (2)  the  posterior  aorta  (A.  visceralis). 

The  anterior  aorta  conveys  blood  to  the  anterior  part  of  the  body  (head,  pharynx, 
proboscis,  oesophagus,  stomach,  copulatory  organs)  and  to  the  mantle,  and  gives  off 
among  others  the  important  arteria  pedalis;  this  latter  soon  breaks  up  into 
separate  arteries,  which  run  longitudinally  through  the  foot.  In  some  cases  the 
cephalic  aorta  is  richly  branched,  breaking  up  into  numerous  fine  vessels  which 
spread  out  in  and  on  the  above-mentioned  organs  ;  in  others,  the  arteries,  without 
branching,  open  into  arterial  sinuses.  Among  these,  the  large  cephalic  sinus  into 
which  the  anterior  aorta  opens  (e.g.  in  Haliotis]  deserves  special  mention.  Where 
the  cephalic  aorta  runs  beyond  the  oesophageal  ring  formed  by  the  central  ganglia 
and  their  commissures,  it  passes  through  this  ring. 

The  aorta  visceralis  supplies  the  organs  which  lie  in  the  visceral  dome,  especially 
the  digestive  gland,  the  genital  glands,  and  the  mid-gut. 

The  venous  blood  collects  in  the  lacunar  spaces  of  all  parts  of  the  body,  and 
flows  into  a  large  venous  sinus,  i.e.  into  the  space  in  which  the  stomach,  salivary 
glands,  intestine,  digestive  gland,  and  genital  organs  lie.  This  space  or  primary 
body  cavity  is  somewhat  spacious  round  the  stomach,  but  very  limited  in  the 
visceral  dome,  where  the  lobes  of  the  digestive  gland,  the  walls  of  the  intestine, 
and  the  genital  glands  with  their  accessory  parts  are  so  crowded  together  as  to  leave 
very  narrow  spaces  between  them. 


vii  MOLLUSCA—THE  CIRCULATORY  SYSTEM  203 

The  blood  passes  out  of  the  large  venous  sinus  back  into  the  heart  by  three 
channels. 

1.  A  large  part  of  it  flows  through  lacunte  or  vessels  into  the  paired  or  unpaired 
branchial  artery  (afferent  branchial  vessel).     In  the  course  of  branchial  respiration 
the  blood  becomes  arterial,  and  collects  in  an  efferent  branchial  vessel  (cf.  section  on 
the  respiratory  organs,  p.  84),  which,  as  branchial  vein,  conducts  it  to  the  auricle 
of  the  heart.     Where  there  are  two  gills,  there  are  naturally  two  branchial  arteries 
and  two  branchial  veins,  the  latter  conducting  the  arterial  blood  to  the  two  auricles. 

2.  Another  part  of  the  venous   blood   flows  through  the  kidney,  then  again 
collects  in  lacunae  or  vessels  which  lead  to  the  gills,  and  finally  reaches  the  heart 


FIG.  169.— Circulatory  system  of  Paludina  vivipara  (after  Leydig).  The  animal  is  seen  from 
the  left  side.  1,  Eye  ;  2,  cerebral  ganglion  ;  3,  efferent  branchial  vessel  (branchial  vein)  ;  4,  gill 
(ctenidium) ;  5,  afferent  branchial  vessel ;  •>,  kidney  ;  7,  aorta  visceralis,  winding  up  close  to  the 
columella ;  8,  ventricle ;  9,  auricle ;  10,  aorta  cephalica  ;  11,  venous  sinuses  of  the  body ;  !•_', 
auditory  vesicle  ;  13,  pedal  ganglion. 

through  the  branchial  veins.  Less  frequently,  the  venous  blood,  after  passing 
through  the  kidney,  enters  the  auricle  more  or  less  directly,  i.e.  without  passing 
through  the  gills,  and  there  mixes  with  the  arterial  blood  coming  from  the  gills. 

3.  A  certain  part  of  the  venous  blood,  passing  by  both  the  kidney  and  the  gill, 
flows  direct  into  the  branchial  veins  leading  to  the  auricle. 

The  arterial  blood  in  the  heart  is  thus  mixed  with  venous  blood. 

(b)  Pulmonata.— (Examples  :  Helix  pomatia,  Limax,  Figs.  170,  171,  95,  p.  100). 
The  blood  vascular  system  is  like  that  of  the  Monotocardia.  The  only  important 
deviation  is  caused  by  the  occurrence  of  pulmonary,  respiration.  Various  veins  col- 
lect the  venous  blood  out  of  the  large  body  sinus  and  the  lacunar  system,  and  unite 
to  form  one  large  vein,  which  accompanies  the  hind-gut,  and,  as  vena  circularis, 


204 


COMPARATIVE  ANATOMY 


CHAP. 


runs  along  the  thickened  edge  of  the  mantle  which  concresces  with  the  nuchal  integu- 
ment.    From  this  vein  spring  numerous  venous  vessels  which  spread  out  on  the 


FIG.  170.— Pulmonary  veins,  heart,  and  arterial  system  of  Helix  (after  Howes).  The  mantle 
(roof  of  pulmonary  cavity)  is  cut  open  and  turned  back.  1,  Pulmonary  vein  (efferent  pulmonary 
vessel) ;  2,  kidney ;  3,  auricle ;  4,  ventricle ;  5,  rectum,  cut  through  ;  6,  hermaphrodite  gland  ; 
7,  columellar  muscle  ;  8,  aorta  visceralis  ;  9,  salivary  glands  ;  10,  aorta  cephalica. 

under  surface  of  the  mantle,  i.e.  on  the  roof  of  the  mantle  cavity,  and  there  form  a 
delicate  respiratory  network.     In  this  network  the  blood  becomes  arterial,  and  is 


FIG.  171. -Vascular  system  of  Limax,  after  drawings  combined  by  Leuckart  from  Delle  Chiaje 
and  Simroth.  The  veins  carrying  the  venous  blood  out  of  the  body  into  the  lungs  are  black. 
A,  auricle  ;  V,  ventricle  ;  VR,  venous  circular  sinus  of  the  pulmonary  cavity  ;  Ax,  aorta  cephalica  ; 
Ay,  aorta  visceralis  ;  M,  muscular  stomach  ;  ZD,  hermaphrodite  gland  ;  II,  digestive  gland  ;  7,  in- 
testine  ;  AL,  respiratory  aperture  ;  .Y,  arteria  genitalis. 

next  conducted  through  many  vessels  into  the  large  pulmonary  vein  (vena  pulmon- 
aris),  which  runs  back  almost  parallel  to  the  rectum  along  the  roof  of  the  mantle 


vii  MOLLUSCA—THE  CIRCULATORY  SYSTEM  205 

cavity,  to  enter  the  auricle.  The  vessels  of  the  respiratory  network  form  projecting 
ribs  on  the  surface  of  the  mantle.  The  pallia!  epithelium  in  the  mantle  cavity  is 
ciliated. 

The  efferent  pulmonary  vessels,  which,  near  the  kidney,  run  along  the  right  side 
of  the  pulmonary  vein,  first  enter  the  kidney  and  break  into  a  fine  vascular  network 
before  passing  into  that  vein. 

The  cephalic  aorta  does  not  pass  through  the  cesophageal  ring,  but  runs  between 
the  pedal  and  visceral  ganglia  ;  this  is  said  to  be  the  case  in  most  Opisthobranchia. 

In  Opistho2meumonic  Pulmonata  (e.g.  Daudebardia,  Testacella),  in  which  the 
small  or  rudimentary  visceral  dome  has  shifted  to  the  posterior  end  of  the  body,  and 
the  organs  elsewhere  found  in  the  dome  (liver  and  genital  organs)  now  lie  in  the  body 
cavity  above  the  foot,  and  thus  in  front  of  the  posteriorly  placed  heart,  the  posterior 
aorta  (A.  visceralis)  is  much  reduced,  but  the  anterior  aorta  (A.  cephalica)  is  strongly 
developed.  The  posterior  aorta  supplies  only  the  posterior  lobes  of  the  liver  and  the 
hermaphrodite  gland,  and  the  anterior  aorta  (cephalic  aorta,  A.  ascendens)  has  thus 
to  supply  the  anterior  lobes  and  even  part  of  the  genital  organs,  which  usually  receive 
their  blood  from  the  posterior  aorta. 

In  Onddium,  there  is  an  arteria  visceralis  corresponding  with  the  posterior 
aorta,  which  branches  oft'  soon  after  the  aorta  leaves  the  heart,  but  it  here  runs 
anteriorly. 

(c)  Opisthobranchia. — Here  also  the  arrangement  is  essentially  the  same  as  in 
the  Prosobranchia,  though  modified  by  the  different  position  of  the  gills,  as  has  been 
already  briefly  noted. 

Gastropteron  affords  a  good  illustration  of  the  circulatory  system  of  the  Tecti- 
branchia.  The  heart,  which  is  enclosed  in  a  spacious  pericardium,  lies  to  the  right, 
in  front  of  and  above  the  base  of  the  gill.  It  lies  transversely,  the  larger  and  more 
muscular  ventricle  to  the  left,  the  auricle  to  the  right.  Out  of  the  ventricle  springs 
the  aorta,  which  at  once  divides  into  a  posterior  and  an  anterior  aorta.  The  anterior 
aorta  enters  the  cephalic  cavity,  giving  off  as  its  principal  arteries  :  (1)  the  arteiy  of 
the  copulatory  organ.  (2)  The  two  large  pedal  arteries,  each  of  which  again  soon 
divides  into  two  branches,  viz.  (a)  an  anterior  artery,  which  branches  richly  in  the 
parapodia  ;  (b]  a  posterior  artery,  which  runs  back  on  each  side  parallel  to  the 
median  line  of  the  foot.  (3)  The  arteries  of  the  cephalic  disc.  (4)  The  arteries 
of  the  cesophageal  bulb  and  of  the  oesophagus.  (5)  The  anterior  end  of  the  aorta 
itself  branches  in  the  tissues  surrounding  the  mouth.  The  following  are  the  chief 
branches  of  the  posterior  aorta  :  (1)  The  gastric  artery.  (2)  The  hepatic  arteries. 
(3)  The  genital  arteries.  The  venous  blood  flows  back  from  all  parts  of  the  body 
through  richly  -  branched  channels  into  two  large  venous  sinuses,  one  of  which 
represents  the  cephalic  and  the  other  the  body  cavity.  AVide  but  short  vessels 
convey  the  venous  blood  out  of  these  sinuses  into  the  kidney,  which  contains  a 
rich  venous  lacunar  system.  From  the  kidney  it  flows  direct  into  the  afferent 
branchial  vessel,  becomes  arterial  in  the  gills,  and  collects  in  the  efferent  branchial 
vessel,  which,  as  the  branchial  vein,  soon  enters  the  auricle. 

All  the  venous  blood  in  Gastropteron,  therefore,  on  its  way  back  to  the  heart, 
passes  first  through  the  kidney  and  then  through  the  gill,  so  that  only  arterial  blood 
flows  through  the  heart. 

This  is,  however,  not  by  any  means  the  case  in  all  Tectibrancliia.  For  example. 
in  Pleurobranchus,  a  large  part  of  the  venous  blood  passes  from  a  dorsal  circular 
sinus  through  a  very  short  but  wide  passage  direct  into  the  branchial  vein  close  to 
its  point  of  entrance  into  the  auricle,  passing  by  both  the  kidney  and  the  gill. 

Dorididse. — Without  going  into  details  as  to  the  circulatory  system  of  this  group, 
it  may  be  mentioned  that  part  of  the  venous  blood  passes  directly  through  two 
lateral  vessels  into  the  auricle.  Another  part  flows  into  an  inner  venous  circumanal 


206  COMPARATIVE  ANATOMY  CHAP. 

sinus,  which  lies  at  the  base  of  the  circle  of  gills.  From  this  the  blood  rises  into  the 
gills,  becomes  arterial,  flows  back  into  an  outer  circumanal  vessel,  and  thence  back 
through  the  branchial  vein  into  the  auricle  (Fig.  93,  p.  98). 

Nudibranchia. — The  heart,  enclosed  in  the  pericardium,  almost  always  lies  in 
front  of  the  centre  of  the  body,  in  the  median  line.  The  aorta,  which  springs  from 
the  ventricle,  divides  into  an  anterior  and  a  posterior  aorta,  each  of  which  breaks  up 
into  an  arterial  system,  the  arteries  having  walls  of  their  own.  The  finer  branches  of 
these  arteries  open  into  the  lacunar  system  of  the  body,  which  occasionally  forms 
canals  resembling  vessels,  and  is  connected  with  the  large  cephalic  and  visceral 
sinuses.  Veins,  apparently  with  walls  of  their  own,  run  from  the  lacunar  system  of 
the  dorsal  appendages  or  the  integument,  and  carry  the  arterial  blood  back  to  the 
auricle.  The  blood  usually  finally  enters  the  heart  through  three  "branchial"  veins, 

two  lateral  and  one  median  posterior, — which  open  into  the   posteriorly-placed 

auricle. 

3.  Scaphopoda. 

The  circulatory  system  of  Dentalium,  but  for  the  recently-discovered  rudimentary 
heart,  is  entirely  lacunar,  consisting  of  systems  of  canals,  sinuses,  and  spaces,  the 
special  arrangement  of  which  cannot  here  be  described. 

The  pericardium  with  the  heart  lies  on  the  posterior  side  of  the  body,  dorsally  to 
the  anus.  If  we  imagine  the  intestine  of  Dentalium  straight  and  horizontal,  the 
heart  would  occupy  the  typical  position  on  the  dorsal  side  of  the  hind-gut.  It  has 
no  auricles,  and  is  merely  a  sac-like  bulging  into  the  pericardial  cavity  of  its 
anterior  wall.  It  is  connected  by  fine  slits  with  the  surrounding  sinuses  of  the  body. 

4.  Lamellibranehia. 

The  Heart. — In  nearly  all  bivalves,  the  heart,  which  is  traversed  by 
the  hind-gut,  possesses  two  lateral  auricles,  and  lies  in  a  pericardium. 

There  are,  however,  isolated  exceptions  to  this  rule.  In  Nucula, 
Area,  and  Anomia,  the  ventricle  lies  over  (dorsally  to)  the  hind-gut. 
This  dorsal  position  must  be  regarded  as  the  primitive  position  of  the 
Lamellibranchiate  heart,  since  the  above  genera  are  among  the  most 
primitive  bivalves,  and,  further,  since  the  heart  of  the  Amphineura,  the 
Scaphopoda,  and  the  Cephalopoda  also  lies  over  or  behind  the  hind-gut. 
The  perforation  of  the  heart  by  the  hind-gut  must  have  arisen  by  the 
bending  of  the  ventricle  down  round  the  latter. 

The  heart  in  the  above-mentioned  genera  is  further  distinguished  by  the  fact 
that  the  ventricle  is  more  or  less  elongated  in  the  transverse  direction,  its  lateral 
ends  being  swollen,  while  the  central  part,  which  lies  above  the  intestine,  becomes 
narrower  and  thinner.  This  modification  goes  furthest  in  Area  Noce,  where  there 
seem  to  be  two  lateral  ventricles  unconnected  by  a  central  portion.  This  separation 
of  the  ventricle  into  two  lateral  parts  has  here  brought  about  a  separation  of  the  two 
aorta.  The  two  anterior  as  well  as  the  two  posterior  branches,  however,  after  a 
comparatively  short  separate  course,  unite  to  form  an  unpaired  anterior  and  an  un- 
paired posterior  aorta. 

Although  these  genera  have,  as  a  rule,  a  heart  lying  above  the  hind-gut,  in  some 
specialised  forms  the  heart  is  placed  under  the  hind-gut,  e.g.  Meleagrina,  Ostrea, 
Teredo.  The  cause  of  this  modification  must  lie  in  the  increasing  distance  between 
the  base  of  the  gills  and  the  original  region  of  the  heart,  the  auricles  and  the  ventricle 
having  shifted  with  the  latter.  The  auricles,  however,  no  longer  lie  laterally  to  the 


VII 


MOLLUSCA—THE  CIRCULATORY  SYSTEM 


207 


ventricle,  but  are  drawn  down  to  its  lower  side,  where  they  grow  together,  communi- 
cating through  a  more  or  less  large  aperture.  Pinna,  Aricula,  and  Perna  exhibit 
the  consecutive  stages  in  the  displacement  of  the  heart  to  the  lower  side  of  the  hind- 
gut.  The  shifting  of  the  gills  from  the  original  region  of  the  heart  just  mentioned 
is  caused  by  the  shifting  forward  of  the  posterior  adductor,  which  grows  more  and 
more  massive  and  finally  reaches  a  median  position  on  the  shell  valve.  It  has  already 
been  mentioned  that  this  posterior  adductor,  by  the  continuous  reduction  and  final 
disappearance  of  the  anterior  adductor,  becomes  the  one  adductor  of  the  Mono- 
myaria. 

In   Teredo  also,  the  heart  lies  on  the  under  side  of  the  hind-gut.     This  is  con- 
nected with  the  approximation  of  the  hind-gut  with  the  anus  to  the  mouth  dorsally, 


TO 


FIG.  173. 

FIG.  172.— Transverse  section  through  Anodonta,  to  illustrate  the  course  of  the  circulation 
of  the  gills  and  the  kidneys,  and  the  branchial  veins  (after  Howes),  br,  Gills ;  bre,  efferent 
branchial  vessel  (branchial  vein)  which  opens  into  the  large  branchial  vein  brei,  running  along  the 
base  of  the  gills,  and  here  cut  through  transversely  ;  pi;  pallial  vein  ;  vc,  large  venous  sinus  of  the 
body  ;  kb,  pericardial  gland  ;  a  HI,  auricle  ;  j-j,  rectum  ;  v,  ventricle  ;  rv  and  /TJ,  renal  vessels  ;  bra\, 
afferent  branchial  vessel  (branchial  artery),  running  along  the  base  of  the  gills  ;  bra,  lateral  branches 
of  the  same  running  in  the  gills.  The  veins  or  sinuses  conveying  venous  blood  are  black. 

FIG.  173. — Another  section  through  Anodonta  (after  Howes).  Lettering  as  in  Fig.  17%2.  «». 
auricle ;  sbc,  spaces  at  the  base  of  the  gills,  bathed  by  the  water  and  communicating  with  the 
mantle  cavity,  between  the  ascending  and  descending  branchial  lamellae. 

while  the  gills,  remaining  in  their  original  position,  retain  the  heart  on  the  lower 
side  of  the  hind-gut. 

Circulation  (Fig.  25,  p.  17).— The  arteries  have  walls  of  their  own,  and  branch 
into  fine  vessels,  which  discharge  the  blood  into  the  lacunar  system  of  the  body. 
The  venous  system  seems  to  have  no  distinct  vessels  with  walls  of  their  own. 
although  it  forms  more  or  less  wide  channels  resembling  true  vessels. 

An  anterior  and  a  posterior  aorta  spring,  as  a  rule,  from  the  ventricle.  The 
anterior  aorta  runs  forward  above  the  intestine  and  breaks  up  into  various  arteries. 
The  arteria  visceralis  supplies  the  intestine,  the  digestive  gland,  and  the  genital 
gland  ;  the  pedal  artery  supplies  the  foot ;  the  anterior  pallial  artery  spreads  out 
over  the  anterior  part  of  the  mantle  and  the  oral  lobes  (labial  palps). 


208  COMPARATIVE  ANATOMY  CHAP. 

The  posterior  aorta  leaves  the  ventricle  posteriorly  and  runs  along  the  lower  side 
of  the  hind-gut.  It  soon  divides  into  two  large  lateral  arteries, — the  posterior  pallial 
arteries.  The  principal  branches  of  the  anterior  and  posterior  pallial  arteries  run 
along  the  free  edge  of  the  mantle  on  each  side  and  then  unite,  forming  together  the 
arteries  of  the  pallial  edge.  From  the  roots  of  the  posterior  pallial  artery  smaller 
arteries  spring,  which  supply  with  blood  the  hind-gut,  the  pericardium,  the  posterior 
adductor,  the  retractors  of  the  siphons,  etc.  The  venous  blood  is  collected  out  of 
the  lacunar  system  of  the  body  through  converging  channels  into  one  longitudinal 
venous  sinus  ;  this  lies  under  the  pericardium  (Fig.  172). 

From  this  sinus,  the  greater  part  of  the  blood  flows  through  the  complicated 
system  of  venous  canals  in  the  kidneys,  after  which  it  is  collected  on  each  side  into 
a  branchial  artery  which  runs  along  the  base  of  the  gills,  and  thence  enters  the 
two  branchial  lamellae.  It  becomes  arterial  through  respiration  in  the  gills,  flows  as 
arterial  blood  into  a  branchial  vein  parallel  with  the  branchial  artery,  and  thence 
into  the  auricle. 

Part  of  the  venous  blood,  however,  passes  by  direct  channels  out  of  the  venous 
sinus  into  the  branchial  artery  (passing  by  the  kidneys),  and  part  even  flows  direct 
into  the  pericardium.  In  this  way  some  venous  blood  comes  to  be  mixed  with  the 
arterial  blood  flowing  through  the  heart  from  the  gills. 

Not  all  Lamellibranchia  have  an  anterior  and  a  posterior  aorta  springing  out 
of  the  heart.  In  the  lower  groups  of  the  Protobranchia  and  Filibranchia  there  are 
numerous  forms  (Nucula,  Solenomya,  Anomia,  Mytilidaz)  in  which  only  one  anterior 
aorta  leaves  the  ventricle  ;  this  soon,  however,  gives  off  the  arteria  visceralis,  which 
supplies  blood  to  those  parts  which,  in  other  Lamellibranchia,  are  fed  by  the  aorta 
posterior.  In  their  possession  of  a  single  aorta  rising  from  the  ventricle,  the  above 
lower  Lamellibranchiates  agree  with  Chiton  and  the  Gastropoda.  The  rise  of  this 
aorta  from  the  posterior  end  of  the  ventricle  in  the  Prosobranchia  and  in  most 
Pulmonata  is  a  secondarily  acquired  arrangement,  caused  by  the  shifting  forward 
of  the  pallial  complex. 

It  must  further  be  noted  that  in  a  very  specialised  bivalve,  Teredo,  the  posterior 
aorta  fuses  with  the  anterior,  and  thus  the  two  leave  the  heart  as  one  vessel. 

In  those  Lamellibranchiates  which  have  siphons,  a  muscular  and  contractile 
widening  occurs  in  the  posterior  aorta  near  the  point  where  it  leaves  the  ventricle  ; 
this  is  called  the  bulbus  arteriosus.  Its  special  function  is  perhaps  that  of  bringing 
about  pressure  of  blood,  to  assist  in  the  extension  of  the  siphons.  The  backward 
flow  of  the  blood  into  the  ventricle  in  the  contraction  of  the  bulbus  arteriosus 
(systole)  is  prevented  by  a  linguiform  valve  which  projects  from  its  anterior  wall. 

5.  Cephalopoda. 

Heart  (Figs.  127,  168,  pp.  147,  199,  and  174).— We  must  here  again  point  out  the 
important  fact  that  Nautilus  has  a  heart  with  four  auricles,  while  the  Decapoda  and 
Octopoda  a  heart  with  only  two  auricles.  This  difference  is  connected  with  the 
difference  in  the  number  of  the  ctenidia  :  four  in  Nautilus  (Tetrabranchia),  two 
in  the  Decapoda  and  Octopoda  (Dibranchia). 

In  Nautilus,  the  heart  is  an  almost  square  sac  drawn  out  to  two  points  on  each 
side  ;  the  four  auricles  which  open  into  the  four  points  of  the  ventricle  are  long 
tubes,  more  like  widened  branchial  veins  than  auricles. 

The  strongly  muscular  ventricle  of  the  Dibranchia  is  almost  always  elongated 
into  a  tube.  In  the  Octopoda  it  lies  transversely,  the  two  auricles  being  in  the  same 
plane  with  the  ventricle.  In  the  Ocgopsidce,  the  ventricle  lies  along  the  longitudinal 
axis  of  the  body,  i.e.  it  is  elongated  dorso-ventrally,  and  the  auricles  are  at  right 


VII 


MOLLUSC  A— THE  CIRCULATORY  SYSTEM 


209 


angles  to  it.  The  heart  of  the  Myopsidce  occupies  a  position  halfway  between  those 
just  mentioned. 

The  heart  here  described  is  the  arterial  heart,  which  corresponds  with  the  heart 
of  the  other  Mollusca.  It  is  called  arterial  to  distinguish  it  from  the  venous  hearts, 
which  will  be  described  below. 

Circulation. — It  is  important  to  note  that  the  circulatory  system  is  at  least 
partially  closed.  There  is  not  only  a  richly -branched  arterial,  but  a  richly-branched 
venous  system,  the  vessels  of  which  have  walls  of  their  own.  These  two  systems 
pass  into  one  another  in  certain  parts  of  the  body,  e.g.  the  integument  and  certain 
muscle  layers,  through  a  system  of  capillary  vessels.  In  other  parts,  however,  the 
arterial  branches  conduct  the  blood  into  a  lacunar  system  ;  when  it  has  become 


FIG.  174.— Circulatory  system,  venous  appendages  of  the  nephridial  system,  and  gills  of 
Sepia  officinalis,  anterior  view  (after  Hunter).  1,  Aorta  cephalica  ;  2,  ctenidium ;  3,  vein  leading 
to  the  ctenidium ;  4,  branchial  heart ;  5,  appendage  of  the  branchial  heart  (pericardial  gland) ; 
6,  venous  appendages  of  the  nephridial  system ;  7,  aorta  abdominalis ;  8,  vena  abdominalis  ;  9, 
lateral  veins  ;  10,  vena  cephalica  ;  11,  auricles  ;  12,  ventricle  (cf.  Fig.  186). 

venous,  the  blood  collects  out  of  this  into  sinuses  (especially  into  a  peripharyngeal 
cephalic  sinus),  and  flows  to  the  gills  through  veins  with  walls  of  their  own. 

Two  aorta  rise  from  the  ventricle :  (1)  the  aorta  cephalica,  which  runs  downward 
(upwards  in  the  figure)  to  the  head,  and  (2)  the  aorta  abdominalis,  which  runs  up 
towards  the  apex  of  the  visceral  dome.  The  former  is  much  stronger  than  the  latter. 
The  aorta  cephalica  first  gives  off  branches  to  the  mantle  and  to  the  anterior  wall  of 
the  body,  and  then  provides  the  stomach,  the  pancreas,  the  digestive  gland,  the 
oesophagus,  the  salivary  glands,  and  the  funnel  with  arteries.  After  accompanying 
the  oesophagus,  it  divides  in  the  head  into  two  branches,  which  run  to  the  bases  of 
the  arms,  and  there  break  up  into  as  many  arterise  brachiales  as  there  are  arms. 

The  aorta  abdominalis  supplies  with  arteries  the  hind-gut,  the  ink-bag,  the 
genital  organs,  the  dorsal  part  of  the  body  wall,  and  the  fins,  \vhen  these  latter  are 
present. 

Only  in  the  Oegopsidce  are  the  aorta  limited  to  the  two,  above  described,  springing 
from  the  heart.  In  the  Odopoda  and  the  Myopsidce,  there  are  other  arteries  rising  out 
of  the  ventricle,  and  running  to  the  same  part  of  the  body  as  the  aorta  abdominalis 
VOL.  II  P 


210  COMPARATIVE  ANATOMY  CHAP. 

in  the  Oegopsidce  ;  among  these  are  the  arteria  genitalis,  which  runs  to  the  genital 
glands,  and,  in  the  Myopsidce,  a  fine  vessel  called  the  arteria  anterior. 

At  certain  places,  the  arteries  may  swell  out  to  form  small  muscular  and  con- 
tractile widenings,  called  peripheral  arterial  hearts. 

In  the  venous  system  of  Sepia,  the  venous  blood  in  each  arm  collects  (partly 
through  capillaries  and  partly  through  lacunae)  into  a  vein  running  down  the  inner 
side  of  the  arm.  All  the  brachial  veins  convey  their  blood  to  a  circular  cephalic 
sinus  surrounding  the  buccal  mass,  which  is  the  reservoir  for  collecting  the  venous 
blood  from  the  whole  head  region.  Out  of  this  sinus  springs  the  large  vena 
cephalica,  which  runs  up  along  the  posterior  side  of  the  oesophagus  and  the  liver 
into  the  visceral  dome,  collecting  on  the  way  venous  blood  from  the  liver,  the  funnel, 
etc.  A  little  below  the  stomach  it  forks,  forming  the  two  venae  cavse,  which  open 
into  the  two  contractile  venous  hearts  at  the  bases  of  the  gills.  From  the  upper 
part  of  the  visceral  dome  the  blood  collects  into  several  abdominal  veins,  the  most 
important  of  which  are  an  unpaired  vena  abdominalis,  opening  into  the  vena 
cephalica  exactly  at  the  point  where  it  divides  into  the  vense  cavse,  and  two  lateral 
abdominal  veins,  which  open  into  the  latter  near  their  point  of  entrance  into  the 
branchial  hearts. 

In  the  region  of  the  heart,  all  these  veins  carry  acinose  or  lobate  appendages 
(venous  appendages),  which  are  hollow,  and  communicate  at  many  points  with  the 
veins,  so  that  they  are  richly  supplied  with  blood.  The  cavity  into  which  these 
appendages  project  is  that  of  the  renal  sacs,  and  the  epithelium  which  covers  them 
belongs  to  the  epithelial  wall  of  the  kidneys  (cf.  Fig.  186,  p.  224).  We  thus  see 
that  here  the  blood  flowing  back  from  the  body  has  abundant  opportunity  of  giving 
off  its  excretory  constituents  to  the  kidneys. 

Appendages  are  found  on  both  the  branchial  hearts  ;  these  are  the  pericardial 
glands,  which  will  be  further  described  later.  The  two  branchial  hearts,  by  their 
contraction,  drive  the  venous  blood  into  the  afferent  branchial  vessel.  The  blood, 
which  has  become  arterial  in  the  gills,  flows  through  the  efferent  branchial  vessel 
(the  so-called  branchial  veins)  into  the  auricles  of  the  heart,  and  thence  into  the 
ventricle  (on  the  branchial  circulation,  cf.  p.  96). 

In  the  Cephalopoda,  unlike  the  other  Mollusca,  the  whole  of  the  blood,  in 
returning  from  the  body,  flows  through  the  gills,  so  that  the  heart  contains  only 
arterial  blood.  By  far  the  greater  part  of  the  blood,  before  entering  the  gills,  conies 
into  contact  with  the  kidneys  in  the  venous  appendages. 

In  the  Octopoda,  the  venous  system  shows  some  not  unimportant  modifications. 
In  Octopus,  two  veins,  connected  with  one  another  by  anastomoses,  run  along  the 
outer  side  of  each  arm  and  collect  the  venous  blood.  At  the  bases  of  the  arms  these 
veins  become  connected  in  pairs,  and  unite  later  in  such  a  way  as  to  form  on  each 
side  a  lateral  cephalic  vein. 

These  two  veins  unite  to  form  the  large  vena  cephalica,  which  runs  up  in  front 
of  the  funnel  and  behind  the  oesophagus.  The  brachial  veins  do  not  here,  as  in 
Sepia,  convey  their  blood  first  to  the  venous  cephalic  circular  sinus,  but  are  directly 
connected  with  the  cephalic  vein.  A  cephalic  sinus  nevertheless  exists  in  Octopus  ; 
it  is  not,  however,  connected  with  the  vena  cephalica,  but  with  a  large  sinus  which 
fills  the  whole  visceral  dome,  and  is,  in  fact,  the  primary  body  cavity,  in  which 
the  viscera  lie  bathed  by  the  venous  blood.  The  latter  flows  out  of  this  large 
venous  sinus  through  two  wide  veins,  the  so -called  peritoneal  tubes,  into  the 
upper  part  of  the  vena  cephalica,  near  the  point  where  this  divides  into  the  two 
vense  cavse. 

Nautilus  is  chiefly  distinguished  by  the  absence  of  the  branchial  hearts. 
Further,  each  of  the  two  vense  cavae  divides  into  two  branches,  which  run,  as 
afferent  vessels,  to  the  gills. 


vii  MOLLUSCA—THE  BODY  CAVITY  211 

XVIII.  The  Body  Cavity. 
Primary  and  Secondary  Body  Cavity,  Pericardium,  Pericaxdial  Gland. 

The  Mollusca  are  said  to  have  a  primary  and  a  secondary  body 
cavity.  The  former  is  the  system  of  laeunse  and  sinuses,  into 
which  the  arteries  open,  and  out  of  which  the  veins,  where  these  are 
present,  draw  their  blood.  It  has  no  epithelial  walls  of  its  own,  its 
boundaries  are  formed  by  connective,  nerve,  or  muscle  tissue,  or  by 
epithelia,  which,  however,  belong  to  other  organs,  such  as  the  intestine, 
the  kidneys,  or  the  body  wall. 

The  so-called  secondary  body  cavity  or  eoelom  is,  in  most  Mollusca, 
very  much  reduced,  usually  consisting  of  only  two  small  cavities,  the 
pericardium  and  the  cavity  of  the  gonads  (testes,  ovaries,  or  her- 
maphrodite glands).  The  ccelom  is  always  lined  by  an  epithelium  of 
its  own,  the  ccelomic  epithelium,  and  corresponds  with  the  true  eoelom 
of  the  Annelida,  which  also  possesses  such  an  epithelium.  Like  the 
latter,  it  is  connected,  by  means  of  the  nephridial  funnel,  with  the 
nephridia,  which  lead  to  the  exterior,  and  in  Molluscs  are  usually 
found  only  in  one  pair.  A  probe  can  therefore  be  introduced  through 
the  kidney  into  the  eoelom,  i.e.  into  that  part  of  it  which,  containing 
the  heart,  is  called  the  pericardium.  The  germinal  layers  must  be 
considered  as  proliferations  of  the  coelomic  endothelium.  The  epi- 
thelium of  the  pericardium  is,  in  very  many  Molluscs,  differentiated 
into  glands,  called  the  pericardial  glands ;  these  probably  may  be 
classed  together  with  the  kidney  as  excretory. 

We  should  be  justified  in  assuming,  a  priori,  that  the  lumen  of  the 
genital  glands  of  the  Mollusca  is  part  of  a  true  eoelom,  and  that 
the  germinal  layers  themselves,  i.e.  that  complex  of  cells  which  yields 
the  eggs  and  spermatozoa,  are  outgrowths  of  the  endothelial  wall  of 
this  eoelom.  Direct  support  is,  however,  given  to  this  assumption  by 
the  fact  that  in  the  Solenogastres,  Sepia,  and  Nautilus,  the  sac  of  the 
genital  glands  is  in  open  communication  with  the  rest  of  the  eoelom, 
forming,  in  fact,  an  only  partly  distinct  division  of  the  same. 

In  the  Solenogastres  (e.g.  Proneomenia),  the  hermaphrodite  gland  lies  above  the 
mid -gut  as  a  long  tube,  which  in  transverse  section  appears  heart-  or  kidney-shaped, 
as  its  lower  part  bulges  out  on  each  side.  Its  shape  is  determined  by  the  fact  that 
the  mid-gut  forms  dorsally  a  narrow  but  deep  furrow,  which  cuts  into  this  glandular 
tube  from  below.  The  tubular  gland  is  divided  into  two  lateral  spaces  by  a  partition, 
whose  endothelial  wall  is  the  place  of  formation  of  the  eggs  ;  these  lateral  chambers 
may  again  be  traversed  by  septa,  on  which  the  genital  products  develop.  This 
division  is  especially  distinct  at  the  posterior  part  of  the  tube,  the  two  chambers 
being  there  completely  isolated,  and  entering  the  pericardium  separately  as  genital 
ducts. 

If  the  secondary  body  cavity  of  Proneomenia  is  compared  with  that  of  an  Annelid, 
we  find  the  following  differences  : 


212 


COMPARATIVE  ANATOMY 


CHAP. 


In  Proneomenia,  the  dorsal  vessel  is  wanting  in  the  region  of  the  mid-gut.  The 
ccelom  is  much  less  spacious,  and  instead  of  surrounding  the  intestine  lies  only  on 
its  dorsal  side.  It  is  developed  merely  as  a  hermaphrodite  glandular  sac,  its  endo- 
thelial  wall  yielding  the  genital  products. 

In  the  region  of  the  hind -gut,  the  vessel  lying  in  the  dorsal  mesentery  is  developed 
as  a  heart,  the  coelom  being  here  represented  by  the  pericardium. 


4  — 


&' 


FIG.  175.— Diagrammatic  sections  through  an  Annelid  (A)  and  a  Solenogastrid  (B  and  C),  to 
illustrate  the  relation  of  the  coelom  to  the  genital  glands  and  nephridia.  B,  Eegion  of  the  cloaca  ; 
C,  region  of  the  mid-gut ;  1,  dorsal  mesentery  ;  2,  dorsal  vessel  or  heart ;  3,  germinal  epithelium  ; 

4,  coelom— in  B= pericardium,  in  C= hermaphrodite  gland  (in  the  coelom  are  genital  products); 

5,  nephridia  ;  6,  intestine  ;  7,  cloaca. 

The  pericardium  is  connected  with  the  cloaca  by  two  canals  ;  these  may  be 
considered  as  the  morphological  equivalents  of  nephridia  (cf.  Fig.  175). 

As  the  genital  glands  have  been  recognised  as  part  of  the  ccelom  in  the  Soleno- 
gastres,  Nautilus,  and  Sepia,  they  must  necessarily  fall  under  the  same  category  in 
all  other  Molluscs,  even  when  no  longer  in  direct  connection  or  in  open  communica- 
tion with  the  same. 

In  the  Chitonidw,  the  coelom  is  large,  and  falls  into  three  distinct  divisions.  One 
contains  the  intestine  and  digestive  gland  (liver),  which  are  accordingly  outwardly 


n  V 

FIG.  176.— Diagrammatic  longitudinal  section  through  Chiton,  to  illustrate  the  relation 
between  the  various  parts  of  the  coelom  (after  Haller).  1-8,  Position  of  the  eight  dorsal  shell- 
plates;  M,  anterior  portion  of  the  dorsal  integument;  L,  snout;  TO,  mouth;  /,  digestive  gland 
(liver);  d,  intestine;/,  foot;  ti,  kidney;  p,  pericardium;  c,  portion  of  the  coelom  surrounding  the 
intestine ;  h,  heart ;  Ip,  band  connecting  pericardium  and  genital  gland ;  gdr,  genital  gland ;  la,  band 
connecting  the  genital  gland  and  the  posterior  portion  of  the  coelom  which  surrounds  the  intestine. 

(i.e.  on  the  side  turned  to  the  ccelom)  covered  with  an  endothelium.  The  mesen- 
teries, however,  which  originally  attached  the  intestine  to  the  body  wall,  and 
along  which  the  parietal  endothelium  passed  into  the  visceral  endothelium  of  the 
intestine  and  liver,  have  disappeared,  with  the  exception  of  portions  retained  on 
the  hind-gut.  The  two  other  divisions  of  the  ccelom  are  :  (1)  the  pericardium,  and 


VII 


MOLLUSCA—THE  BODY  CAVITY 


213 


(2)  the  genital  gland.  Certain  bands,  by  means  of  which  the  three  divisions  are 
connected  together,  have  been  regarded  as  the  constricted  remains  of  communications 
between  the  three  divisions  of  the  originally  single  coelom  (Fig.  176). 

The  Cephalopoda  may  with  advantage  be  considered  in  connection  with  the 
AmpJiineura.  In  NautihLS  and  the  Decapoda  (e.g.  Sepia,  Fig.  177)  a  spacious 
secondary  body  cavity  is  found  in  the  dorsal  part  of  the  visceral  dome.  It  is  incom- 
pletely divided  by  a  projecting  dorsal  septum  into  two  cavities,  one  lying  above  the 
other  ;  the  lower  of  these  contains,  as  pericardium,  the  heart  with  the  arteries  and 
veins  running  out  of  and  into  it,  the  branchial  hearts,  and  the  pericardial  glands  ; 
while  the  upper  holds  the  stomach  and  the  genital  glands.  This  double  cavity, 


FIG.  177. — Diagram  showing  the 
secondary  body  cavity  of  Sepia  (after 
Grobben).  Median  longitudinal  section 
through  the  body,  in  which,  however,  some 
organs  are  represented  which,  being  paired 
and  symmetrical,  do  not  properly  come 
into  the  plane  of  the  section.  The  outlines 
of  the  coelom  are  indicated  by  thicker  lines. 
1,  Female  germinal  body,  with  eggs  (2)  pro- 
jecting into  the  genital  cavity  (the  ovarial 
division  of  the  coelom) ;  3,  shell ;  46,  an- 
terior portion  of  the  renal  sac ;  5,  pancreatic 
appendage  of  the  efferent  duct  (bile  duct) 
of  the  digestive  gland  (liver)  ;  4a,  anterior 
venous  appendage  of  the  renal  system  ;  6, 
aperture  (funnel)  of  the  kidney  into  the 
coelom  ;  7,  outer  or  pallial  aperture  of  the 
kidney ;  8,  digestive  gland  (liver) ;  9, 
"head"  (Kopffuss);  10,  funnel;  11,  end  of 
the  oviduct  with  female  genital  aperture  ; 
12,  mantle  cavity ;  13,  mantle ;  14,  posterior 
portion  of  the  renal  sac ;  15,  intestine ;  14j, 
posterior  venous  appendage  of  the  renal 
system  (pericardial  gland);  IS,  fold,  in- 
completely dividing  the  coelom  into  an 
upper  and  a  lower  portion ;  19,  stomach ; 
20,  upper  division  of  the  coelom  (principally 
genital  cavity);  21,  pigment  gland  (ink- 
bag)  ;  22,  aperture  of  the  oviduct  into  the 
genital  cavity ;  rf,  dorsal ;  v,  ventral ;  a, 
anterior;  p,* posterior. 


22 


which  is  called  the  viscero-pericardial  cavity,  is  covered  by  endothelium,  which  also 
covers  the  organs  within  it.  It  is  connected  by  two  ciliated  funnels  with  the  two 
renal  sacs.  In  Nautilus  it  also  opens  direct  into  the  mantle  cavity  by  two  canals, 
whose  apertures  lie  close  to  the  renal  apertures. 

While  the  ccelom  in  Nautilus  and  the  Decapoda  is  very  spacious,  in  the  Oetopoda, 
on  the  contrary,  it  is  very  much  reduced.  It  consists  merely  of  a  narrow  system  of 
canals,  which,  however,  have  thick  walls  ;  this  was  formerly  called  the  water  vascular 
system.  The  organs,  which  in  Nautilus  and  the  Decapoda  lie  in  the  coelom,  viz. 
the  arterial  heart  with  its  afferent  and  efferent  vessels,  the  branchial  hearts  and  the 
stomach,  are  no  longer  found  within  the  body  cavity,  but  outside  of  it,  and  are 


214 


COMPARATIVE  ANATOMY 


CHAP. 


— \ 19 


therefore  no  longer  covered  with  endothelium.  Nevertheless  this  canal  system  of 
the  Octopoda  shows  the  same  morphologically  important  characteristics  as  the  ccelom 
of  the  Dccapoda.  There  are,  for  instance,  on  each  side  three  canals  which  open 
together,  one  entering  the  renal  sac,  the  second  widening  round  the  pericardial 
gland  (appendage  of  the  branchial  heart)  to  form  a  flask-shaped  capsule,  and  the 
third  running  to  the  genital  gland  to  be  continued  into  its  wall.  [In  so  far  as  in  the 
Octopoda  the  heart  is  excluded  from  the  coelom,  which  has  been  reduced  to  the  ' '  water 
canal  system,"  the  reduction  of  this  cavity  has  gone  further  in  these  Mollusca  than 
in  any  others,  which  all  retain  at  least  the  heart  in  one  portion  of  the  ccelom,  the 
pericardium. 

In  the  Lamellibranchia  and  Gastropoda,  the  only  part  of  the  ccelom  retained, 
besides  the 'genital  glands,  is  the  pericardium.     The  pericardium  and  the  gonad  are, 
•  however,    entirely    separated 

from  one  another.  In  Lamelli- 
branchs,  there  is  in  the  peri- 
cardium, besides  the  heart,  a 
part  of  the  hind  -  gut  which 
traverses  it ;  in  the  Gastropoda 
(except  in  those  Diotocardia 
in  which  the  hind-gut  pene- 
trates the  heart),  only  this 
latter  organ.  Rarely  (<\g. 
Phyllirhoe)  the  auricle  also 
is  excluded  from  the  peri- 
cardium. 

The  pericardial  gland  is 
found  in  most  Mollusca.  It 
is  a  glandular  differentiation 
of  the  endothelial  wall  of  the 
pericardium,  and  perhaps,  as 
already  suggested,  shares  the 
excretory  functions  of  the 
kidney.  Its  position  in  the 
pericardium  varies,  but  it 
seems  in  all  cases  shut  off  from 
the  blood  vascular  system, 
with  which  it  is,  however, 
functionally  connected.  Its 
secretions  or  excretions  must  be  discharged  into  the  pericardium,  and  thence  out- 
wards through  the  kidney. 

Among  the  Prosobranchia,  in  the  Diotocardia,  the  pericardial  gland  is  found  on 
the  auricle,  its  walls  forming  dendriform  branched  outgrowths  into  the  pericardial 
cavity,  these  being  covered  with  pericardial  endothelium.  Where  pericardial  glands 
are  found  in  the  Monotocardia,  they  lie  on  the  wall  of  the  pericardium  itself. 
Similar  lobate  formations  occur  among  the  Opisthobranchia,  in  Aplysia,  and 
Notarchus,  on  the  anterior  aorta  which  runs  along  the  pericardial  wall ;  in  Pleuro- 
branchus  and  Plcurobranchcea  on  the  lower,  in  Doridopsis  and  Phyllidcea  on  the  dorsal 
pericardial  wall.  The  lateral  furrows  of  the  pericardium  of  Doris  form  niches,  which 
may  again  have  accessory  niches.  These  enlargements  of  the  surface  of  the  peri- 
cardial epithelium  have  also  been  considered  as  pericardial  glands. 

Pericardial   glands  are  much  more  common  among  the  Lamellibranchia  than 
among  the  Gastropoda,  but  are  wanting  in  the  most   primitive  forms   (Nucula, 
Anomia).     The  gland  is  usually  of  a  rusty  red  colour,  and  occurs  in  two 


FIG.  ITS.— Eledone  moschata.  This  figure  corresponds  with 
Fig.  177  of  Sepia  (after  G-robben).  Si,  Efferent  duct  of  the 
digestive  gland;  17a,  pericardial  gland  (appendage  of  the 
branchial  heart) ;  23,  water  canals. 


viz  MOLLUSCA—THE  NEPHRIDIA  215 

forms,  consisting  either  of  glandular  protrusions  of  the  endothelial  wall  of  the 
auricles  into  the  pericardial  cavity,  or  of  glandular  tubes  protruding  from  the 
anterior  corner  of  the  pericardium  into  the  mantle  Keber's  organ,  red-brown 
organ).  The  first  form  is  found  specially  strongly  developed  in  Mytilus,  Lithodomus, 
and  Saxicava,  more  or  less  developed  in  Dreissena,  Unio,  Anodonta,  Venus,  Car- 
dium,  Scrobicularia,  Solen,  Pholas,  and  Teredo,  and  more  or  less  rudimentary  in 
Pecten,  Spondylus,  Lima,  Ostrea.  The  second  form  has  been  observed  in  Unio, 
Anodonta,  Venus,  Cardium,  Scrobicularia,  Solen,  Pholas,  Montacuta,  and  Dreissensia. 
Pericardial  glands  may  also  occur  singly  in  other  parts  of  the  pericardium,  as  in 
Meleagrina  (as  a  projecting  ruff  in  the  posterior  base  of  the  pericardium),  and  in 
Ohama  on  the  ventricle,  etc. 

The  pericardial  gland  of  the  Cephalopoda  is  the  so-called  appendage  of  the 
branchial  hearts.  This  is  a  structure  connected  with  the  branchial  heart,  and 
covered  with  peritoneal  endothelium,  which  projects  into  the  viscero-pericardial 
cavity,  or,  in  the  Octopoda,  into  a  flask-like  widening  of  the  water-canal  system 
(which  has  been  recognised  as  a  division  of  the  cffilom).  In  Sepia  this  appendage  is 
conical.  A  deep  furrow  on  the  surface  which  projects  into  the  viscero-pericardial 
cavity  leads  into  a  richly-branched  system  of  canals,  the  glandular  epithelium  of 
which  is  a  continuation  of  the  peritoneal  epithelium.  Blood  sinuses  from  the 
branchial  heart  penetrate  in  between  the  canals  of  this  system.  In  other  Cephalo- 
poda, the  pericardial  gland  varies  in  form  and  structure  ;  details  of  these  variations 
cannot,  however,  be  here  given.  Nautilus  possesses  two  pairs  of  pericardial  glands  ; 
this  fact  is  connected  with  its  possession  of  two  pairs  of  gills,  with  their  two  pairs 
of  afferent  vessels,  and  on  these  the  two  pairs  of  pericardial  glands  occupy  positions 
corresponding  with  those  of  the  branchial  hearts. 


XIX.  The  Nephridia. 
Kidney,  Organ  of  Bojanus. 

The  organs  which  serve  for  excretion  are  homologous  in  all 
Mollusca. 

They  consist  typically  of  two  symmetrical  sacs,  which,  on  the  one 
hand,  open  into  the  mantle  cavity,  through  the  two  outer  renal 
apertures,  and  on  the  other  are  connected  by  two  inner  apertures 
(renal  funnels,  ciliated  funnels)  with  the  pericardium  or  ccelom.  The 
nephridia  always  lie  near  the  pericardium.  Their  walls  are  richly 
vascularised,  indeed  a  large  part  of  the  venous  blood,  in  returning 
from  the  body,  flows  through  the  renal  walls  and  gives  off  excretory 
matter  before  it  enters  the  respiratory  organs.  The  renal  walls 
are  traversed  exclusively  by  venous  blood. 

The  nephridia  are  paired  in  all  symmetrical  Molluscs,  and  also  in 
those  Gastropoda  which  have  paired  gills  and  two  auricles  (Diotocardia). 

In  all  other  Gastropoda,  along  with  the  original  right  ctenidium 
(which,  in  the  Prosobranchia,  lies  to  the  left),  and  the  corresponding 
auricle,  only  one  kidney  (the  corresponding  one)  is  retained. 

Nautilus,  which  has  four  gills  and  four  auricles,  has  also  four 
kidneys ;  only  two  of  these,  however,  communicate  with  the  viscero- 
pericardial  cavity. 


216 


COMPARATIVE  ANATOMY 


CHAP. 


A  relation  between  the  nephridial  and  genital  systems  similar 
to  that  found  in  the  Annelida  exists  in  the  Solenogastridce,  the 
nephridia  functioning  as  ducts  for  the  genital  products,  the  latter 
passing  from  the  hermaphrodite  gland  (genital  chamber  of  the  coelom) 
into  the  pericardium. 

In  a  few  Lamellibranchia,  Diotocardia,  and  in  the  Scaphopoda,  there 
is  a  relation  between  the  genital  glands  and  the  nephridia,  the  former 
opening  into  the  latter;  so  that  a  certain  part  of  the  nephridium 
functions  not  only  as  renal  or  urinary  duct,  but  also  as  efferent  genital 
duct.  In  all  Diotocardia,  it  is  the  right  nephridium  which  functions  as 
genital  duct.  In  the  Monotocardia,  in  which  the  right  nephridium  of 
the  Diotocardia  has  atrophied  as  such,  its  duct  persists  as  genital  duct. 
In  all  other  Molluscs  the  genital  ducts  are  entirely  distinct  from  the 
urinary  passages. 

A.  Amphineura. 

The  kidneys  of  the  Solenogastridce  and  the  Chitonidcc  differ  greatly  from  one 
another  in  structure. 

1.  In  the  Solenogastridce,  two  canals  spring  from  the  pericardium,  embrace  the 
hind-gut,  and  open  into  the  cloaca  beneath  it  through  a  common  terminal  portion 


FIG.  179.—  Paramenia  impexa.  Posterior  end  of  the  body  ;  the  integument  must  be  supposed 
to  be  reinoved  on  the  right  side,  and  also  a  piece  of  the  wall  of  the  right  nephridium  ;  diagram  (after 
Pruvot).  1,  Integument ;  2,  ovarial  portion  of  the  hermaphrodite  gland ;  3,  testicular  portion  of 
the  same,  near  the  point  where  the  latter  opens  into  the  pericardium  (4) ;  5,  glandular  appendage 
of  the  right  nephridium  ;  6,  dorsal  commissure  of  the  pleurovisceral  cords ;  7,  organ  called  the 
sensory  bud ;  8,  aperture  of  the  hind-gut  into  the  cloaca ;  9,  gill ;  10,  cloaca ;  11,  common  aper- 
ture of  the  nephridia  into  the  cloaca :  12,  lower  portion  of  the  nephridium;  13,  upper  portion  of 
the  right  nephridium,  which  opens  above  into  the  pericardium  ;  14,  hind-gut. 

(Fig.  179).  These  canals  function  as  ducts  for  the  genital  products.  It  is  also 
certain  that  they  correspond  morphologically  with  the  kidneys  of  other  Molluscs, 
even  though  their  excretory  activity  has  not  been  proved.  They  are  covered  with 
an  extraordinarily  deep  epithelium  of  long  filiform  glandular  cells. 

In  some  Solenogastridce,  an  accessory  gland  opens  into  each  nephridial  canal. 

2.  In  the  Chitonidcc,  the  strongly- developed  paired  nephridia  function  exclusively 
as  excretory  organs. 

Each  nephridium   (Fig.  180)  consists  of  a  wide  canal  shaped  like  a  long  Y, 


VII 


MOLLUSCA—THE  NEPHRIDIA 


217 


the  diverging  portions  being  directed  backward,  and  the  undivided  portion 
forward.  These  Y-shaped  kidneys  run  longitudinally  along  each  side  of  the  body 
through  its  whole  length.  One  of  the  paired  limbs  of  the  Y  opens  outward  into 
the  posterior  part  of  the  mantle  cavity,  the  other  into  the  pericardium,  which  also 
lies  in  the  posterior  part  of  the  body.  In  this  way  the  pericardial  and  outer  aper- 


3- 


10 


11 


FIG.  180. — Nephridial  and  genital  systems  of  Chiton,  diagrammatic,  from  above,  after  the 
figures  and  accounts  of  various  authors.  1,  Mouth  ;  2,  gills  ;  3,  unpaired-portion  of  the  nephridium 
which  runs  forward,  with  its  lateral  branches  ;  4,  gonad  ;  5,  efferent  ducts  of  the  gonad  ;  6,  portion 
of  the  'nephridium  running  to  the  outer  aperture  (10) ;  7,  portion  running  to  the  reno-pericardial 
aperture  (9) ;  8,  genital  apertures ;  9,  reno-pericardial  funnel ;  10,  nephridial  aperture ;  11,  peri- 
cardium, indicated  only  in  outline  ;  12,  anus. 

tures  of  the  kidney  lie  near  one  another.  The  third  limb  of  the  Y  ends  blindly 
anteriorly.  Secondary  lobules  or  lobed  canals  open  into  all  the  three  parts  of  the 
kidney,  and  are  specially  abundant  in  its  anterior  portion.  Except  in  the  terminal 
portion  of  the  efferent  branch,  the  epithelium  of  the  limbs  as  well  as  that  of  the  lobes 
is  cubical  and  ciliated. 

B.  Gastropoda. 

1.  Prosobranchia.  (a)  Diotocardia.  —  Among  all  the  Gastropoda,  Fissurella 
alone  possesses  a  symmetrical  excretory  apparatus,  in  the  sense  of  having  two 


218 


COMPARATIVE  ANATOMY 


CHAP. 


nephridia  opening  into  the  mantle  cavity  to  the  right  and  left  of  the  anus.  The 
left  nephridium  is,  however,  much  reduced,  while  the  right,  which  is  strongly 
developed,  sends  its  lobes  everywhere  into  the  spaces' between  the  lobes  of  the  liver, 
the  intestine,  and  the  genital  organs.  There  are  no  reno-pericardial  openings.  The 
genital  gland  does  not  open  direct  into  the  mantle  cavity,  but  through  the  right 
kidney. 

In  Haliotis,  Turbo,  and  Trochus,  both  nephridia  are  present.  The  left  nephri- 
dium has,  however,  almost  entirely  lost  its  excretory  function,  but  is  still  connected 
both  with  the  pericardium  and  the  mantle  cavity.  It  is  called  the  papillar  sac,  its 
walls  projecting  into  its  cavity  in  the  form  of  numerous  large  papilla*.  The  blood 
lacunre  which  penetrate  into  the  papilla*  communicate  direct  with  the  auricles,  and 
are  thus  supplied  with  arterial  blood.  In  these  lacuna;  of  the  papilla*  a  crystalloid 
substance  (albumen  ?)  is  deposited.  It  has  been  thought  that  these  papillar  sacs 
serve  as  reservoirs  of  nutritive  material  (in  the  form  of  the  crystalloids  just  men- 
tioned), and  when  needed  yield  it  up  to  the  blood. 

The  right  nephridium  is  exclusively  excretory  in  function.  It  is  divided  into 
two  lobes,  one  behind  the  other,  which  communicate  by  means  of  a  wide  aperture  ; 
the  anterior  lobe  lies  under  the  floor  of  the  mantle  cavity,  bulging  it  upward.  A 
spongy  network,  covered  with  excretory  epithelium,  rises  from  part  of  its  wall  into 
the  cavity  of  the  nephridial  sac.  The  meshes  of  the  network  are  penetrated  by  a 
system  of  vessels  with  walls  of  their  own.  Nearly  all  the  venous  blood,  before 

reaching  the  gills,  passes  through  the 
vascular  system  thus  developed  on  the  walls 
of  the  kidneys.  The  right  nephridium  is 
in  no  Avay  connected  with  the  pericardium. 
The  Neritidse  have  only  one  nephridium 
to  the  right  of  the  heart,  which  opens 
through  a  slit  in  the  base  of  the  mantle 
cavity.  The  renal  sac  is  traversed  by  trabe- 
cul«,  many  of  which  reach  from  one  wall  to 
the  other,  forming  a  spongy  structure.  The 
trabeculae  are  covered  by  a  glandular  epithe- 
lium on  the  surfaces  turned  to  the  spaces  of 
the  sac. 

Patella  (Fig.  181)  still  has  two  nephridia, 
both  functioning  as  excretory  organs.     The 
apertures  lie  at  the  two  sides  of  the  anus. 
FIG.  I8l.-Diagram  of  the  two  nephridia    The  right  kidney  is   however,  much  larger 

"D*k+Al1«»     /n-ft-^**    T  AVkl»-sk«+A-M\          7.««         A«J-««:«»,  * 


of  Patella  (after  Lankester).    ksa,  Anterior 
and  upper  lobe  of  the  large  right  kidney  Jcsl ; 


,,  in,       m,        ,    .-,    v     .     , ,       •  ,        - 

tlian  tlie  left"     The?  b°th  lie  t0  the  nSht  °f 

Tcsi,  lower  subvisceral ;  ksp,  posterior  lobe  of  tne  pericardium,  but  there  are  no  reno-peri- 
the  same  ;  /,  subanal  tract  of  the  large  right  cardial  apertures.  The  internal  structure 
kidney  ^analjjapilla  with  the  portion  of  the  of  the  right  kidney  is  spongy,  but  the  left 

forms   a   simple   cavity,   into  which   folds 
project  from  the  walls.     A  lacunar  system 


rectum  which  runs  to  it ;  h,  papilla  with  the 
aperture  of  the  left  kidney  (which  is  not 
drawn) ;  /,  the  same  of  the  right  kidney  ;  I, 


pericardium,  indicated  by  a  dotted  outline  ;    without  special  walls  traverses  the  trabeeular 


the  existence  of  the  reno-pericardial  aperture 
figured  near  /,  is  now  denied. 


network  of  the  right  kidney,  but  is  com- 
pletely cut  off  from  its  cavity  ;  the  venous 
blood  from  the  body  passes  through  this 
The  lacunar  system  of  the  left  kidney  communicates 


system  before  entering  the  gills, 
directly  with  the  auricle. 

In  Haliotis  and  Patella  also  the  genital  products  pass,  as  in  Fissurella  and  the 
Diotocardia  generally,  out  of  the  genital  gland  into  the  right  kidney,  and  are  ejected 
through  the  right  renal  aperture. 


vii  MOLLUSC  A— THE  NEPHEIDIA  219 

(b)  Monotocardia. — The  Monotocardia  have  only  one  nephridium  functioning  as 
an  excretory  organ,  viz.  the  left  of  the  Diotocardia.  This  takes  the  form  of  a  sac 
lying  immediately  below  the  mantle  cavity  on  the  right  side  of  the  pericardium, 
directly  under  the  integument.  It  is  generally  found  to  the  left  of  the  hind- 
gut  ;  less  frequently  (Cassidari",  Tritoniidce)  the  kidney  is  traversed  by  the  rectum, 
or  the  latter  runs  forward  below  it.  The  slit-like  pallial  aperture  of  the  kidney, 
however,  is  always  found  to  the  left  of  the  hind-gut,  quite  at  the  base  of  the  mantle 
cavity.  This  position  of  the  kidney,  and  especially  of  its  outer  apertures,  had 
already  led  to  the  assumption  that  the  Monotocardian  nephridium  corresponds  with 
the  left  kidney  of  the  Diotocardia,  before  this  fact  was  established.  The  assump- 
tion was  all  the  more  plausible  because  of  the  occurrence  of  a  gland  called  the 
anal  kidney  in  a  few  Monotocardia  (e.g.  Dolium)  ;  this  gland  opens  to  the  right 
near  the  anus,  and  might  represent  the  right  kidney  of  the  Diotocardia. 

The  kidney  is  always  connected  by  means  of  a  canal  (the  reno-pericardial  canal) 
with  the  pericardium. 

Lamellse  or  trabeculse,  covered  with  the  glandular  epithelium  of  the  kidney, 
project  inward  from  the  lateral  walls  of  the  renal  sac.  -These  are  especially 
strongly  developed  in  fresh -water  Prosobranchia  (excepting  Valvata),  traverse 
the  whole  kidney,  and  impart  to  it  a  spongy  structure.  The  venous  blood  always 
flows  through  the  whole  of  the  glandular  part  of  the  kidney,  either  in  special 
vessels  or  in  lacuna,  before  passing  on  to  the  gills  ;  but  an  open  communication 
with  the  renal  cavity  is  never  found. 

In  the  Tcenioglossa  Proboscidifera  the  kidney  forms  two  lobes  similar  in  struc- 
ture. In  Natica  and  Cyprcca  the  lobes  begin  to  differ,  and  among  the  Stenoglossa 
this  difference  becomes  more  and  more  marked  in  a  way  which  need  not  here  be 
described. 

In  Paludina  and  Valvata  the  kidney  no  longer  opens  into  the  posterior  base  of 
the  mantle  cavity,  but  is  continued  as  a  urinary  duct  (ureter),  which  runs  forward 
in  the  mantle  and  opens  at  its  edge. 

The  above-mentioned  theory  that  the  single  kidney  of  the  Monotocardia  corre- 
sponds with  the  left  kidney  of  the  Diotocardia  has  recently  been  ably  opposed, 
another  theory  being  put  forward  in  its  place.  Attention  is  specially  drawn  to  the 
fact  that  in  the  Diotocardia  the  left  kidney  is  always  the  smaller,  that  in  Patella  it 
is  shifted  to  the  right  side  of  the  pericardium,  and  that  in  Haliotis,  Turbo,  and 
Trochus  (as  papillar  sac)  it  is  not  excretory  in  function.  In  Haliotis,  Turbo, 
Trochus,  and  Po.tdla  the  lacunar  system  developed  in  the  wall  of  the  left  kidney  is 
in  direct  communication  with  the  auricles. 

In  most  Monotocardia  there  is  a  differentiated  part  of  the  kidney,  viz.  that 
which  is  called  the  nephridial  gland.  This  consists  of  two  principal  parts :  (1) 
canals,  covered  with  ciliated  epithelial  cells  and  opening  into  the  kidney.  These  are 
merely  protrusions  of  the  renal  wall,  which  project  into  the  organ  ;  their  epithelium 
is  a  continuation  of  the  renal  epithelium.  (2)  Between  these  canals,  the  organ  is  filled 
with  cells  of  connective  tissue  and  muscles,  and  contains  blood  lacunae,  one  of  these 
being  specially  large  and  communicating  with  the  auricle.  This  latter  portion  of 
the  organ  perhaps  plays  the  part  of  a  blood-forming  gland. 

This  nephridial  gland  may  perhaps  be  the  persistent  excretory  portion  of  the  lost 
nephridium,  i.e.  the  right  of  the  Diotocardia,.  The  duct  of  this  lost  nephridium  is 
now  known  to  persist  as  genital  duct.  As  we  saw  above,  all  Diotocardia  discharge 
the  genital  products  through  the  right  nephridium. 

2.  Pulmonata  (Fig.  182). — The  Pulmonata  have  only  one  kidney,  which  lies 
in  the  mantle  at  the  base  of  the  pallial  cavity,  between  the  rectum  and  the  peri- 
cardium. The  renal  sac  is  of  the  so-called  parenchymatous  type,  the  excretory 
epithelium  of  its  wall  projecting  into  the  cavity  in  the  form  of  numerous 


220 


COMPARATIVE  ANATOMY 


CHAP. 


folds  and  lamellae  in  such  a  way  as  to  leave  hardly  any  central  free  space.  The 
kidney  always  communicates  by  means  of  a  ciliated  canal  (renal  funnel  or  renal 
syringe,  "  Nieren-Spritze  ")  with  the  pericardium.  The  position  of  the  kidney  and 
the  morphology  of  the  urinary  duct  have  already  been  explained  (pp.  74-78). 

3.  Opisthobranchia— Tectibranchia. — Only  one  kidney  is  found  in  the  usual 
position  on  the  right  side  of  the  body,  with  the  pericardium  in  front  of  it  and  the 
hind-gut  behind  it.  It  is  of  the  parenchymatous  type,  and  is  connected  by  a 
ciliated  canal  with  the  pericardium.  It  opens  at  the  base  of  the  gill  in  front  of  the 
anus. 

In  the  Pteropoda  the  delicate-walled  kidney  is  not  parenchymatous,   but  is  a 


---6 


FIG.  182.—  Nephridium  and  pericardium  of  Daude- 
bardia  rufa,  from  above,  diagram  (after  Plate).  1,  Peri- 
cardium ;  2,  reno-pericardial  aperture  (renal  funnel) ;  3, 
nephridium  ;  4,  primary  ureter  ;  5,  rectum  ;  6,  secondary 
ureter  (cf.  Fig.  74,  p.  77). 


FIG.  183.— Nephridium  of  Bornella  (after  Hancock). 
1,  Kidney  ;  2,  part  connecting  it  with  the  reno-pericardial 
aperture  (pyriform  vesicle,  renal  syringe) ;  3,  part  of  the 
pericardial  wall ;  4,  ureter  ;  5,  nephridial  aperture. 


simple  hollow  cavity  lined  with  epithelium,  and  always  communicates  with  the 
pericardium,  against  which  it  lies. 

Nudibranchia  (Fig.  183).— The  kidneys  of  the  Nudibranchia  are  strikingly 
different  in  form  from  those  of  the  Tectibranchia.  The  unpaired  kidney  is  here 
somewhat  similar  to  the  paired  kidney  of  the  Chitonidic.  It  is  a  somewhat  wide 
tube  (renal  chamber)  traversing  the  cavity  of  the  body,  to  a  greater  or  less  extent ; 
branches  entering  it  from  all  sides.  This  tube  is  connected  at  one  end  with  the 
pericardium  by  a  duct  (renal  syringe,  pyriform  vessel),  which  varies  in  length,  and 
at  the  other  opens  outward  through  a  ureter  at  the  base  of  or  near  the  anal 
papilla. 

It  is  said  that  Pleurobranchcm,  a  Tcctibranchiate,  from  which  the  Nudibranchia 
may  perhaps  be  derived,  possesses  a  Nudibranchiate  kidney. 

In  Plnjllirhoe,  the  urinary  chamber  has  no  branchings  ;  it  runs  back  from  the 


VII 


MOLLU8CA—THE  NEPHRWIA 


221 


pericardium  as  a  simple  median  tube.  Anteriorly  it  is  connected  with  the  peri- 
cardium by  a  funnel,  and  near  the  middle  communicates  with  the  exterior  by  means 
of  a  lateral  urinary  duct  (Fig.  19,  p.  12). 

C.  Scaphopoda  (Fig.  165,  p.  193). 

Dentalium  has  a  pair  of  symmetrical  kidneys,  one  on  each  side  of  the  hind-gut. 
Each  nephridium  consists  of  a  sac  provided  with  short  diverticula.  The  two  nephri- 
dia  are  connected  by  a  tube  above  the  anus,  and  open  into  the  mantle  cavity  by 
two  apertures  at  the  sides  of  the  anus.  If,  as  maintained  by  all  authorities,  there 
are  no  reno-pericardial  apertures,  the  Scaphopoda  would  be  the  only  group  of  Molluscs 
in  which  these  apertures  are  entirely  absent.  Apart  from  the  symmetry  of  the 
kidneys,  a  fact  to  be  specially  noted  is  that  the  genital  products  pass  out  of  the 
genital  gland  into  the  right  kidney  (either  by  the  bursting  of  the  wall  between  the 
two  organs  or  through  an  aperture),  and  only  reach  the  exterior,  i.e.  the  mantle 
cavity,  through  the  right  renal  aperture. 

It  must,  further,  be  noted  that  near  the  anus  on  each  side,  between  it  and  the 
renal  aperture,  a  pore,  the  water-pore,  occurs,  the  function  of  which  is  still  doubt- 
ful. If  these  pores  really  lead  into  the  blood  lacunar  system  of  the  body,  as  was 
formerly  maintained,  and  is  still  held  to  be  possible,  this  would  be  the  only  known 
case  of  the  direct  imbibition  of  water  into  the  blood. 


D.  Lamellibranchia. 

The  nephridium  (organ  of  Bojanus  is  always  paired  and  symmetrical,  and  lies 
below  the  pericardium  and  in 
front  of  the  posterior  adductor. 
Each  nephridium  is  tubular  or 
sac-like,  opening  at  one  end 
through  a  funnel  into  the  peri- 
cardium, and  at  the  other  into 
the  mantle  cavity.  This  com- 
munication of  the  kidney  with 
the  mantle  cavity  always  takes 
place  above  the  cerebrovisceral 
connective. 

The  lowest  Lamellibranchia 
(Protobranchia,  Nucula,  Leda. 
Solenomya)  are  distinguished 
in  two  ways.  (1)  Each  nephri- 
dium is  a  simple  tube,  with  a 
free  cavity  not  traversed  by 
trabeculse  or  lamellte.  This 
tube  consists  of  two  portions 
which  unite  posteriorly  at  an 
angle  ;  the  anterior  end  of  one 
of  these  portions  enters  the 


FIG.  1S4.— Transverse  section  through  the  body  of  £ao— 
donta.  showing  the  pericardium,  the  heart,  and  the  kidneys, 
combined  and  diagrammatised  from  figures  by  Griesbach. 
Not  all  the  parts  represented  occur  on  the  same  section.     1, 


pericardium  through  the  renal  Pericardium;  2,  ventricle;  3,  auricles;  4,  hind -gut;  5, 
funnel,  the  other  end  opens  venous  sinus  ;  6,  reno-pericardial  aperture  (funnel)  ;  7,  renal 
into  the  mantle  cavity.  (2)  sac  or  cavity ;  8,  vestibular  cavity,  which  at  9  enters  the 
The  paired  genital  glands  do  ^ntle  cavity  through  the  nephridial  aperture ;  10,  genital 

aperture  ;  11,  base  of  the  foot, 
not  open  outward  directly,  but 

enter  the  kidneys  near  their  pericardial  funnel — a  fact  which  is  very  important  in 


222  COMPARATIVE  ANATOMY  CHAP. 

connection  with  the  arrangement  in  the  Solenogastridcc,  the  lower  Prosobranchia  (i.e. 
the  Diotocardia),  and  the  Scaphopoda. 

In  other  Lamellibranchia  also  there  is  a  relation  between  the  genital  glands  and 
the  kidneys.  In  the  Pectinidce  and  the  Anomiidce  the  genital  gland  opens  into  the 
kidney,  but  near  its  outer  aperture.  In  Area,  Ostrcea,  Cyclas,  and  Montacuta,  the 
kidney  and  the  genital  gland  open  on  each  side  into  the  base  of  a  common  depres- 
sion (urogenital  cloaca) ;  in  all  other  bivalves  the  outer  nephridial  and  genital  aper- 
tures are  separate. 

The  simple  structure  of  the  Protobranchiate  kidney  becomes  complicated  in 
other  Lamellibranchia  in  the  following  manner  : — 

1.  That  portion  of  the  renal  tube  which  opens  outward  forms  an  external  cavity 
(vestibular  cavity,   external   sac)  ;    this   cavity   has   no   excretory   epithelium  ;    it 
encircles  the  outer  side  of  the  pericardial  portion  of  the  kidney,  the  renal  sac  (Fig. 
184).     The  latter  alone  is  developed  as  an  excretory  organ.     Folds  or  trabeculte, 
covered  with  glandular  epithelium,  project  inward  from  its  walls,  forming  a  paren- 
chymatous  or  spongy  structure.     The  renal  sac  is  connected  with  the  pericardium 
by  means  of  a  nephridial  funnel  of  varying  length. 

2.  The  two  renal  sacs  communicate  freely  in  the  median  plane.     The  connecting 
part  is  widest  in  the  most  specialised  bivalves  (Pholadacea,  Myacea,  Anatinacea, 
Septibranchia). 

In  Anomia,  where  all  the  parts  are  asymmetrical,  the  two  kidneys,  which  do 
not  communicate  with  one  another,  are  also  asymmetrical. 

Venous  blood  flows  through  the  kidneys  on  its  way  to  the  gills.  The  afferent 
renal  vessels  seem  to  have  walls  of  their  own,  but  the  efferent  vessels  appear  to  be 
lacunar.  Open  communication  between  the  blood  vascular  system  and  the  kidneys 
is  nowhere  found. 

E.  Cephalopoda. 

(Of.  Figs.  185,  186,  and  the  sections  on  the  ccelom  and  the  blood 
vascular  system,  pp.  213  and  208). 

The  Cephalopoda  have  two  (Dibranchia)  or  four  (Tctrabrancliid)  spacious  sym- 
metrical renal  sacs,  in  the  posterior  and  upper  part  of  the  visceral  dome.  These 
communicate  in  the  typical  way  at  the  one  end  with  the  coelom,  and  at  the  other 
with  the  exterior  (mantle  cavity).  Only  one  of  the  two  pairs  of  kidneys  in  Nautilus, 
however,  possesses  coelomic  funnels. 

The  large  veins  returning  from  the  body  to  the  heart  run  along  the  anterior  wall 
of  the  urinary  sac.  These  veins  bulge  out  into  the  cavity  of  the  sac  to  form  the 
venous  appendages  already  mentioned.  The  epithelium  of  the  urinary  sac  which 
covers  these  appendages  is  no  doubt  the  seat  of  the  excretory  function.  The  excretory 
matter  is  discharged  into  the  urinary  sac  (the  wall  of  which  is  otherwise  smooth), 
and  passes  out  thence  through  a  ureter  of  varying  length  into  the  mantle  cavity. 
The  renal  aperture  is  found  on  the  median  side  of  the  base  of  the  gill,  and  in  Nautilus, 
the  Ocgopsidce,  and  Sepioteuthis  among  the  Myopsidce,  it  is  simple  and  slit-like  ;  in  the 
other  Myopsidce  and  in  the  Octopoda,  however,  it  lies  at  the  end  of  a  renal  papilla 
which  projects  freely  into  the  mantle  cavity. 

The  two  renal  sacs  in  the  Octopoda  are  entirely  distinct.  Near  the  point  where 
the  renal  sac  passes  into  the  ureter  lies  the  renal  funnel,  which  corresponds  with 
the  pericardial  aperture  of  other  Molluscs,  and  which  here  leads  to  the  coelomic 
cavity,  now  reduced  to  the  "  water  vascular  system." 

In  the  Decapoda,  the  two  renal  sacs  communicate  with  one  another  in  the  median 
plane.  In  Sepia,  there  are  two  points  of  communication,  one  above  and  the  other 
.below.  The  lower  junction  is  bulged  out  to  form  a  large  sac,  which  rises  towards 


VII 


MOLLUSCA—THE  NEPHRIDIA 


223 


the  apex  of  the  visceral  dome  on  the  anterior  side  of  the  paired  renal  sacs  (cf.  Fig. 
177,  p.  213).  The  veins  returning  from  the  body  to  the  heart  run  in  the  partition 
between  the  unpaired  anterior  and  the  paired  posterior  sacs,  and  may  here  bulge  out 
to  form  venous  appendages,  not  only  posteriorly,  i.e.  into  the  cavities  of  the  two 
paired  renal  sacs,  but  also  anteriorly,  into  that  of  the  unpaired  connecting  sac.  Near 


FIG.  1S5.— Renal  sac,  ccelom,  genital  organs,  etc.,  of  Sepia.  A,  female;  B,  male.  The 
visceral  dome  is  seen  from  behind  ;  the  mantle,  the  body  wall,  the  ink-bag,  and  in  A  the  hind-gut 
and  the  nidamental  gland  are  removed  (after  Grobben).  o,  Heart ;  5,  genital  vein ;  c,  genital 
artery ;  <1,  stomach  ;  e,  female  germinal  body ;  /,  aperture  of  the  oviduct  in  the  ovarial  cavity ; 
y.  oviduct ;  h,  unpaired  anterior  renal  sac  ;  i,  abdominal  vein ;  fc,  appendage  of  the  branchial 
heart  (pericardial  gland);  I,  branchial  heart;  m,  paired  posterior  renal  sac;  «,  gill;  o,  canals  of 
the  coelom  leading  to  the  kidneys  ;  />,  gland  of  the  oviduct ;  q,  female  genital  aperture  ;  r,  renal 
aperture.  In  B,  1,  testes  ;  -2  (the  indicator  points  rather  beyond  the  right  place),  aperture  of  the 
male  germinal  body  into  the  genital  cavity  or  capsule  ;  /,  aperture  of  the  seminal  duct  into  the 
male  genital  capsule  ;  3,  section  of  the  coelom  containing  the  vas  deferens  (peritoneal  sac)  ;  5,  anus  ; 
6.  rectum  ;  q,  male  genital  aperture. 

the  point  where  each  renal  sac  is  produced  into  the  ureter,  the  reno-pericardial  canal 
springs  from  it,  opening  into  the  secondary  body  cavity  which  contains  the  heart, 
and  corresponds  with  the  pericardium  of  other  Molluscs. 

The  form  of  the  renal  sac  is  at  least  partly  determined  by  the  form  and  position 
of  the  surrounding  viscera,  the  stage  of  maturity  of  the  genital  organs,  and  the 
different  shape  of  these  organs  in  the  two  sexes.  All  viscera  which  press  against  the 
renal  wall  from  without,  bulging  it  inward,  are  naturally  covered  at  the  points  of 


224 


COMPARATIVE  ANATOMY 


CHAP. 


contact  with  the  epithelium  of  the  renal  sacs.  The  same  is  the  case  with  all  organs 
which,  like  the  stomach,  the  gastric  ccecum,  and  the  efferent  ducts  of  the  digestive 
glands  in  the  Decapoda  (Sepia),  apparently  lie  inside  the  spacious  renal  sacs.  These 
organs  really  lie  outside  of  them,  being  only  suspended  into  them,  like  the  intestine 
of  an  Annelid,  which  apparently  lies  within  the  body  cavity,  but  is  entirely  separated 
from  it  by  the  peritoneal  endothelium. 

It  has  been  already  mentioned  that  only  one  of  the  two  pairs  of  renal  sacs  of 
Nautilus,    viz.    the   upper   pair,   has   reno-pericardial    apertures.      This   fact   was 


FIG.  186.— Diagram  showing  the  posterior  paired  renal  sacs  of  Sepia  officinalis,  and  the  vein 
running  along  its  anterior  wall  with  its  venous  appendages,  from  behind  (after  Vigelius).  vc, 
Vena  cava ;  rno,  right  nephridial  aperture ;  y\,  left  reno-pericardial  aperture,  the  outlines  of  the 
secondary  body  cavity  are  indicated  by  a  dotted  line ;  vg,  vena  genitalis  ;  rvc,  right  branch  of  the 
vena  cava  ;  vpd,  right  pallial  vein  ;  va,  right  vena  abdominalis ;  vba,  vein  of  the  ink-bag ;  vas,  left 
vena  abdominalis  ;  cv,  section  of  the  secondary  body  cavity  (capsule  of  the  branchial  heart),  which 
surrounds  the  branchial  heart  cb,  and  the  appendage  of  the  same  (pericardial  gland)  x ;  vps,  left 
pallial  vein ;  Ivc,  left  branch  of  the  vena  cava  cephalica ;  vm,  left  vena  genitalis ;  vpc,  secondary 
body  cavity  (viscero-pericardial  sac) ;  y,  left  reno-pericardial  aperture  (renal  funnel)  (cf.  Fig.  174). 

brought  forward  in  support  of  the  view  that  the  two  pairs  of  renal  sacs  arose  by 
the  division  of  one  single  pair,  corresponding  with  that  of  the  Dibranchia.  Accord- 
ing to  this  view,  the  lower  pair  of  gills,  and  the  two  auricles  are  also  to  be  considered 
to  be  new  acquisitions.  Indeed,  the  whole  question  of  the  original  metamerism  of  the 
Molluscan  body,  which  has  so  often  been  asserted,  rests  on  very  weak  foundations. 
It  gains  no  support  from  the  Chitonidce,  where,  in  spite  of  large  numbers  of  pairs  of 
gills,  only  two  auricles  occur,  and  where  no  relation  exists  between  the  number  of  the 
shell  plates  and  that  of  the  gills. 


vii  MOLL USCA— GENITAL  ORGANS  225 

XX.  Genital  Organs. 

A.  General. 

In  treating  of  the  genital  organs  of  the  Mollusca,  we  shall  have  to 
consider — (1)  the  gonads  or  germinal  glands,  those  most  important 
organs,  in  which  the  reproductive  cells  (eggs  and  spermatozoa)  are 
formed ;  (2)  the  duets  through  which  these  cells  reach  the  exterior ; 
and  (3)  the  eopulatory  organs. 

1.  The  gonads  or  germinal  glands  have  already,  in  Section  XVIII., 
been  recognised  as  completely  or  incompletely  demarcated  portions 
of  the  secondary  body  cavity,  and  have  been  described  in  their 
relation  to  the  other  divisions  of  that  cavity. 

The  gonads  are  paired  and  symmetrical  in  the  Lamellibranchia  and 
Solenogastres,  occurring  in  one  pair.  In  all  other  Mollusca,  only  one 
unpaired  gonad  is  found.  In  very  rare  cases,  such  as  that  of  some 
hermaphrodite  Lamellibranchs,  which  will  be  described  later,  there  are 
two  pairs  of  gonads,  one  female  and  one  male. 

The  sexes  are  separate,  among  the  Amphineura  in  the  Chitonidce 
and  Chcetoderma,  in  many  Lamellibranchs,  in  the  Scaphopoda,  among  the 
Gastropoda  in  the  Prosobranchia  (excepting  a  few  Marseniadce  and  the 
Falvata),  and  in  all  Cephalopoda.  Hermaphroditism  prevails  among 
the  Amphineura  in  Proneomenia,  Neomenia,  and  allied  forms ;  in  many 
Lamellibranchs,  among  the  Gastropoda  in  the  Pulmonata,  Opisthobi'anchia, 
and  in  the  Prosobranchiate  family  of  the  Marseniadce. 

In  hermaphrodite  animals,  it  is  the  rule  that  the  same  gland,  the 
hermaphrodite  gland,  produces  both  eggs  and  spermatozoa,  but  in 
exceptional  cases  there  are  in  the  same  individual  distinct  male  and 
female  gonads  (testes  and  ovaries).  This  is  the  case,  as  already 
mentioned,  in  certain  bivalves,  viz.  the  Anatinacea  and  the  Septi- 
branchia,  which  possess  two  testes  and  two  ovaries. 

Position  of  the  gonads. — The  long  tubular  hermaphrodite  glands 
of  the  Solenogastres,  which  are  separated  from  one  another  by  a  median 
septum,  lie  in  the  anterior  prolongation  of  the  pericardium,  over  the 
intestine.  In  the  Chitonidce,,  the  gonads  are  found  in  a  similar 
position,  but  are  not  in  open  communication  with  the  pericardium. 
In  the  Gastropoda  they  lie  in  the  visceral  dome,  usually  in  its  upper- 
most part,  between  the  lobes  of  the  digestive  gland.  Where  the 
visceral  dome  has  disappeared,  the  gonad  with  the  intestine  and  the 
digestive  gland  shift  back  into  the  primary  body  cavity  above  the 
foot.  The  gonads  in  the  Scaphopoda  occupy  a  position  similar  to  that 
of  the  Gastropodan  gonads,  lying  dorsally  in  the  high  visceral  dome, 
above  the  anus  and  the  kidneys.  The  same  is  the  case  in  the 
Cephalopoda.  The  paired  much-lobed  genital  glands  of  the  Lamelli- 
branchia  lie  in  the  typical  position  in  the  primary  body  cavity,  above 

VOL.  II  Q 


UNIVERSITY 


226  COMPARATIVE  ANATOMY  CHAP. 

the  muscular  part  of  the  foot,  between  the  coils  of  the  intestine. 
They  may  lie  behind  the  "  liver,"  or  else,  passing  between  its  lobes, 
spread  out  at  the  sides  of  and  below  the  kidney. 

The  epithelium  which  lines  the  gonads  is,  morphologically,  the 
endothelium  of  the  secondary  body  cavity.  The  reproductive  cells 
may  either  be  produced  from  any  part  of  the  epithelium  of  the  gonad, 
or  from  definite  areas  of  this  epithelium  (Cephalopoda),  which  areas 
may  then  be  called  germinal  epithelium  or  germinal  layers.  It  may 
then  appear  as  if  the  germinal  gland  lay  in  or  on  a  special  sac, 
whereas  this  sac  is,  in  reality,  the  gonad  itself,  and  the  germinal 
gland  is  only  the  much-developed  germinal  layer  of  the  gonad. 

The  ripe  reproductive  cells  become  detached  from  their  place  of 
formation,  and  fall  into  the  cavity  of  the  gonad,  i.e.  into  a  part  of  the 
secondary  body  cavity,  from  which  they  pass  out  in  various  ways. 

2.  The  gonads  either  have  separate  ducts  (Chitonidce,  Mo7iotocardia, 
Pulmonata,  Opisthobranchia,  Cephalopoda,  many  Lamellibranchia)  or  they 
utilise  the  nephridia  as  ducts.  In  the  latter  case  the  genital  products 
either  pass  direct  into  the  kidney,  and  reach  the  exterior  through 
the  nephridial  aperture  (all  Diotocardia,  the  Scaphopoda,  and  many 
Lamellibranchia),  or  they  first  pass  into  the  pericardium,  and  then  are 
ejected  through  the  nephridia  (Solenogaslres).  Where  the  gonads 
open  into  the  kidneys,  their  apertures  may  lie  in  various  parts  of 
these  organs ;  either  in  the  proximal  part,  which  communicates  with 
the  pericardium  by  means  of  the  renal  funnel,  and  is  usually  widened 
into  the  renal  sac,  or  in  the  distal  part  (ureter)  which  opens  externally, 
or  into  a  shallow  urogenital  cloaca. 

The  gonads  therefore  open  into  : — 

a.  The  pericardium  (Solenogastres). 

b.  The  proximal  or  pericardial  part  of  the  kidney. 

c.  The  distal  part  or  ureter  of  the  kidney. 

d.  The  urogenital  cloaca.     Or  : — 

e.  They  open  externally,  quite  apart  from  the  kidney. 

Paired  gonads  have  paired  ducts  (Solenogastres,  Lamellibranchia). 
Where  there  is  a  single  unpaired  gonad,  there  is  either  a  single 
efferent  renal  duct,  or  a  single  renal  duct  is  made  use  of  (Gastropoda, 
Scaphopoda,  Cephalopoda,  etc.);  the  duct  is  then  always  asymmetrical 
and  usually  lies  on  the  right  side.  A  paired  duct,  belonging  to  an 
unpaired  genital  gland,  is,  however,  found  in  the  Chitonidce  and  in 
many  Cephalopoda. 

When  the  genital  glands  have  special  efferent  ducts,  various 
sections  of  the  latter  may  be  differentiated  into  accessory  sacs  and 
glands,  copulatory  apparatus,  etc.,  which,  especially  in  the  Pulmonata, 
Opisthobranchia,  and  Cephalopoda,  transform  the  ducts  into  a  very 
complicated  apparatus.  In  males,  this  complication  arises  through 
the  development  of  copulatory  organs,  and  of  special  glands  which 
form  the  capsules  of  the  spermatophores,  and  of  seminal  vesicles,  etc.  ; 
in  females,  through  the  development  of  albuminous  glands,  shell 


VII 


MOLLUSC  A— GENITAL  ORGANS 


227 


glands,  receptacula  seminis,  vagina,  etc.  Since,  in  hermaphrodite 
Molluscs,  both  kinds  of  complication  occur  simultaneously  in  the 
same  genital  apparatus,  the  most  complicated  arrangement  is  found 
in  the  (hermaphrodite)  Pulmonata  and  Opisthobranchia. 

3.  Copulatory  organs  are  wanting  in  many  Molluscs,  such  as  the 
Amphineura  (see  below),  nearly  all  Diotocardia,  the  Scaphopoda,  and 
all  Lamellibranchia.  They  are  present  in  the  Monotocardia,  the 
Pulffioiutta,  Opisthobranchia,  and  Cephalopoda.  In  the  Gastropoda,  in 
the  nuchal  region,  to  the  right,  there  is  a  male  apparatus,  consisting 
sometimes  of  a  freely  projecting  muscular  penis,  sometimes  of  an 
organ  which  can  be  protruded  or  evaginated  through  the  genital 
aperture.  In  the  Cephalopoda,  this  is  a  definite  arm  in  the  male, 
which  is  specially  modified  (hectoeotilised),  sometimes  in  a  very 
remarkable  manner,  and  which  plays  a  more  or  less  important  part  in 
copulation. 

B.  Special. 

a.  Gonads.  (1)  Amphineura. — The  long  hermaphrodite  gland  of  Proneomenia 
and  allied  forms  has  been  called  paired.  As  a  matter  of  fact  it  is  divided  into  two 
more  or  less  distinct  lateral  tubes,  by  a  median  much-folded  septum.  In  the  lower 
portion  of  each  tube,  that 

which  lies  next  the  intestine,  3-,  .1 

the  germinal  epithelium  pro- 
duces spermatozoa,  in  the 
upper  portion  eggs.  Pos- 
teriorly, these  tubes  sepa- 
rate for  a  certain  distance, 
and  open  as  a  pair  of  dis- 
tinct ducts  into  the  anterior 
end  of  the  pericardium. 

The  male  or  female  gonad 
of  the  Chitonidce  lies  as  a 
long  unpaired  sac  on  the 
dorsal  side  of  the  intestine,  in 
front  of  and  partly  under  the 
pericardium.  In  the  ovary, 
numerous  pear-shaped  tubes 
(Fig.  187)  project  from  the 
epithelial  wall  into  the 
cavity.  Each  of  these  tubes  FIG.  1ST.— Section  through  the  wall  of  the  ovary  of  Chiton 
is  a  stalked  follicle,  with  egg  (diagram  after  Haller).  1,  Eggs  at  different  stages  of  develop- 
cells  surrounded  by  follicular  ment  '•  2>  §ei™inal  epithelium  ;  3,  egg  sac  or  tubes  ;  4,  follicular 
cells.  These  follicles  are  epithelium  ;  5,  egg  tube  after  the  discharge  of  the  egg. 
found  in  all  sizes  and  at  all  stages  of  development.  Each  egg  is  at  first  a  simple 
ovarial  epithelial  cell,  which  is  distinguished  by  its  size  from  the  surrounding 
epithelial  cells.  As  it  grows  and  becomes  more  and  more  rich  in  yolk,  it  sinks 
down  under  the  ovarial  epithelium,  bulging  out  this  latter  towards  the  ovarial 
cavity,  and  thus  forming  a  young  follicle.  The  wall  of  the  pear-shaped  testicle 
also  rises  into  its  cavity  in  the  form  of  numerous  folds,  in  which  the  epithelium 
becomes  inultilaminar,  and  produces  the  mother  cells  of  the  spermatozoa. 

The  fact  that  the  gonad  of  Chiton  has  two  ducts  makes  it  probable  that  it  was 


228  COMPARATIVE  ANATOMY  CHAP. 

originally  paired.  The  two  ducts,  i.e.  the  two  seminal  ducts  in  the  male  and  the 
two  ovarial  ducts  in  the  female,  open  into  the  mantle  furrow  on  each  side,  somewhat 
in  front  of  the  renal  aperture  (Fig.  180,  p.  217). 

(2)  Gastropoda.  — The  gonads  of  the  Prosobranchia  offer  but  few  points  of 
interest  to  the  comparative  anatomist.     In  the  Pulmonata  and  Opisthobranchia,  the 
germinal  gland   is  a  hermaphrodite   gland,   in   which   spermatozoa   and   eggs   are 
produced  simultaneously.     This  gland  is  much  lobed,  or  else  consists  of  numerous 
converging  diverticula  ;  the  spermatozoa  and  eggs  arise  intermingled  on  the  walls, 
become  detached  at  one  of  the  stages  of  their  development,  and  then  lie  free  in  the 
cavity  of  the  gonad.     The  same  applies  to  the  large  hermaphrodite  gland  of  the 
TectibrancMa,  which  varies  much  in  its  outer  form.     It  lies  in  the  posterior  part  of 
the  body,  on  the  digestive  gland,  penetrating  at  times  between  its  lobes  ;  it  is  itself 
more  or  less  lobed,  its  lobes  consisting  of  secondary  lobes  (vesicles  or  acini).     In 
all  these  acini,  spermatozoa  and  eggs  are  simultaneously  produced.     It  is  only  in 
the  Pleurobranchcea  and  allied  forms  that  the  parts  of  the  gland  which  produce 
spermatozoa  and  those  which  produce  eggs  are  localised  ;  this  arrangement  resembles 
that  in  the  Nudibranchia,  which  will  presently  be  described.     The  constituent  lobes 
or  vesicles  are  either  male  or  female,  the  former  producing  only  spermatozoa,  the 
latter  only  eggs.      This  is   the  arrangement   found   also   in   some  Nudibranchia 
(Amphorina,    Capellinia),    but  in  most  Nudibranchs  the    male   and    the  female 
germinal  regions  become  separated  in  such  a  way  that  the  terminal  acini  yield  eggs 
only,  but  open  in  groups  into  lobes  of  the  gland  which  produce  only  spermatozoa. 
Each  lobe  has  its  duct ;  these  ducts,  uniting  together,  finally  form  the  duct  of  the 
hermaphrodite  gland.     This  gland  thus  forms  an  extensive  organ  spread  out  in  the 
larger  posterior  part  of  the  primary  body  cavity  ;  where  there  is  a  compact  diges- 
tive gland  it  covers  this  organ.     Phyllirhoe  has  2  to  6  (usually  3)  separate  globular 
acini  whose  long  and   thin  ducts  combine  to   form  a   hermaphrodite  duct   (Fig. 
195,  p.  238). 

The  hermaphrodite  gland  of  the  Pteropoda  ( Tcctibranchia  natantia)  always  lies 
in  the  upper  (dorsal)  portion  of  the  visceral  dome  ;  it  is  sometimes  acinose  and 
sometimes  consists  of  converging  tubular  follicles  or  of  laminae  closely  crowded 
together.  The  eggs  are  always  produced  at  the  peripheral  part  of  the  acini,  tubes, 
or  lamellae,  while  the  spermatozoa  arise  in  the  central  parts,  near  the  ducts.  These 
two  parts  are  generally  separated  by  a  membrane,  which  the  eggs  have  to  break 
through  to  reach  the  hermaphrodite  duct.  The  Pteropoda  are  protandrously 
hermaphrodite,  i.e.  the  spermatozoa  are  produced  before  the  eggs,  an  arrangement 
found  in  many  hermaphrodite  Molluscs. 

(3)  Scaphopoda. — The  gonad  (testis,  ovary)  in  these  animals  is  a  long  spacious 
sac,  provided  with  lateral  diverticula  ;  it  lies  above  the  anus,  rising  high  up  into  the 
visceral  dome  along  the  posterior  side  of  the  body.     In  the  Solenopoda  (Siphono- 
dentalium,  etc.)  a  large  part  of  the  gonad  stretches  into  the  mantle.     In  young 
animals,  the  gonad  is  closed  on  all  sides,  but  in  adults  its  wall  appears  to  fuse  with 
the  right  kidney,  and  in  the  partition  wall  so  formed  an  aperture  arises  which 
establishes  communication  between  the  gonad  and  the  right  nephridium. 

(4)  Lamellibranchia. — The  gonads  are  here  found  in  the  form  of  much-branched 
tubular  or  lobate  masses  lying  on  each  side  in  the  primary  body  cavity,  surrounding 
and  partly  penetrating  between  the  other  internal  organs.     In  some  cases  (Anomiidcv, 
Mytilidce),  the  gonad  on  each  side  stretches  into  the  mantle.     In  others  (Axinus, 
Montacuta),  it  bulges  out  the  body  wall  in  such  a  way  that  branched  outgrowths, 
containing  th«  germinal  tubes,  project  from  the  body  into  the  mantle  cavity. 

In  most  Lamellibranchia  the  sexes  are  separate,  but  hermaphroditism  sometimes 
occurs.  There  are  (1)  whole  groups  of  bivalves  which  are  hermaphrodite  ;  e.g.  the 
most  specialised  forms,  such  as  the  Anatinacea  and  Septibranchia ;  (2)  families 


vii  MOLLUSC  A— GENITAL  ORGANS  229 

with  a  few  hermaphrodite  genera  :  Cydas,  Pisidium,  Entovalva  ;  (3)  genera  (Ostrcea, 
Pccten,  Cardium]  with  a  few  hermaphrodite  species  ;  (4)  occasional  cases  of  henna- 
phroditism  in  species  the  sexes  of  which  are  usually  separate :  Anodonta.  The 
hermaphroditism  of  the  Lamellibranchia  is,  however,  always  incomplete  in  the  sense 
that  the  spermatozoa  and  the  eggs  do  not  ripen  simultaneously. 

In  the  Anatinacea  and  Septibratichia,  there  are  on  each  side  entirely  separate  male 
and  female  gonads,  whereas  all  other  hermaphrodite  Lamellibranchs  have  a  her- 
maphrodite gland  on  each  side. 

(5)  Cephalopoda. — The  sexes  are  always  separate  in  this  class.  It  has  already 
been  mentioned  that  the  germinal  sacs  form  a  part  of  the  secondary  body  cavity,  with 
which  they  are  in  open  communication. 

One  single  unpaired  gonad  is  always  found,  lying  in  the  uppermost  part  of  the 
visceral  dome.  It  is  a  variously-formed  sac  (peritoneal  sac  or  genital  capsule),  lined 
on  all  sides  by  an  epithelium  often  to  a  great  extent  ciliated,  which  is  in  reality  the 
peritoneal  epithelium  of  the  secondary  body  cavity.  The  whole  of  the  epithelium 
covering  the  wall  of  the  gonad  is  not,  however,  germinal,  but  only  that  on  its  anterior 
side  (that  turned  to  the  shell).  The  germinal  layer  here  forms  what  may  be  called,  in 
the  narrower  sense,  the  ovary  or  the  testis,  which  is  then  said  to  be  contained  in  a 
peritoneal  sac  or  an  ovarial  or  testicular  capsule,  or  else  to  project  into  or  be  suspended 
in  such  sac  or  capsule.  The  whole  apparatus  is  really  a  gonad,  in  which  the  places 
of  formation  of  the  reproductive  cells  are  localised  on  the  anterior  wall. 

From  this  it  is  clear  why  the  testes  and  ovaries  do  not  appear  to  possess  efferent 
ducts  of  their  own,  but  to  empty  their  products  into  their  respective  capsules,  these 
products  passing  out  into  the  mantle  cavity  through  the  ducts  of  these  capsules 
(oviducts  and  seminal  ducts).  Since,  however,  the  entire  germinal  sac  corresponds 
with  the  genital  gland  of  a  Gastropod  or  a  Lamellibranch,  the  reproductive  pro- 
ducts in  reality  merely  fall  into  the  cavity  of  this  gland  (the  testicular  and  ovarial 
capsules),  and  pass  out  through  the  ovarial  and  seminal  ducts,  which  exactly  corre- 
spond with  the  same  ducts  in  the  Gastropoda,  Lamellibranchia,  and  Chitonidce. 

The  genital  cavity  has  also  another  means  of  communication  with  the  exterior, 
since,  in  the  Cephalopoda,  it  is  in  open  communication  with  the  remaining  part  of 
the  secondary  body  cavity,  whether  the  latter  forms  a  viscero-pericardial  cavity  (Deca- 
poda]  or  is  reduced  to  the  "  water  canal  system  "  (Octopoda).  This  latter  part  of  the 
body  cavity  again  is  connected,  by  means  of  the  nephridia,  with  the  mantle  cavity. 

In  this  way,  the  genital  cavity  communicates  with  the  mantle  cavity  directly  by 
means  of  the  oviduct  or  seminal  duct,  and  indirectly  through  (1)  the  viscero-pericardial 
cavity  or  the  "  water  canal  system,"  and  (2)  the  nephridia.  This  second  way  of 
communication,  however,  is  never  used  for  discharging  the  genital  products. 

The"  female  germinal  layer  or  ovarial  layer  (the  ovary  in  the  narrower  sense)  is 
always  found  on  the  anterior  wall  of  the  gonad,  and  varies  considerably  in  structure  (Fig. 
188).  We  can  always  distinguish  (1 )  the  eggs,  and  (2)  the  ovigerous  wall.  The  former 
are  stalked,  and  project  from  the  wall  into  the  cavity  of  the  gonad  (the  cavity  of  the 
ovarial  capsule).  The  largest  and  oldest  eggs  are  covered  by  a  follicular  epithelium, 
and  this  latter  by  the  general  epithelium  of  the  wall  of  the  gonad,  which  also  covers 
the  stalk.  Each  egg  has  a  separate  stalk.  The  youngest  eggs  are  mere  prominences 
on  the  wall,  which  in  the  process  of  growth  acquire  a  stalk,  by  means  of  which  they 
remain  connected  with  the  wall  from  which  they  project.  This  arrangement  is 
exactly  like  that  in  the  Chiton.  When  the  eggs  are  mature,  the  follicle  bursts,  they 
fall  into  the  genital  cavity,  and  thence  reach  the  exterior  through  the  oviduct. 

In  Nautilus  (Fig.  188,  A)  and  Eledone  the  whole  wall  of  the  gonad,  with  the 
exception  of  the  posterior  surface,  can  produce  eggs  ;  these  stand  out  from  it  all  over 
on  simple  stalks.  In  Argonauta  (Fig.  188,  B]  and  Trernodopus  also,  the  whole  ova- 
rial  capsule  except  the  posterior  wall  produces  eggs,  but  the  egg-bearing  region  (to 


230 


COMPARATIVE  ANATOMY 


CHAP. 


obtain  increase  of  surface)  projects  into  the  genital  cavity  in  the  form  of  numerous 
dendriform  processes,  the  eggs  being  attached  by  simple  stalks  to  the  stems  and 
branches.  In  Parasira  (Tremoctopus)  catenulata  there  is  a  central  region  containing 
more  than  twenty  large  "egg  trees  "  surrounded  by  a  circle  of  smaller  "trees."  On 
the  anterior  wall  of  the  gonad  in  Octopus  there  is  a  single  but  very  richly-branched 
"egg  tree"  (C],  In  Sepia,  Sepiola,  and  Rossia  the  egg-bearing  surface  bulges  out 
in  the  shape  of  a  ridge  on  the  anterior  wall  of  the  gonad.  This  ridge,  in  Loligo, 
becomes  a  narrow  fold,  the  free  edge  of  which  is  produced  into  filaments,  which  carry 
on  all  sides  simply -stalked  eggs.  In  the  Oegopsidce  (Ommastrephes,  Fig.  188,  D, 
Onychoteuthis,  Thysanotcuthis)  the  region  which  carries  the  eggs  is  only  attached  by 
its  upper  and  lower  ends  to  the  wall  of  the  gonad,  and  forms  an  otherwise  free  spindle- 
shaped  body  traversing  the  genital  cavity,  and  beset  all  over  with  stalked  eggs. 

In  Octopus  and  Eledone  all  the  eggs  in  a  given  ovary  are  found  at  the  same 
stage  of  maturity. 

A  peculiar  transformation  of  the  follicular  epithelium  takes  place  in  the  ovarial 
eggs  of  the  Cephalopoda  when  nearly  mature.  An  extraordinary  increase  of  surface 
occurs  in  the  shape  of  numerous  folds,  which  run  longitudinally  along  the  egg,  either 
reticulating  or  remaining  parallel  to  one  another,  and  projecting  far  into  the  yolk 


FIG.  188.— A -D,  Four  diagrams  of  the  female  gonads  of  the  Cephalopoda.  A,  Nautilus  type. 
B,  Argonaut  type,  f ,  Octopus  type.  D,  Ommastrephes  type.  1 ,  Aperture  of  the  oviduct  into 
the  gonad  ;  2,  cavity  of  the  gonad  (a  section  of  the  secondary  body  cavity) ;  3,  egg-carrier. 

of  the  egg  which  they  surround.  This  arrangement  may  be  connected  with  the 
nutrition  of  the  egg. 

The  male  germinal  layer  (germinal  body,  or  testis  in  the  narrower  sense)  is  a 
variously-shaped  (often  globular  or  oviform)  compact,  organ,  which  usually  lies  free 
in  the  genital  cavity,  suspended  to  its  anterior  wall  by  a  thin,  ligament  (mesorchium) 
in  which  the  genital  artery  runs.  The  germinal  body  is  everywhere  covered  with 
epithelium,  which  is  continued  over  the  mesorchium  into  the  epithelium  of  the  wall 
of  the  gonad  (endothelium  of  the  testicular  capsule).  On  the  surface  of  the  germinal 
body  which  is  turned  away  from  the  mesorchium,  there  is  a  funnel-shaped  depression 
(Fig.  189,  A)  ;  towards  this,  from  all  sides,  the  tubular  testicular  canals  which  form 
the  male  germinal  body  converge,  in  order  to  open  into  it.  In  these  testicular  canals, 
between  which  there  is  a  slight  framework  of  connective  tissue,  the  spermatozoa  are 
produced,  and  are  passed  on  to  the  genital  cavity  through  the  depression  into  which 
all  the  canals  open  ;  they  reach  the  exterior  by  means  of  the  seminal  duct.  The 
testicular  canals  originally  possess  a  multilaminar  germinal  epithelium,  which  yields 
the  spermatozoa,  and  which  passes  at  the  common  aperture  into  the  outer  epithelium 
of  the  germinal  body,  and  so  into  the  epithelium  of  the  germinal  sac. 

This  description  applies  to  the  male  germinal  body  of  most  Cephalopoda.     In 


VII 


MOLLUSCA— GENITAL  ORGANS 


231 


Loligo  (B),  however,  the  funnel-shaped  depression  into  which  all  the  testicular 
canals  open  is  replaced  by  a  longitudinal  furrow,  into  which  these  converging  canals 
open.  In  Sepia  (C),  the  germinal  body  has  no  ligament,  but  lies  immediately  in 
front  of  the  anterior  wall  of  the  gonad,  and  is  thus  outside  the  genital  cavity.  The 
germinal  body  here  has  a  central  channel  towards  which  the  radially  arranged 
seminal  canals  converge  from  all  sides,  and  which  they  enter.  This  channel,  again, 
opens  through  an  efferent  duct  into  the  genital  cavity,  from  which  the  spermatozoa 
are  conducted  to  the  exterior  by  the  seminal  duct. 

The  spermatozoa  of  the  Mollusca  are  of  the  common  pin  shape.  In  many  species 
of  Prosobranchia  two  different  forms  of  spermatozoa,  the  hair-shaped  and  the  vermi- 
form, occur  in  one  and  the  same  individual.  This  phenomenon  has  by  some  been 
taken  as  an  indication  of  developing  hermaphroditism,  and  by  others  as  pointing  to 
a  former  hermaphrodite  condition  ;  in  the  first  case  the  vermiform  spermatozoa 
would  be  the  eggs  beginning  to  form,  in  the  second  the  rudiments  of  eggs.  There 
is,  however,  no  solid  foundation  for  either  of  these  views. 

With  regard  to  the  question  whether  the  hermaphrodite  or  the  dioecious  condition 
is  the  original  condition,  the  latter  alternative  may  be  considered  as  the  more  prob- 
able. Of  the  five  classes  of  the  Mollusca,  two,  the  Scaphopoda  and  the  Cephalopoda, 


FIG.  189.— A,  B,  C,  Three  diagrams  of  the  male  gonads  of  the  Cephalopoda.  A,  ordinary 
type.  B,  Loligo.  C,  Sepia.  1,  Seminal  duct ;  2,  cavity  of  the  gonad  ;  3,  space  into  which  all  the 
canals  of  the  testis  open,  and  which  itself  opens  into  the  cavity  of  the  gonad,  in  Sepia,  by  means 
of  a  canal  (4) ;  5,  suspensor  of  the  male  germinal  body,  attaching  it  to  the'anterior  wall  of  the 
gonad. 

are  altogether  dioecious.  Among  the  Amphineura,  the  Chitonidce,  which  most 
recent  observers  hold  to  be  less  specialised  than  the  Solenogastres,  are  sexually 
separate.  Among  the  Lamellibranchia,  the  sexes  are  separate  in  the  Protobranchia, 
which  are  rightly  considered  as  primitive  forms  ;  and  most  other  bivalves  are  also 
dioecious.  Among  the  Gastropoda,  the  sexes  are  separate  in  the  Prosobranchia, 
especially  in  the  Diotocardia,  which  are  universally  considered  to  be  the  lowest  and 
least  specialised  Gastropods. 

b.  The  ducts. — The  manner  in  which  the  sexual  products  are  conducted  to  the 
exterior  in  the  Amphineura,  Scaphopoda,  and  Lamellibranchia  need  not  again  be 
discussed,  as  it  has  already  been  described  in  the  general  part  of  this  section,  and  in 
the  section  on  the  nephridial  system.  "VVe  thus  have  now  only  to  treat  of  the  very 
complicated  ducts  of  the  Gastropoda  and  the  Cephalopoda. 

(1)  Gastropoda.  — It  has  been  seen  that  in  all  Diotocardia  (Haliotis,  Fissurella, 
Patella,  etc.)  the  genital  products  are  ejected  through  the  right  kidney.  In 
the  Monotocardia,  the  right  kidney  has  atrophied  as  such,  but,  according  to  the 
most  recent  investigations,  its  duct  persists  as  genital  duct.  In  the  Pulmonata 
and  Opisthobranchia,  the  genital  aperture  is  no  longer  in  the  mantle  cavity, 


232  COMPARATIVE  ANATOMY  CHAP. 

but  has  shifted  far  forward  along  the  right  side  of  the  neck,  probably  in  con- 
nection with  the  development  of  the  copulatory  apparatus.  The  position  of  this 
aperture  is  thus  not  necessarily  affected  by  any  further  displacement  of  the  pallial 
complex,  or  indeed  of  the  whole  visceral  dome,  which  explains  the  fact  that,  in  Ddude- 
bardia  and  Testacella,  the  common  genital  aperture,  and  in  Oncidium,  the  male 
aperture,  lies  far  forward  on  the  right  side  of  the  body,  although  the  pallial  complex 
has  shifted  completely  to  the  posterior  end  of  the  body. 

In  the  Opisthobranchia  also,  the  single  or  (secondarily)  double  genital  aperture 
lies  to  the  right  in  front  of  the  anus  and  even  in  front  of  the  kidney.  This  position 
seems  inexplicable  except  by  the  supposition  of  a  shifting  back  of  the  pallial  com- 
plex in  which  the  genital  aperture,  emancipated  from  the  complex,  took  no  part, 
thus  coming  to  lie  in  front  of  the  shifted  anal  and  renal  apertures. 

Monotocardia. — Unlike  the  Diotocardia,  which,  with  the  exception  of  the  Neritidce, 
have  no  copulatory  organs,  the  Monotocardia  possess  a  penis,  which,  however,  does 
not  lie  in  the  mantle  cavity  where  the  genital  aperture  originally  lay.  It  wrould  be 
unable  to  function  in  this  position,  and  is  therefore  placed  on  the  right  side  of  the 
head  or  neck  (Fig  71,  p.  73),  and  forms  a  freely  projecting,  extensible,  muscular 
appendage,  which  often  attains  a  considerable  size.  The  male  genital  aperture, 
however,  in  very  many,  perhaps  in  most,  Monotocardia,  remains  in  its  original  posi- 
tion in  the  mantle  cavity,  to  the  right,  near  the  rectum.  In  such  cases,  a  ciliated 
furrow  runs  forward  on  the  floor  of  the  respiratory  cavity,  along  the  right  side  of  the 
neck,  to  the  base  of  the  penis,  to  the  tip  of  which  it  is  continued  as  a  deep  groove. 
This  furrow  conducts  the  semen  to  the  penis  from  the  genital  aperture.  In  some 
cases  the  furrow  closes,  and  forms  a  canal ;  the  penis  then  becomes  tubular,  and  the 
seminal  duct  enters  into  it.  The  genital  aperture  is  thus  shifted  far  forward  from 
its  original  position.  The  seminal  duct,  which  arises  from  the  testis,  usually  forms 
coils  as  it  runs  along  the  columellar  side  of  the  shell.  The  vas  deferens  has  no 
special  appendages,  although  it  may  widen  into  a  vesicle  at  some  point  in  its  course. 

In  the  female,  the  genital  aperture  remains  in  the  mantle  cavity,  lying  to  the 
right  near  the  rectum,  behind  the  anus.  The  duct  remains,  as  a  rule,  more  or  less 
simple  ;  it  is  divided  into  the  following  consecutive  sections  :  (1)  an  oviduct,  rising 
from  the  ovary,  which  may  bulge  out  to  form  one  or  more  receptacula  seminis  ; 
(2)  the  uterus,  a  wider  section  with  thick  glandular  walls,  in  which  the  eggs  are 
provided  with  albumen  and  a  shell :  (3)  a  muscular  sheath,  the  vagina,  which  leads 
to  the  outer  genital  aperture.  In  Paludina,  there  is  a  special  albuminous  gland 
opening  into  the  oviduct. 

In  hermaphrodite  Prosobranchia  (Valvata,  a  few  Marseniadce,  e.g.  Marsenina, 
Onchidiopsis)  a  hermaphrodite  gland  is  found.  This  gland  gives  rise  either  to  one 
duct,  which  divides  later  into  a  vas  deferens  and  an  oviduct,  or  to  a  vas  deferens  and 
an  oviduct  which  are  from  the  first  distinct.  The  vas  deferens  runs  to  the  penis  as 
in  the  males  of  dioecious  Prosobraiichiates ;  the  oviduct  runs  to  the  female  genital 
aperture.  Both  these  ducts  are,  owing  to  the  occurrence  of  accessory  glands,  etc., 
more  complicated  than  in  other  Prosobraiichiates. 

Opisthobranchia  and  Pulmonata. — The  ducts  in  these  orders  are  extremely 
complicated,  both  by  division  into  many  consecutive  sections  and  by  the  develop- 
ment of  various  accessory  organs. 

In  the  following  descriptions  of  several  types  of  genital  ducts  only  the  most 
important  points  can  be  mentioned.  We  give  first  the  type  of  duct  commonly 
found  in  the  Cephalaspidce  (Tectibranchia}. 

1st  Type. — The  hermaphrodite  gland  has  a  single  undivided  efferent  duct, 
opening  out  through  a  single  genital  aperture.  From  this  aperture  the  fertilised 
eggs  pass  out  direct,  but  the  spermatozoa  pass  into  a  ciliated  seminal  furrow  which 
runs  along  in  the  mantle  cavity,  and  by  which  they  are  conducted  to  the  penis. 


VII 


MOLLUSC  A— GENITAL  ORGANS 


233 


This  lies  more  or  less  far  forward  in  front  of  the  genital  aperture,  near  the  right 
tentacle. 

If  w'e  imagine  the  testis  of  a  male  Monotocardian  transformed  into  a  herma- 
phrodite gland,  and  the  vas  deferens  into  a  hermaphrodite  duct,  the  above  condition 
would  be  realised. 

Gastropteron  may  be  chosen  as  a  good  example  of  this  arrangement  (Fig.  190), 
which    is    further  found   in   other 
Cephalaspidcc   (Doridium,    Philine, 
Scaphander,  Bulla)  and  all  Ptero- 
poda. 

The  hermaphrodite  gland  or  ovo- 
testis,  which  lies  between  the  lobes 
of  the  liver  in  the  posterior  part  of 
the  body,  gives  rise  to  a  herma- 
phrodite duct,  which,  after  a  long 
coiled  course,  enters  a  short  but 
much  widened  terminal  section  * 
known  as  the  uterus  or  genital 
cloaca.  This  cloaca  opens  outward 
in  front  of  the  base  of  the  gills 
through  the  genital  aperture.  Into 
the  cloaca  open  :  (1)  the  common 
•efferent  duct  of  two  glands,  one 
of  which,  the  albuminous  gland, 
supplies  the  egg  with  albumen, 
while  the  other,  the  iiidameutal  or 
shell  gland,  yields  its  outer  pro- 
tective envelope  ;  (2)  the  duct  of 
a  globular  vesicle  (receptaculum 
seminis,  Schwammerdam's  vesicle), 
which  receives  the  spermatozoa  dur- 
ing copulation.  From  the  genital 
aperture,  which  has  a  more  or  less 
median  position  on  the  right  side  of 
the  body,  the  seminal  furrow  runs 
forward  to  the  penis.  The  latter  is 
enclosed  in  a  special  sheath,  out  of 
which  it  can  be  protruded,  and  into 


7:      6 


-K) 


FIG.  190.— Genital  organs  of  Gastropteron  Meckelii 
which  it  is  withdrawn  by  means  of  (after  Vayssiere).  The  penis  and  the  seminal  furrow 
a  retractor  muscle.  A  gland  called  are  not  draw?'  *•  Common  genital  aperture  ;  2  genital 

cloaca  ;  3,  albuminous  gland  ;  4,  nidamental  gland  ;  5, 

ie  prostata  opens  into  the  penis.    hermaphrodite  duct;  6,  hermaphrodite  gland;  7,  re- 
The  penis  itself  lies  on  the  right    ceptaculum  seminis. 
anteriorly,  on  the  boundary  between 

the  head  and  the  foot.     When  it  is  at  rest  its  sheath  lies  in  the  cephalic  cavity, 
near  the  buccal  mass. 

The  very  complicated  ducts  of  Aplysia  and  Accra  do  not  essentially  differ  from 
that  above  described.  The  hermaphrodite  duct,  on  reaching  the  region  of  the 
albuminous  gland,  coils  back  upon  itself,  the  ascending  and  descending  portions  of 
this  coil  surrounding  the  albumen  gland  with  their  spiral  coils.  The  penis  has  no 
prostata. 

2nd  Type.  —The  hermaphrodite  gland  gives  rise  to  a  hermaphrodite  duct,  which 
soon  divides  into  two  parts,  the  vas  deferens  or  seminal  duct,  and  the  oviduct.  The 
former  runs  to  the  male  copulatory  apparatus,  the  latter  to  the  female  genital 


234  COMPARATIVE  ANATOMY  CHAP. 

aperture.    The  male  aperture  and  the  penis  lie  in  front  of  the  female,  far  forward  on 
the  head  or  neck  ;  the  two  apertures  are  quite  distinct,  and  both  lie  on  the  right. 

This  second  type  may  be  deduced  from  the  first,  if  we  assume  not  only  that  the 
common  duct  of  the  hermaphrodite  gland  divided  into  a  male  and  a  female  duct, 
but  also  that  the  seminal  furrow  closed  to  form  a  canal  in  continuation  of  the 
male  duct. 

When  the  duct  of  this  second  type  split  into  a  male  and  a  female  duct,  the 
accessory  organs  also  so  divided  that  the  male  opened  into  the  vas  deferens,  the 
female  into  the  oviduct. 

To  this  type  belong,  among  the  Pulmonata,  the  Basommatophora,  a  few  species 
of  Daudcbardia  (D.  Saulcyi,  in  which  the  two  apertures  lie  close  together),  the 
Oncidia,  and  Vaginulidce.  In  both  these  latter  groups,  the  female  aperture  has 
followed  that  part  of  the  pallial  complex  which  shifted  to  the  posterior  end  of  the 
body,  and  lies  near  the  anus.  The  male  aperture  has,  however,  retained  its  anterior 
position  on  the  head,  behind  the  right  cephalic  tentacle.  The  two  apertures  thus 
lie  at  the  opposite  ends  of  the  body.  Among  the  Opisthobranchia,  this  second  type 
is  exemplified  in  Oscinius  (Tectibranchia). 

Taking  Limnaea  stagnalis  and  Oncidium  as  examples,  we  find  in  the  former 
(Fig.  191)  that  the  hermaphrodite  gland  which  lies  embedded  in  the  "  liver,"  high 
up  in  the  visceral  dome,  gives  rise  to  a  thin  hermaphrodite  duct ;  this  soon  divides 
into  a  male  and  a  female  duct.  The  male  duct  first  widens  into  a  flattened  sac, 
then  into  a  large  pear-shaped  glandular  vesicle  (prostata).  From  this  vesicle  it  runs 
as  a  long  thin  vas  deferens  through  part  of  the  pedal  musculature,  and  finally  enters  ' 
the  male  copulatory  apparatus,  which  is,  in  fact,  merely  the  widened  muscular  and 
protrusible  end  of  the  vas  deferens.  A  small  penis  tube  is  first  formed  by  the  vas 
deferens,  and  this  projects  on  a  papilla  into  a  subsequent  larger  tube  (the  penis 
sheath),  which  is  evaginated  during  copulation.  Protractors  are  attached  to  the 
sheath,  and  retractors  to  the  small  tube  ;  the  latter  alone  with  its  papilla  enters  the 
vulva  during  copulation. 

An  albuminous  gland  opens  into  the  female  duct  immediately  after  its  separation 
from  the  male  duct.  It  then  forms  a  uterus  consisting  of  wavy  folds,  and  is  continued 
into  a  large  pear-shaped  body  as  oviduct,  the  narrow  end  of  which  is  the  vagina  and 
leads  to  the  female  genital  aperture.  The  oviduct  receives  a  lateral  accessory  gland 
called  the  nidamental  gland,  and  the  vagina  the  efferent  duct  of  the  globular 
receptaculum  seminis. 

In  Oncidium  celticum  (Fig.  192)  the  hermaphrodite  gland  and  female  accessory 
glands  lie  in  the  posterior  part  of  the  body,  between  the  lobes  of  the  liver  and  the 
coils  of  the  intestine.  From  the  gland  rises  a  hermaphrodite  duct,  which  at  one 
point  carries  a  small  lateral  csecum,  and  opens  into  an  irregularly-shaped  organ,  the 
uteltis.  Within  the  uterus  two  projecting  folds  border  a  channel ;  if  these  folds 
become  apposed,  the  channel  becomes  a  tube.  This  channel  runs  from  the  point  of 
entrance  of  the  hermaphrodite  duct  to  the  point  where  the  seminal  duct  leaves  the 
uterus,  and  serves  for  conducting  the  semen.  The  remaining  wider  portion  of  the 
uterus  serves  as  oviduct  and  egg-reservoir,  and  carries  a  large  caeca!  appendage  ; 
the  ducts  of  the  two  much-lobed  albuminous  glands  also  enter  the  uterus. 

A  comparison  of  Limncea  and  Oncidium  shows  that  in  the  latter  the  male  and 
female  ducts  separate  from  one  another  further  back  than  in  the  former.  The  vas 
deferens  in  Oncidium  is  only  incompletely  separated  as  a  groove  in  the  uterus.  Its 
differentiation  into  a  separate  duct  takes  place  here,  as  in  terrestrial  Pulmonates,  at 
the  distal  end  of  the  uterus.  The  thin  seminal  duct  (vas  deferens)  passes  into  the 
body  wall  to  the  right,  and  runs  forward  along  the  longitudinal  furrow  between  the 
foot  and  back,  passing  again  at  the  anterior  end  of  the  body  into  the  primary  body 
cavity,  where  it  forms  numerous  coils,  and  finally  enters  the  copulatory  apparatus. 


VII 


MOLLUSC  A— GENITAL  ORGANS 


235 


This  apparatus,  in  Limncea,  consists  of  a  large  evaginable  terminal  widening,  into 
which  the  vas  deferens  projects  in  the  form  of  a  papilla.  Blood  pressure  causes  the 
penis  sheath  or  prreputium  to  be  evaginated  through  the  genital  aperture,  into  which 


FIG.  191. 


FIG.  192. 


FIG.  191.— Genital  organs  of  Limnaea  stagnalis  (after  Baudelot).  1,  Male  genital  aperture  ; 
2,  larger  penis  tube  (penis  sheath) ;  3,  protractors  ;  4,  smaller  penis  tube  ;  5,  vas  deferens  ;  6,  pro- 
stata  ;  7,  flattened  widening  of  the  vas  deferens ;  8,  hermaphrodite  duct ;  9,  hermaphrodite  gland  ; 
10,  part  of  the  digestive  gland  (liver)  ;  11,  albuminous  gland  ;  12,  nidamental  gland ;  13,  uterus  ; 
14,  pear-shaped  body  ;  15,  receptaculum  seminis  ;  16,  vagina  ;  17,  female  genital  aperture. 

FIG.  192.  —  Genital  organs  of  Oncidium  celticum  (combined  from  the  figures  of  Joyeux- 
Laffuie),  somewhat  diagrammatic  ;  only  part  of  the  vas  deferens  is  drawn.  1,  Male  genital  aperture ; 
2,  penis  sheath  (pneputium) ;  3,  penis  papilla ;  4,  vas  deferens ;  5,  uterus,  the  seminal  furrow  iu 
the  uterus  is  indicated  by  dotted  lines  ;  6.  caecum  of  the  uterus  ;  7,  oviduct  and  vagina  ;  8,  csecal 
appendage ;  9,  receptaculum  seminis ;  10,  female  genital  aperture ;  11,  albuminous  glands ;  12,  caecum 
of  the  hermaphrodite  duct,  13  ;  14,  hermaphrodite  gland. 

it  is  again  withdrawn  by  means  of  a  retractor.  In  other  species  of  Oncidium ,  the 
copulatory  apparatus  is  complicated  by  the  occurrence  of  accessory  penis  glands  and 
variously-shaped  cartilaginous  armature. 


COMPARATIVE  ANATOMY 


CHAP. 


The  oviduct  which  separates  from  the  vas  deferens  at  the  end  of  the  uterus  is  also 
a  vagina.  It  is  a  simple  tube  which  opens  outward  to  the  right  near  the  anus 
through  the  genital  aperture.  Near  the  middle  of  its  course  it  is  joined  by  the 
stalk-like  duct  of  a  globular  vesicle,  the  receptaculum  seminis  (bursa  copulatrix),  and 
by  a  long  glandular  csecal  appendage. 

3rd  Type.  —We  find  this  in  the  Stylomtnatophora  among  the  Pulmonata,  and  also 


FIG.  193.— Anatomy  of  Helix  pomatia  (after  Leuckart,  Wandtafeln).  The  shell  is  removed 
and  the  mantle  laid  back  to  the  left,  the  organs  of  the  visceral  dome  and  head  are  isolated  and 
separated.  To  the  left  (in  the  figure)  are  the  genital  organs.  L,  Digestive  gland  (liver)  ;  Zd,  her- 
maphrodite gland;  J,  intestine;  N,  kidney;  V,  ventricle;  M,  fore-stomach;  F,  foot;  A,  anus; 
Al,  edge  of  the  mantle  near  the  respiratory  aperture  ;  Mr,  retractor  muscle  ;  G,  cerebral  ganglion  ; 
Fl,  flagellum  ;  Sk,  oesophageal  bulb  (pharynx)  ;  P,  penis  ;  R,  retractor  of  the  tentacle  ;  Ps,  dart  sac  ; 
AD,  digitate  glands  ;  Vd,  vas  deferens  ;  X,  lateral  bulging  of  the  stalk  of  the  receptaculum  seminis 
(Rs) ;  Od,  portion  of  the  uterus  belonging  to  the  oviduct ;  Ed,  albuminous  gland  ;  Zg,  hermaphrodite 
duct. 

in  all  Nudibranchia  and  a  few  Tectibranchia  (e.g.  Pleurobranchcea).  The  herma- 
phrodite gland  gives  rise  to  a  hermaphrodite  duct,  which,  as  in  the  second  type, 
sooner  or  later  divides  into  a  male  and  a  female  duct.  These,  however,  do  not  open 
out  through  distinct  apertures,  but  again  unite  to  form  a  common  atrium  genitale  or 
a  genital  cloaca.  This  third  type  may  be  deduced  from  the  second  by  suppos- 
ing that  the  male  and  female  apertures  became  approximated,  and  finally  opened 
together. 


VII 


MOLLUSCA— GENITAL  ORGANS 


237 


Helix  pomatia   and    Pleurobranchcea   Meckelii   afford   good    examples   of    this 
arrangement. 

Helix  pomatia  (Fig.  193). — From  the  hermaphrodite  gland  a  hermaphrodite 
duct,  in  zigzag  coils,  passes  into  the  long  folded  uterus.  The  straight  band  which 
passes  along  the  folds  of  the  uterus  is  that  portion  of  it  which  belongs  to  the  seminal 
duct ;  the  folds  belonging  to  the  female  ducts.  The  seminal  channel,  however,  is 
merely  a  furrow  within  the  uterus,  divided  from  the  cavity  of  the  latter  by  two 
projecting  folds,  the  edges  of  which  become  superimposed.  A  longitudinal  glandular 
band,  which  is  regarded  as  a  prostata,  accompanies  this  duct.  At  the  point  where 
the  hermaphrodite  duct  passes  into  the  uterus,  the  large  linguiform  albuminous 
gland  opens  into  it.  At  the  end  of  the  uterus,  the  male  and  female  ducts  become 
entirely  distinct.  The  thin  vas  deferens  runs  in  coils  to  the  copulatory  apparatus, 
which  again  opens  into  the  genital  cloaca.  The  copulatory  apparatus  consists  of  a 
protrusible  penis  ;  at  the  point  where  the  vas  deferens  enters  this  organ,  the  latter 
carries  a  long  hollow  appendage,  the  flagellum,  the  glandular  epithelium  of  which 
perhaps  yields  the  substance  of  the  spermatophoral  capsules.  At  the  same  point  a 
retractor  muscle  is  attached  to  the  penis.  The 
short  oviduct  widens  before  opening  into  the 
genital  cloaca.  The  widened  portion  has  the 
following  appendages  :  (1)  a  long  stalked  pear- 
shaped  receptaculum  seminis,  lying  close  to  the 
uterus, — the  stalk  has  a  lateral  bulging,  which 
is  sometimes  rudimentary ;  (2)  two  tassel-shaped 
organs,  the  digitate  glands,  the  milky  secretion 
of  which  contains  calcareous  concretions,  and 
no  doubt  assists  in  the  formation  of  the  outer 
envelope  of  the  egg  ;  (3)  the  dart  sac,  which 
lies  close  to  the  cloaca,  and  contains  a  pointed 
calcareous  rod,  the  spiculum  amoris,  which  is 
thrust  by  each  individual  into  the  tissue  of  the 
other  as  an  excitant  during  copulation. 

The  common  outer  genital  aperture  lies  in 
the  nuchal  region  behind  the  right  optic 
tentacle. 

Pleurobranchsea  Meckelii  (Fig.  194).— The 
hermaphrodite  duct,  which  rises  from  the  gland, 
forms  a  long  ampulla  or  widening,  and  then 
divides  into  a  male  and  a  female  duct.  The 
vas  deferens  runs  in  coils  to  the  penis  sheath, 
which  it  enters,  coiling  up  in  it  almost  like  a 
watch-spring,  and  then  forms  the  evaginable 
widened  end  portion  which  is  called  the  penis, 
and  which  can  be  invaginated  by  a  retractor 
muscle.  The  oviduct  has  a  shorter  course,  and 
receives  the  short  efferent  duct  of  a  globular  the  ^n,e;  5  vas  deferens ;  6  nidamental 

gland  ;    , ,  albuminous  gland  ;  8,  genital 

receptaculum  seminis.  The  widened  terminal  cloaca;  9>  oviduct;  10,  recepteculum 
portion  of  the  oviduct  (the  vagina),  which  enters  seminis  ;  11,  widening  and  caecal  appen- 
the  genital  cloaca  with  the  penis,  receives  the  daSe  of  the  oviduct ;  12,  hermaphrodite 
ducts  of  the  albuminous  and  nidamental  glands  duct ;  13'  hermaphrodite  gland. 
(shell  and  slime  glands)  ;  the  second  of  these  may  be  regarded  as  the  homologue  of 
the  digitate  gland  of  Helix. 

There  is  a  general  agreement  between  the  ducts  of  the  Nudibranchia  and  those 
just  described  ;  in  details,  however,  extraordinary  variety  prevails.     The  male  and 


FIG.  194.— Genital  organs  'of  Pleuro- 
branchaea  Meckelii  (after  Mazzarelli). 
1,  Common  genital  aperture ;  2,  penis 
sheath  ;  3,  penis  ;  4,  retractor  muscle  of 


238 


COMPARATIVE  ANATOMY 


CHAP. 


\ 


female  ducts  nearly  always  unite  in  the  base  of  a  genital  cloaca,  which  often  lies 
anteriorly  on  the  right,  on  a  papilla.  The  male  and  female  apertures  are  rarely 
separate  ;  when  they  are  so,  they  lie  close  together  (cf.  Fig.  195  of  Phyllirhoe}. 
The  penis  is  often  armed  in  various  ways. 

The  important  subject  of  the  mutual  relations  of  the  three  types  of  genital  ducts 
in  hermaphrodite  Gastropoda  has  been  much  discussed,  but  no  satisfactory  con- 
clusion has  been  reached.  Ontogenetic  research  has  been  appealed  to  so  far  in  vain. 

It  is  thus  not  at  present  known  whether  the 
^  single   hermaphrodite   duct   has   arisen   by  the 

fusing  of  separate  male  and  female  ducts,  or 
whether  the  separate  ducts  have  come  into  exist- 
ence by  the  splitting  of  an  originally  single 
hermaphrodite  duct.  The  difficulty  is  increased 
by  the  fact  that  the  genetic  significance  of  the 
hermaphrodite  gland  is  uncertain. 

Fertilisation  is  mutual  in  hermaphrodite 
Gastropods.  It  is,  however,  certain  that,  in  the 
Pulmonata  at  least,  when  copulation  does  not 
take  place,  self  -  fertilisation  can  occur.  The 
hermaphrodite  duct  not  infrequently  carries  one 
or  two  lateral  cseca  or  vesicuhe  seminales,  in 
which  an  animal  can  store  up  its  own  sperm  to 
be  used  in  fertilising  its  own  eggs  if  cross-fertil- 
isation does  not  take  place.  The  eggs  and  the 
sperm  are  often  not  ripe  at  the  same  time. 

(2)  Cephalopoda. — Although  the  gonad  in  all 
extant  Cephalopoda  is  unpaired,  the  ducts  are 
originally  paired  in  both  sexes.  In  Nautilus, 
the  OegopsidcK,  and  the  Octopoda,  there  is  one 
pair  of  ducts  in  the  female  ;  but  in  the  males  a 
paired  seminal  duct  occurs  only  in  Nautilus  and 
Philonexis  carence  (Trcrnoctopus),  In  Nautilus, 
in  which  both  sexes  possess  paired  ducts,  the 
left  duct  is  in  both  cases  rudimentary  and  no  longer  functions.  It  is  the  so-called 
pear-shaped  vesicle,  which  is  attached  on  one  side  to  the  heart  and  the  lower  end 
of  the  gonad,  and  on  the  other  opens  into  the  mantle  cavity  at  the  "base  of  the 
lower  gills. 

Where  only  one  duct  is  retained,  it  is,  in  both  sexes,  the  one  on  the  left,  as  in 
Loligo,  Sepia,  Sepiola,  Rossia,  Sepioteuthis,  Chiroteuthis,  Cirrhoteuthis,  etc. 

The  genital  ducts  rise  on  the  wall  of  that  part  of  the  secondary  body  cavity  which 
is  known  as  the  genital  cavity  (peritoneal  sac,  genital  capsule),  and  open  into  the 
mantle  cavity  at  the  sides  of  the  anus,  between  the  nephridial  aperture  and  the  base 
of  the  gills. 

Male  ducts,  seminal  duct. — In  the  more  complicated  form  of  male  duct, 
such  as  that  of  Sepia  (Fig.  196),  four  principal  divisions  may  be  distinguished. 
From  the  testicular  capsule  rises  a  vas  deferens,  which  runs  along  in  close  coils,  and 
then  widens  into  a  vesicula  seminalis,  the  highly  developed  and  much  folded  epi- 
thelium of  which  plays  an  important  part  in  the  formation  of  the  spermatophores. 
The  vesicula  seminalis  is  continued  as  a  thin  vas  deferens  to  the  last  division,  the 
spermatophoral  pouch  (Needham's  pouch),  which  serves  as  a  reservoir  for  the 


FIG.  195. — Genital  organs  of  Phylli- 
rhoe (after  Souleyet).  1,  Vas  deferens  ; 
•1,  penis ;  3,  oviduct ;  4,  male,  5,  female 
genital  aperture ;  6,  vagina  ;  7,  herma- 
phrodite gland  ;  8,  hermaphrodite  duct ; 
9,  receptaculum  seminis. 


VII 


MOLLUSC  A— GENITAL  ORGANS 


ITY 
239 


spermatophores.  This  pouch  is  flask-shaped  and  projects  freely,  with  the  end  which 
corresponds  to  the  neck  of  the  flask,  at  which  the  male  genital  aperture  lies,  into 
the  mantle  cavity.  The  vas  etferens  receives  (1)  the  short  duct  of  an  oviform  gland, 
the  prostata,  and  (2)  a  simple,  lateral,  non-glandular  caecum.  The  prostata  takes 
part,  like  the  vesicula  seminalis,  in  the  formation  of  the  spermatophores.  The 
prostata,  csecuni,  and  vesicula  seminalis,  in  their  natural  position,  form  a  coil, 


Fin.  196. 

FIG.  190.— Hale  genital  organs  of  Sepia  officinalis.  1,  Genital  aperture ;  2,  spermatophoral 
pouch  ;  3,  vas  efferens  ;  4.  caecum  ;  5,  prostata  ;  6,  canalicule,  opening  into  that  part  of  the  body 
cavity  which  surrounds  the  male  duct ;  7,  vesicula  seminalis ;  8,  9,  vas  deferens ;  10,  gonad,  a 
portion  of  the  posterior  wall  is  removed,  the  genital  cavity  is  revealed,  and  on  its  anterior  wall  is 
seen  the  aperture  of  the  male  germinal  body  (12) ;  11,  aperture  of  the  seminal  duct  into  the  genital 
cavity. 

FIG.  197.— Male  genital  organs  of  Octopus  vulgaris  (after  Cuvier).  1,  Penis  ;  2,  muscle,  cut 
through  ;  3,  spermatophoral  pouch  ;  4,  veslcula  seminalis  ;  5,  prostata  ;  6,  vas  deferens  ;  7,  opened 
genital  cavity,  on  whose  anterior  wall  the  testicular  canals  of  the  germinal  body  (8)  are  seen  ; 
9,  aperture  of  the  seminal  duct  into  the  genital  sac. 

which  lies  in  a  special  division  of  the  secondary  body  cavity,  the  peritoneal  sac.  It 
is  remarkable  that  the  vas  deferens  is  in  open  communication  with  this  peritoneal 
sac  by  means  of  a  narrow  tube. 

The  male  efferent  apparatus  of  Octopus  (Fig.  197),  as  compared  with  that  of 
Sepia,  is  distinguished  chiefly  by  the  absence  of  a  separate  vas  efferens.  The  long 


240  COMPARATIVE  ANATOMY  CHAP. 

vesicula  seminalis  opens  into  the  large  prostata  near  the  point  where  the  latter  enters 
the  spermatophoral  pouch.  This  point  lies,  not  in  the  base,  but  in  the  neck  of  the 
pouch,  where  the  latter  is  produced  into  the  long  fleshy  penis,  the  point  of 
which  projects  into  the  mantle  cavity.  The  penis  is  provided  with  a  lateral 
caecum. 

It  has  already  been  mentioned  that,  as  far  as  we  know  at  present,  only  two  living 

ventraL 


-4 


O— --, 


dorsal. 

FIG.' 198. —Female  genital  organs  ol  Sepia  officinalis  (chiefly  after  Brock).  The  mantle 
cavity  is  opened,  the  posterior  integument  of  the  visceral  dome  removed,  the  ink-bag  laid  some- 
what to  one  side,  and  the  oviduct  uncovered.  The  complex  of  organs  thus  exposed  is  seen  from 
behind.  1,  Funnel ;  2,  edge  of  the  aperture  of  the  funnel ;  3,  cartilaginous  locking  apparatus ; 
4,  left  ganglion  stellare ;  5,  glandular  terminal  portion  of  the  oviduct  with  the  female  genital 
aperture ;  6,  left  lateral  lobe  of  the  accessory  nidamental  gland  ;  7,  gland  of  the  oviduct ;  8,  left 
gill;  9,  oviduct  filled  with  eggs  which  are  seen  through  its  wall ;  10,  left  nidamental  gland  ;  11, 
mantle  ;  12,  ovarial  sac,  opened  from  behind,  the  stalked  ovarial  eggs  are  seen  on  its  anterior  wall ; 
13,  ink-bag  (pigment  gland) ;  14,  stomach ;  15,  right  nidamental  gland  ;  16,  central  portion  of  the 
accessory  nidamental  gland  ;  17,  right  lateral  lobe  of  the  same  ;  18,  right  gill ;  19,  right  renal 
aperture ;  20,  anus. 

Cephalopods,  Nautilus  and  Philonexis  carence,  have  paired  male  ducts.  In  Nautilus, 
the  left  duct  is  rudimentary.  Whether  the  two  ducts  of  Philonexis  carence  correspond 
with  the  two  ducts  which  we  may  assume  that  the  Cephalopoda  originally  possessed 
is  very  doubtful.  The  two  vasa  deferentia  of  Philonexis,  which  arise  out  of  the 
testicular  capsule,  and  differ  considerably  in  structure,  unite  together  later,  and 


vii  MOLLUSCA— GENITAL  ORGANS  241 

both  lie  on  the  left  side.  It  is  also  remarkable  that  the  spermatophoral  pouch  has 
two  apertures,  and  that  there  are  thus  two  genital  apertures. 

Female  genital  organs — Sepia  (Fig.  198). — The  complicated  female  efferent 
apparatus  consists  of  two  entirely  distinct  parts,  opening  separately  into  the  mantle 
cavity:  (1)  an  unpaired  oviduct  (to  the  left),  the  position  and  aperture  of  which 
correspond  with  those  of  the  seminal  duct  in  the  male  ;  and  (2)  the  nidamental 
glands.  The  two  large  nidamental  glands  are  pear-shaped  organs,  lying  just 
beneath  the  integument  in  the  posterior  part  of  the  visceral  dome,  symmetrically,  at 
the  sides  of  and  anterior  to  the  descending  efferent  duct  of  the  ink-bag.  They  open 
into  the  mantle  cavity  at  their  ventral  ends.  Each  gland  appears  symmetrically 
divided  by  a  series  of  glandular  lamellae,  traversing  it  from  side  to  side.  The  spaces 
between  the  lamellae  open  into  the  central  slit-like  duct ;  this  structure  is  to  be  seen 
even  on  the  exterior  of  the  gland.  Besides  these  two  nidamental  glands  there  is  an 
accessory  nidamental  gland  lying  below  and  in  front  of  the  former.  It  is  of  a 
brick-red  colour,  and  consists  of  a  central  part  and  two  lateral  lobes.  It  consists  of 
numerous  coiled  glandular  canalicules,  which  open  into  a  glandular  area  in  the 
mantle  cavity.  This  glandular  area  forms  a  depression  between  the  central  and 
lateral  lobes.  As  the  aperture  of  the  large  nidamental  gland  also  lies  in  this 
depression,  the  secretions  of  the  two  glands  here  mingle. 

The  oviduct  which  rises  from  the  ovarial  sac  is,  during  the  reproductive  season, 
so  full  of  eggs,  that  it  becomes  much  distended,  especially  at  the  part  which  opens 
into  the  ovarial  sac.  Before  this  duct  opens  outward  into  the  mantle  cavity  at  the 
same  point  and  in  a  similar  manner  as  the  seminal  duct  in  the  male,  it  becomes  con- 
nected by  means  of  a  freely  projecting  portion, with  a  doubly-lobed  or  heart-shaped 
accessory  gland,  the  gland  of  the  oviduct,  which  repeats  the  structure  of  the  nida- 
mental gland.  The  terminal  portion  also  (from  the  point  of  entrance  of  this  gland 
to  the  aperture  of  the  oviduct)  is  glandular,  two  symmetrical  rows  of  perpendicular 
glandular  leaflets  projecting  from  its  wall  into  its  lumen. 

The  secretions  of  the  nidamental  glands,  accessory  nidamental  glands,  and  the 
glands  of  the  oviducts  yield  the  outer  envelopes  of  the  ovarial  eggs. 

Nidamental  glands  occur,  among  the  Cephalopoda,  (1)  in  the  Tetrabranchia 
(Nautilus)  ;  (2)  in  the  Dibrandiia,  among  the  Decapoda,  in  the  Myopsidce  (Sepia, 
Sepiola,  fiossia,  Loligo,  Sepioteuthis,  etc.)  ;  in  a  few  Oegopsidce  (OmmastrepJies, 
Onycoteuthis,  Thysanoteuthis).  They  are  wanting  in  the  Octopoda  and  in  some 
Oegopsidce  (Enoploteuthis,  Chiroteuthis,  Owenia). 

Nautilus  is  distinguished  from  all  other  living  Cephalopoda  (1)  by  the  possession 
of  only  one  nidamental  gland,  and  (2)  by  the  fact  that  this  gland  does  not  lie  in  the 
visceral  dome  but  in  the  mantle. 

Accessory  nidamental  glands  are  found  only  in  the  Myopsidce.  The  two  glands 
are  either  separate  (fiossia,  Loligo,  Sepioteuthis)  or  fused  together  (Sepia,  Sepiola). 

Glands  of  the  oviduct  occur  in  all  Cephalopoda,  but  vary  in  position  and  in 
structure. 

Outgrowths  of  the  oviduct,  which  function  as  receptacula  seminis,  occasionally 
occur  (Tremoctopiis,  Parasira'). 

In  all  Cephalopoda,  certain  quantities  of  spermatozoa  are  collected  in  extremely 
complicated  envelopes,  the  spermatophores.  The  substance  of  these  large  fila- 
mentous spermatophores  is  yielded  by  the  prostata  and  the  vesicula  seminalis,  but 
the  mechanism  by  which  so  complicated  a  case  is  produced  is  still  unknown. 
When  touched,  or  when  they  reach  water,  the  spermatophores  burst  at  definite 
points,  and  scatter  their  contents.  At  the  reproductive  season  the  spermatophoral 
pouch  is  entirely  filled  with  spermatophores.  In  Philonexis  carence,  however,  only 
one  very  long  spermatophore  is  produced. 

VOL.  II  R 


242 


COMPARATIVE  ANATOMY 


CHAP. 


c.  The  copulatory  apparatus — Hectocotylisation  in  the  Cephalopoda. — The 

copulatory  apparatus  of  the  Gastropoda,    and  the  penis  which  projects  into  the 
mantle  cavity  in  certain  Cephalopoda  have  already  been  described. 

One  of  the  most  remarkable  and  enigmatical  phenomena  in  connection  with 
the  Cephalopoda  is  their  hectocotylisation.  This  consists  in  the  transformation  of 
one  of  the  oral  arms  of  the  male  into  a  copulatory  organ  and 
spermatophore-carrier.  This  arm  is  said  to  be  hectocotylised  ; 
during  copulation  it  becomes  detached,  and  finds  its  way  into 
the  mantle  cavity  of  the  female. 

Typical  hectocotylisation  (Fig.  200)  is  found  only  in  the 
Octopodan  genera  Argonauta,  Philonexis,  and  Tremoctopus.  In 
Tremoctopus  and  Philonexis  (Parasira)  the  third  arm  on  the 
right  is  the  one  transformed,  in  Argonauta  the  third  on  the  left. 
The  arm  is  at  first  enclosed  in  an  outwardly  pigmented  sac  (Fig. 
200  A),  when  this  bursts,  the  arm  becomes  free,  and  then  its 
special  form  can  be  recognised  (B).  The  folds  which  formed 
the  sac  bend  back  so  as  to  form  a  new  sac,  which  receives  the 
spermatophores  and  is  now  inwardly  pigmented.  An  aperture 
leads  from  this  sac  into  a  seminal  vesicle  inside  the  hectocotylised 
arm  ;  this  vesicle  is  continued  into  a  long  thin  efferent  duct, 
which  runs  the  whole  length  of  the  arm  and  opens  outwardly 
at  its  end.  The  end  of  the  arm  is  transformed  into  a  long 
filamentous  penis,  which  at  first  is  also  enclosed  in  a  special  sac. 
When  the  penis  is  evaginated  the  sac  remains  as  an  appendage 
at  its  base. 

The  spermatophores  then  pass  from  the  pigmented  sac  into 
the  seminal  vesicle,  and  are  ejected  through  the  efferent  duct 
which  opens  at  the  tip  of  the  penis. 

It  is  probable  that  Cephalopods  grasp  one  another,  during 
copulation,  with  their  arms,  in  such  a  way  that  their  mouths  face 
each  other.  In  this  position  the  hectocotylised  arm  of  the  male 
becomes  detached,  and  in  some  way  or  other  forces  its  way 
into  the  mantle  cavity  of  the  female.  Detached  arms  are  often 
found  in  the  mantle  cavity  of  the  female,  as  many  as  four  have 
been  found  at  one  time. 

We  still  do  not  know  (1)  how  the  hectocotylised  arm  fertilises 
the  eggs  of  the  female,  or  (2)  how  the  spermatophores  reach  the 
hectocotylised  arm. 

_  ,  The  males  and  females  in  the  above-mentioned  genera  differ 

FIG.  199.— Spermato-  .  ° 

phore  of  Sepia  (after    fr°m  one  another,  apart  from  the  sexual  dimorphism  caused  by 
Milne  Edwards),    a,    the  development  of  the  hectocotylised  arm.      The  males  are 
Outer  case ;  5,  inner    much  smaller,  and  in  Argonauta  the  female  only  has  a  shell, 
case ;  c,  sperraatozoal         It  ig  v        probable  that  the  detached   hectocotylised   arm 
sac ;  d,  e,  f.  g,  various  J 

parts  of  the  ejacula-    can  be  replaoed  by  a  new  one. 

tory  apparatus.  Although  a  true  hectocotylised  arm,  which  can  be  detached, 

is  only  developed  in  the  three  genera  above  mentioned,  it  has 
been  proved  that  in  all  other  Cephalopoda  (even  Nautilus,  cf.  p.  117),  a  certain  arm 
or  portion  of  the  head  in  the  male  is  in  some  way  modified,  differing  in  some 
(often  unimportant)  manner  from  the  other  arms.  Such  an  arm  is  said  to  be 
hectocotylised,  and  it  is  assumed  that  it  plays  some  part  in  copulation,  although 
its  exact  function  is  unknown.  In  Sepia  and  Nautilus  it  is  even  difficult  to 
imagine  what  part  it  can  take  in  copulation.  The  constant  occurrence  of  a 
hectocotylised  arm  is  the  more  remarkable  as  it  is  by  no  means  always  the 


VII 


MOLLUSC  A— GENITAL  ORGANS 


243 


same  arm  that  is  thus  transformed.     In  the  Octopoda,  as  a  rule,  it  is  the  third 
on  the  right  side,  but  in  the  Octopodan  subgenus  Scceurgus  and  in  Argonauta 


FIG.  200.— Male  of  Argonauta  argo  (after  H.  Miiller).  (Female,  Figs.  35,  36,  pp.  24,  25.)  A, 
With  the  hectocotylised  arm  enclosed  in  the  sac  (d).  B,  with  the  arm  free,  a,  Funnel ;  5,  edge  of 
the  mantle  fold  ;  c,  left  eye  ;  d,  sac  ;  di,  hectocotylised  arm  ;  e,  mouth. 

it  is  the  third  on  the  left.     In  the  Decapoda  the  hectocotylised  arm  is  generally 
the  fourth  on  the  left,  but  in  the  genus  Enoploteuthis  it  may  be  the  fourth  on 


FIG.  201.  —  Hectocotylus  of  Philonexis 
(Octopus)  carenae  (after  Leuckart).  a, 
Spennatophoral  pouch ;  b,  seminal  vesicle  ; 
c,  efferent  duct  of  the  same ;  d,  appendage 
=  remains  of  the  penis  sac  ;  e,  penis ;  /, 
sucker. 


the  right,  or  even  in  one  and  the  same  species  of  Ommastrephes,  it  is  sometimes 
the  fourth  on  the  left  and  sometimes  the  fourth  on  the  right.     In  Sepiola  and 


244 


COMPARATIVE  ANATOMY 


CHAP. 


Rossia,  it  is  the  first  arm  which  is  hectocotylised.  Finally,  both  the  arms  of  one 
pair  may  be  thus  transformed  ;  in  Idiosepion  and  Spirula,  this  is  the  case  with  the 
fourth  pair,  in  fiossia  with  the  first. 

The  difference  in  size  between  the  male  and  the  female,  Avhich  has  been  mentioned 
as  occurring  in  those  forms  which  have  true  hectocotylised  arms,  is  also  found, 
though  not  to  the  same  degree,  in  many  other  Cephalopoda,  in  which  the  male  is 
slightly  smaller  than  the  female. 


XXL  Parasitic  Gastropoda. 

1.  Thyca  ectoconcha  (Fig.  202)  is  a  Prosobranchiate  Gastropod  which  is  parasitic 
on  the  Star-fish  Linckia  multiforis.  The  chief  points  in  its  organisation  are  shown 
in  Fig.  202,  a  longitudinal  section  in  which,  however,  several  organs  which  lie 
laterally  to  the  section  are  also  represented.  The  organisation  of  the  Gastropod  is  as 
yet  little  influenced  by  its  parasitic  manner  of  life.  It  possesses  a  shell,  shaped 


ml 


oc 


FIG.  202.— Longitudinal  section  through  Thyca  ectoconcha  (after  P.  and  F.  Sarasin).  Some 
organs  not  actually  belonging  to  the  section  are  included,  cer,  Cerebral  ganglion ;  d,  alimentary 
canal ;  fl,  folds ;  /*,  foot ;  k,  gill ;  I,  liver  ;  ml,  mantle  ;  oc,  eye  ;  ot,  otocyst ;  ped,  pedal  ganglion ;  pr, 
proboscis  ;  sf,  false  foot ;  si,  oesophageal  bulb  ;  vl,  cephalic  fold. 

somewhat  like  a  Phrygian  cap.  In  the  mantle  cavity  lies  the  gill.  The  alimentary 
and  nervous  systems  also  are  in  no  way  remarkable.  It  has  eyes  and  auditory 
organs,  and  a  short  powerful  snout,  and  muscular  cesophageal  bulb,  which  penetrates 
the  Star-fish  between  the  calcareous  parts  of  its  integument  into  the  tissues.  There 
is  no  radula.  The  base  of  the  snout  is  surrounded  by  a  muscular  disc  consisting 
of  an  anterior  and  a  posterior  part.  This  disc,  the  so-called  false  foot,  is  the 
grasping  organ  by  which  the  animal  attaches  itself  to  the  integument  of  its  host 
so  firmly  that  it  cannot  be  torn  away  without  injury.  The  rudiment  of  a  foot 
(/«)  occurs  without  an  operculum. 

2.  The  Gastropodan  organisation  is  somewhat  more  strongly  modified  in 
Stilifer  Linckia  (Fig.  203),  which  is  parasitic  on  the  male  Linckia.  The  whole 
body  of  this  parasite  penetrates  into  the  calcareous  layer  of  the  integument  of  the 
host,  on  which  it  raises  pathological  globular  swellings,  and  further  causes  the 
peritoneum  to  bulge  inwards  towards  the  body  cavity.  The  parasite  communicates 


VII 


MOLLUSCA— PARASITIC  GASTROPODA 


245 


with  the  outer  world  only  by  means  of  a  small  aperture  at  the  tip  of  the  swelling. 
The  parasite,  thus  established  in  the  integument  of  its  host,  is  surrounded  on  all 
sides  by  a  fleshy  envelope  (sm).  This  envelope  is  only  broken  through  by  an 
aperture  at  the  point  where  the  apex  of  the  dextrally  twisted  shell  lies  ;  this 
aperture  corresponds  in  position  with  the  aperture  above  mentioned  as  occurring 
at  the  tip  of  the  pathological  swelling.  This  envelope  is  called  the  false  mantle, 
and  corresponds  morphologically  with  the  false  foot  of  Thyca,  much  increased  in 
size  and  bent  back  on  to  the  shell.  There  occur  besides  a  true  mantle,  a  gill,  a 
rudimentary  foot  without  an  operculum,  eyes,  auditory  organs,  and  a  typical 
Prosobranchiate  nervous  system.  The  development  of  the  remarkable  false  mantle 


jel 


V 

FIG.  203.— Longitudinal  section  through  Stilifer  Linckiae  (after  P.  and  F.  Sarasin).  be, 
Buccal  ganglion  ;  U,  blood  sinus  ;  ccr,  cerebral  ganglion  ;  d,  alimentary  canal ;  fs,  foot ;  k,  gill ;  7, 
liver  ;  ml,  mantle  ;  n,  proboscidal  nerve  ;  oc,  eye  ;  ot,  otocyst ;  ped,  pedal  ganglion  ;  pr,  proboscis  ; 
sm,  false  mantle  ;  sub,  subintestinal  ganglion ;  sup,  supraintestinal  ganglion. 

no  doubt  signifies  that,  although  the  animal  is  embedded  deep  in  the  integument 
of  the  host,  communication  with  the  exterior  is  retained.  "Water  for  respiration  can 
enter  and  flow  out  of  the  mantle  cavity,  and  the  faecal  masses  and  genital  products, 
and  perhaps  also  the  larvse  can  pass  into  the  cavity  of  the  false  mantle  and  be 
ejected  through  its  aperture.  The  sexes  are  separate.  The  snout  has  lengthened 
into  a  very  long  proboscidal  tube  which  pierces  the  tissues  of  the  integument  of  the 
Star-fish,  which  are  rich  in  blood,  and  draws  from  them  the  necessary  nourishment. 
Both  O3sophageal  bulb  and  radula  are  wanting. 

3.  The  two  parasites  just  described  are  typical  Gastropods,  and  are  easily 
recognised  as  such  when  carefully  examined ;  there  are,  however,  two  other 
parasitic  Gastropods  in  which  the  typical  organisation  is  so  much  modified  that  it 


246 


COMPARATIVE  ANATOMY 


CHAP. 


would  be  difficult  to  recognise  them  as  Gastropods,  or  even  as  Molluscs,  were  it  not 
proved  that  the  larvse  of  one  of  these  forms  at  least  are  distinctly  Gastropodan 
larvae.  The  incomplete  state  of  our  knowledge  of  the  development  of  these  two 
parasites,  and  the  absence  of  any  transition  forms  between  them  and  the  typical 
organisation,  make  them  very  difficult  to  understand. 

Entocolax  Ludwigii  inhabits  endoparasitically  the  body  cavity  of  a  Holothurian 
(Myriotrochus  Rinkii],  one  end  of  its  vermiform  body  being  attached  to  the  body 
wall  of  its  host.  Its  organisation,  a  scheme  of  which  is  given  in  Fig.  205,  can  be 
best  studied  with  the  help  of  some  hypothetical  transition  forms,  through  which  a 


FIG.  204.— A,  B,  0,  D,  Hypothetical  transition  stages  between  Thyca  and  Stilifer  on!,the 
one  side  and  Entocolax  (Fig.  205)  on  the  other  (after  Schiemenz).  a,  Anus ;  fd,  pedal  gland ; 
I,  liver  (digestive  gland);  Id,  hepatic  intestine ;  m,  mouth ;  mag,  stomach;  o,  ovary  ;  of,  aperture  of 
the  false  mantle  ;  sf,  false  foot ;  sm,  false  mantle ;  u,  uterus ;  w,  body  wall  of  the  host. 

Gastropod  of  the  type  of  Thyca  or  Stilifer  might  pass  in  developing  into  an 
endoparasitic  parasite  like  Entocolax.  Fig.  204  A  shows  the  first  stage,  which  still 
much  resembles  Thyca,  and  is  still  ectoparasitic  ;  Fig.  204  B,  C,  D  are  further 
stages  in  development.  In  proportion  as  the  animal  becomes  endoparasitic,  and 
gives  up  its  relations  to  the  external  world,  do  the  sensory  organs,  the  shell,  and  the 
mantle  cavity  with  the  gill  disappear.  The  stomach,  as  a  separate  section  of  the 
intestine,  degenerates,  the  digestive  gland  (liver)  becomes  a  simple  unbranched 
diverticulum  of  the  intestine,  which  loses  the  rectum  and  anus.  All  organs  for  the 
purpose  of  mastication  at  the  anterior  end  of  the  alimentary  canal  are  lost.  The 


VII 


MOLLUSC  A— PARASITIC  GASTROPODA 


247 


false  mantle  becomes  larger  and  larger,  and  envelops  the  small  visceral  dome,  which 
gradually  becomes  rudimentary,  and  finally  contains  merely  the  genital  organs.  At 
the  stage  D  the  whole  animal  already  projects  freely  into  the  body  cavity  of  the 
host,  attached  to  its  wall  by  a  displaced  portion  of  the  false  foot,  and  connected  with 
the  exterior  only  by  the  aperture  of  the  false  mantle.  If  this  last  means  of 
communication  with  the  exterior  is  also  abandoned,  i.e.  if  the  whole  false  mantle 
with  its  aperture  becomes  enclosed  in  the  body  cavity  of  the  host,  we  have  a  form 
corresponding  with  the  endoparasite  Entocolax  Ludwigii  (Fig.  205).  In  this  form, 
the  cavity  enclosed  by  the  false  mantle,  into  which  the  ovary  and  its  receptacula 


...fc 


FIG.  205. 


FIG.  206. 


FIG.  205.  — Entocolax  Ludwigii,  sketch 
after  Voigt.  Lettering  the  same  as  that  in 
the  preceding  figure. 


FIG.  206.— Entoconcha  mirabilis,  sketch 
by  Schiemenez  (after  Baur).  Lettering  as 
in  Fig.  204.  hod,  Testes? 


seminis  open,  serves  as  a  receptacle  for  the  fertilised  eggs,  which  were  found  in  it  in 
their  first  stage  of  segmentation  in  the  one  (female)  specimen  discovered. 

Entoconcha  mirabilis,  an  endoparasite  which  has  been  found  in  a  Holothurian, 
Synapta  digitata,  is  even  more  deformed  than  Entocolax.  The  body  of  this  parasite 
is  a  long  vermiform  coiled  tube,  attached  by  one  end  to  the  intestine  of  the  host, 
while  the  rest  of  the  tube  floats  freely  in  the  body  cavity  of  the  latter.  Its 
organisation  has  as  yet  been  imperfectly  investigated.  Fig.  206  is  a  very  simple 
diagram,  which  is  introduced  for  comparison  with  Fig.  205  of  Entocolax.  It  is 
impossible  to  say  how  far  such  a  comparison,  which  the  lettering  is  intended  to 
facilitate,  is  justifiable.  Up  to  the  present  time,  no  aperture  leading  from  the  ovary 
into  the  brood-chamber,  which  is  thought  to  be  the  cavity  of  the  false  mantle,  and  is 


248  COMPARATIVE  ANATOMY  CHAP,  vn 

filled  with  embryos  (not  represented  in  the  figure),  has  been  observed.  In  a  widening 
of  the  tube  near  its  attached  end,  a  number  of  free  "  testicular  vesicles  "  have  been 
found,  but  their  real  significance  can  only  be  discovered  by  further  research. 

The  embryos  found  in  the  brood  cavity  of  Entoconcha  have  the  same  general 
structure  as  Gastropodan  larvae.  They  have  a  spirally  twisted  shell,  into  which 
the  body  can  be  withdrawn  ;  an  operculum,  a  small  velum,  the  rudiments  of  two 
tentacles,  two  auditory  vesicles,  [a  foot,  and  an  intestine,  which,  according  to  one 
observer  (the  most  recent),  consists  of  only  a  mouth,  pharynx,  oesophagus,  and  the 
rudiment  of  a  liver,  but  according  to  an  older  authority  is  complete.  There  is, 
further,  a  branchial  cavity  with  a  transverse  row  of  long  cilia.  Nothing  further  is 
known  of  the  development  and  life  history  of  Entoconcha. 

Some  details  of  parasitic  Lamellibranchiate  larvse  (Unionidce)  will  be  given  in  the 
section  on  Ontogeny. 


XXII.  Attached  Gastropoda. 

Of  the  several  forms  of  attached  Gastropods  known,  only  Vermetus,  whose  inner 
organisation  has  been  carefully  investigated,  can  be  shortly  described  in  this  place. 
Vermetus  has  a  shell  which,  instead  of  being  coiled  like  the  well-known  shell  of  the 
snail,  is  a  calcareous  tube,  which  rises  freely  from  the  bottom  of  the  sea,  to  which  its 
tip  is  cemented.  This  shell  is  very  like  the  calcareous  tubes  of  tubicolous  worms 
such  as  Serpula.  The  larva  of  this  form,  however,  possesses  a  typically  coiled  shell, 
and  even  the  young  animal,  after  it  has  attached  itself,  has  such  a  shell.  In  the 
course  of  growth,  however,  the  coils  become  loosened,  and  the  shell  finally  grows  out 
as  a  tube. 

The  typical  organisation  of  the  Monotocardian  Prosobranchiates,  to  which  Vermetus 
belongs,  is  little  affected  by  the  attached  manner  of  life.  The  visceral  dome,  like 
the  shell,  is  much  elongated  and  almost  vermiform.  The  intestine,  the  circulatory 
system,  the  kidneys,  the  mantle,  the  gill,  and  the  nervous  system  are  typically 
developed.  The  sexes  are  separate,  and  copulatory  organs,  which  could  not  be 
used  by  attached  animals,  are  wanting.  The  head  is  well  developed,  and  the 
pharynx  well  armed.  When  the  animal  is  slightly  irritated,  it  is  said  not  to 
withdraw  at  once  into  its  shell,  like  other  Gastropods,  but  to  bite.  The  foot  has 
the  form  of  a  truncated  cylinder,  and  is  directed  anteriorly,  ventrally  to  the  head. 
It  cannot,  of  course,  function  as  a  locomotory  organ,  but  carries  the  operculum  for 
closing  the  shell,  and,  by  means  of  the  pedal  gland,  secretes  mucus.  Vermetus  is 
said  to  produce  great  quantities  of  this  secretion,  which  it  allows  to  float  in  the 
water  for  a  time  like  a  veil,  and  then  swallows  together  with  all  that  has  become 
attached  to  it.  In  this  way  it  fishes  for  the  small  organisms  which  form  its  food. 


XXIII.  Ontogeny. 

A.  Amphineura. 

1.  Ontogeny  of  Chiton  Polii  (Fig.  207).  The  egg  possesses  little  nutritive  yolk. 
The  segmentation  is  total  and  somewhat  unequal ;  a  ccelogastrula  is  formed  by 
invagination. 

(a)  The  blastopore  of  the  gastrala  larva  marks  its  posterior  end.  >  A  pair  of 
endoderm  cells  near  the  dorsal  edge  of  the  blastopore  are  specially  large.  A 
longitudinal  section  shows  two  dorsal  and  two  ventral  ectodermal  cells  with  larger 


FIG.  207.— Development  of  Chiton  Polii  (after  Kowalevsky).  A-F,  Six  stages  in  the  develop- 
ment of  the  gastrula  into  the  young  Chiton  ;  sections  nearly  median.  G-,  frontal  section  through 
stage  C,  oblique,  from  the  upper  part  of  the  velum  to  the  blastopore.  H,  I,  K,  L,  transverse 
sections  of  four  stages  of  development  behind  the  mouth.  1,  Blastopore  ;  2,  archenteron  or 
midgut ;  3,  mesoderm  ;  4,  ectoderm  ;  5,  velum  or  preoral  ciliated  ring ;  6,  stomodaeum  or  oesophagus  ; 
7,  mouth ;  8,  radular  sac  ;  9,  body  cavity ;  10,  pedal  gland,  in  I  oesophagus ;  11,  foot ;  12,  anus 
with  proctodteum ;  13,  cerebral  ganglion  ;  14,  pretrochal  tuft ;  15,  pleurovisceral  cords  ;  16,  pedal 
cords  ;  17,  mantle  furrow  ;  18,  eye ;  c,  shell ;  crc7,  the  seven  shell  plates  first  formed. 


250  COMPARATIVE  ANATOMY  CHAP. 

nuclei ;  these  belong  to  a  double  row  of  cells  on  which  is  developed  the  preoral 
ciliated  ring  which,  in  Molluscs,  is  called  the  velum  (Fig.  207  A). 

(6)  At  a  later  stage,  the  blastopore  appears  shifted  somewhat  towards  the  ventral 
side,  and  an  inward  growth  of  ectodermal  cells  begins  at  its  edge  ;  this  is  the 
commencement  of  the  formation  of  the  ectodermal  stomodseum.  At  the  posterior 
and  upper  edge  of  the  blastopore,  there  is,  in  the  figure,  a  cell  lying  between  the 
endoderm  and  the  ectoderm ;  this  is,  no  doubt,  a  mesodermal  cell  (B). 

(c)  The  larva  elongates ;  a  distinct  stomodseum  (embryonic  oesophagus),  leading 
through  the  blastopore  into  the  archenteron,  is  formed  by  the  continuous  growth 
inward  of  the  ectodermal  cells  ;  this  organ  becomes  shifted  still  further  forward 
along  the  ventral  surface  (C). 

(d)  Fig.  207  G  is  an  oblique  section  from  an  anterior  upper  to  a  posterior  lower 
point  through  a  slightly  older  larva,  which  shows  the  stomodseum,  and,  at  the  sides 
of  the  blastopore,  the  first  mesoderm  cells.     These  are  probably  derived  from  the 
endoderm,  and  are  symmetrically  placed  at  the  two  sides  of  the  blastopore. 

(e)  A  median  section  through  the  next  stage  (D)  shows  no  mesoderm  cells  as  yet 
in  the  median'  plane.     The  mouth,  however,   appears  shifted  forward  along  the 
ventral  side  as  far  as  the  ciliated  ring  or  velum,  the  double  row  of  cells  in  the  latter 
being  very  clear. 

(/)  Transverse  section  of  an  older  stage  (H).  The  mesoderm  cells  have  increased 
in  number,  and  are  arranged  in  two  groups  at  the  sides  of  the  stomodseum,  between 
the  ectoderm  and  the  endoderm. 

(g]  At  a  later  stage,  a  longitudinal  section  of  which  is  given  in  Fig.  207  E,  the 
principal  feature  is  a  stronger  development  of  the  mesoderm,  in  which  a  space,  the 
body  cavity,  now  appears.  A  bulging  backward  of  the  stomodeeum  forms  the  first 
rudiment  of  the  radular  sac.  Behind  the  mouth,  a  sac-like  depression  is  formed, 
evidently  by  the  ectoderm  ;  this  has  been  called  the  pedal  gland,  although  it  has 
not  yet  been  discovered  what  becomes  of  it  in  the  adult  animal. 

(h)  When  the  body  cavity  forms,  the  cells  of  the  mesoderm  become  divided  into 
two  layers,  the  inner  visceral  layer  becoming  applied  to  the  intestine,  and  the  outer 
parietal  layer  to  the  ectoderm  (cf.  Fig.  207  I).  In  the  transverse  section,  we  see, 
deep  down  in  the  ectoderm,  the  first  rudiments  of  the  pleurovisceral  cords.  The 
pedal  cords  arise  in  the  same  way,  and  anteriorly,  in  the  cephalic  area,  which  is 
encircled  by  the  preoral  ciliated  ring,  the  rudiments  of  the  supra-cesophageal 
central  nervous  system  form  as  a  neural  plate,  i.e.  as  a  thickening  of  the  ectoderm, 
which  carries  a  tuft  of  long  cilia. 

(i)  At  later  stages  (F,  K,  L),  the  central  nervous  system  with  the  pleurovisceral 
and  pedal  cords  become  detached  from  the  ectoderm  and  take  up  their  mesodermal 
position.  The  rudiments  of  seven  shell-plates  appear  on  the  back  as  cuticular 
formations  ;  the  eighth  only  appears  later.  A  posterior  invagination  of  the  ectoderm 
represents  the  rudiment  of  the  proctodeum  (the  embryonic  hind-gut  with  the  anus). 
The  first  teeth  appear  in  the  radular  sac.  The  whole  of  the  cephalic  area  and  the 
region  of  the  foot  become  covered  with  cilia.  On  the  dorsal  ectoderm,  on  the  parts 
that  are  not  covered  by  the  shell-plates,  the  first  calcareous  spines  appear.  In  the 
posterior  part  of  the  body,  a  great  accumulation  of  mesodermal  elements  evidently 
marks  the  position  of  a  formative  mesodermal  zone. 

At  this  stage,  the  larva  leaves  the  egg  envelope,  and  swims  about  freely,  and,  on 
the  degeneration  of  the  ciliated  ring,  sinks  to  the  bottom  transformed  into  a  young 
Chiton.  During  this  last  transformation  two  lateral  larval  eyes  appear  on  the 
anterior  ventral  side  of  the  body.  The  development  of  the  circulatory  system,  the 
nephridia,  the  genital  organs  and  the  ctenidia  has  not  been  followed. 

2.  Solenogastres. — The  ontogeny  of  this  order  is  as  yet  only  known  through 
a  very  incomplete  account  of  the  development  of  Dondersia  banyulensis.  The 


VII 


MOLL  USG A— ONTOGENY 


251 


segmentation  is  unequal  and  total,  and  takes  place  through  the  formation  of 
micromeres.  The  process  of  gastrulation  seems  to  occur  in  a  manner  half  way 
between  epibole  and  invagination.  The  blastopore  marks  the  posterior  end  of  the 
larval  body,  which  is  divided  by  two  circular  furrows  into  three  consecutive  regions. 
The  anterior  region  consists  of  two  circles  of  cells,  and  evidently  corresponds  with 
the  pretrochal  area.  It  is  partially  ciliated,  and  carries  in  the  middle  a  group  of 
longer  cilia,  one  of  which  is  sometimes  to  be  distinguished  from  the  rest  as  a  flagellum. 
The  second  region,  which  consists  of  a  single  row  of  cells,  carries  a  circle  of  long 
cilia,  and  evidently  represents  the  velum.  The  third  region  consists  of  two  rows 
of  cells  carrying  short  cilia ;  the  second  row  edges  the  blastopore.  At  an  older 
stage,  the  posterior  part  of  the  larva  appears  to  be  withdrawn  into  an  invagination 
of  the  anterior  part.  The  whole  or  by  far  the  greater  part  of  the  body  of  Dondersia 
is  said  to  be  produced  from  this  posterior  part  (the  "  embryonic  cone  ")  alone.  On 
this  embryonic  cone,  there  appear,  first  of  all,  on  the  two  sides  of  the  middle  line, 
three  pairs  of  consecutive  imbricated  spiculse,  still  retained  in  their  formative  cells. 


Fig.  208.— Dondersia  banyulensis.    A,  Larva  36  hours  old.    B,  Larva  100  hours  old.    C,  Young 
Dondersia  immediately  after  transformation  (7th  day),  after  Pruvot. 

They  soon  break  through  these  cells,  and  their  number  is  increased  by  the  appear- 
ance of  new  ones  anteriorly.  The  embryonic  cone  lengthens,  becomes  curved  ven- 
trally.  The  anterior  part  of  the  body  with  the  velum  and  the  pretrochal  area  becomes 
reduced  and  finally  appears  as  a  sort  of  collar  at  the  anterior  end  of  the  body.  The 
larva  sinks  to  the  bottom,  and  throws  off  the  whole  anterior  part  of  the  body  with 
the  velum  and  the  pretrochal  area.  Such  throwing  off  or  resorption  of  parts  of  the 
body  which  have  been  of  great  physiological  importance  in  the  larva  is  very  common 
in  the  animal  kingdom  ;  see  sections  on  the  ontogeny  of  the  Worms  (e.g.  Nemertina, 
Phoronis,  etc.,  vol.  i.  p.  272),  of  the  Arthropoda  (Metamorphism  of  Insects,  vol.  i. 
p.  490),  and  of  the  Echinodermata. 

On  the  dorsal  region  of  the  young  Dondersia,  seven  consecutive,  imbricated,  but 
only  slightly  overlapping,  calcareous  plates  can  now  be  distinguished,  consisting 
of  rectangular  rods  lying  close  alongside  of  one  another  (Fig.  208,  C).  This 
fact  is  very  significant  with  regard  to  the  shell  of  the  Chiton,  which  in  the  adult 
consists  of  eight,  but  in  the  larva  of  only  seven  plates.  If  it  could  be  proved  that 
the  Solcnogastridce  pass  through  a  Chiton  stage,  the  view  that  they  are  more 
specialised  animals  than  the  Polyplacophora,  and  are  to  be  derived  from  Chiton-like 
forms,  would  receive  almost  decisive  support. 

Besides  the  seven  dorsal  calcareous  plates,  the  young  Dondersia  has  numerous 


252 


COMPARATIVE  ANATOMY 


CHAP. 


circular  calcareous  spicules,  covering  it  laterally  ;  the  ventral  side  is,  however, 
naked.  A  mouth  is  still  wanting,  the  endodermal  mass  is  not  yet  hollow,  and  on 
each  side,  between  the  endoderm  and  the  integument,  there  is  a  solid  mesodermal 
streak. 

B.  Gastropoda. 

As  a  type  of  the  development  of  the  Gastropoda,  we  may  take  Paludina  mvipara 
(Figs.  209  and  210),  the  ontogeny  of  which  has  recently  been  again  very  carefully 
investigated.  Development  here  takes  place  within  the  body  of  the  mother.  The 
egg  is  comparatively  poor  in  yolk.  A  ccelogastrula  is  formed  by  invagination,  the 
blastopore  of  which  marks  the  posterior  end  of  the  germ,  and  becomes  the  anus. 
No  proctodseum  is  formed.  The  whole  of  the  intestine  from  the  stomach  to  the 


FIG.  209.— Development  of  Paludina  vivipara  (after  v.  Erlanger).  A  and  B,  Stage  after 
gastrulation,  with  the  rudiments  of  the  mesoderm  and  the  ccelom  as  outgrowths  of  the  archenteron. 
A,  Median  optical  longitudinal  section.  B,  Horizontal  optical  longitudinal  section.  C,  Horizontal 
optical  longitudinal  section  through  the  embryo,  after  the  entire  separation  of  the  ccelomic  sac 
from  the  intestine.  D,  Sagittal  optical  longitudinal  section  through  an  embryo,  in  which  the 
mesoderm  has  brolien  up,  the  cells  becoming  spindle-shaped.  1,  Velum  ;  2,  segmentation  cavity  ; 
3,  archenteron  ;  4,  coelom  ;  5,  blastopore  ;  6,  mesoderm  cells  ;  7,  shell-gland. 

anus  proceeds  from  the  endoderm.  The  mesoderm  arises  as  a  ventral  hollow  out- 
growth of  the  archenteron,  which  soon  becomes  constricted  from  the  intestine,  and 
lies  between  the  intestine  and  the  ectoderm  in  the  segmentation  cavity  as  a  vesicle 
with  two  points  directed  forward  (Fig.  209  A,  B,  C).  This  vesicle  spreads  out  to 
the  right  and  left  dorsally  round  the  intestine,  finally  closing  round  it  dorsally.  Its 
outer  wall  of  cells,  which  becomes  applied  to  the  ectoderm,  forms  the  parietal 


vii  MOLLUSGA— ONTOGENY  253 

layer,  while  its  inner  wall,  which  is  applied  to  the  intestine,  forms  the  visceral 
layer  of  the  mesoderm.  The  cells  of  the  mesoderm  soon  become  detached  from  one 
another  (Fig.  209,  D)  ;  they  assume  the  spindle  shape  and  finally  fill  the  segmenta- 
tion cavity  like  a  network. 

In  the  meantime  the  velum  has  appeared,  and,  between  it  and  the  anus,  the  shell- 
gland  forms.  The  oesophagus  arises  as  an  invagination  of  the  ectoderm,  which  soon 
becomes  connected  with  the  midgut.  By  the  addition  of  a  paired  primitive  kidney, 
the  typical  Molluscan  Trocophora  is  formed  ;  this  at  first  is  quite  symmetrical, 
the  anus  lying  posteriorly  in  the  middle  line. 

After  the  development  of  the  oesophagus,  a  mass  of  mesoderm  cells  collects  on 
each  side  of  and  below  the  intestine,  this  mass  soon  becoming  hollow.  In  this  way 
two  inesodermal  sacs  are  formed  which  approximate  towards  the  middle  line  till 
they  touch,  and  then  fuse  to  form  one  sac,  the  double  origin  of  which  is  still,  for  a 
time,  evidenced  by  the  presence  of  a  median  septum.  The  sac  which  thus  arises  is 
the  pericardium.  Fig.  210  A  shows  a  somewhat  further  developed  embryo  seen 
from  the  right  side.  Below  and  behind  the  mouth  are  seen  the  projecting  rudiment 
of  the  foot,  on  which  to  the  right  and  left  the  auditory  vesicles  have  arisen  as 
invaginations  of  the  ectoderm.  In  the  pretrochal  area,  protuberances  to  right  and 
left  represent  the  rudiments  of  the  tentacles,  at  the  bases  of  which  the  eyes  have 
appeared  as  ectodermal  pits.  The  shell  gland  has  secreted  a  shell.  The  greater 
growth  of  that  side  of  the  body  which  is  covered  by  the  shell  has  caused  a  bending 
by  which  the  anus  is  shifted  towards  the  ventral  side.  Immediately  behind  the 
anus,  the  ectoderm  bulges  out  to  form  the  rudiment  of  the  mantle  fold,  so  that  the 
anus  comes  to  lie  in  a  shallow  depression,  the  rudiment  of  the  pallial  or  respiratory 
cavity.  It  is  important  to  note  that  at  this  outwardly  symmetrical  stage,  the  mantle 
cavity  and  the  anus  lie  posteriorly.  The  fore-gut  (oesophagus)  has  greatly  lengthened. 
The  digestive  gland  has  grown  out  from  the  stomach  ventrally  in  the  form  of  a 
wide  sac,  but  is  still  connected  with  the  latter  by  a  wide  aperture.  The  pericardium, 
in  which  the  septum  is  still  visible,  has  already  somewhat  shifted  from  below  the 
stomach  to  its  right  side.  The  rudiments  of  the  definite  nephridia  next  form  in 
the  following  way  (Fig.  210,  D).  In  each  division  of  the  pericardium  (the  left 
division  being  smaller  than  the  right)  the  wall  bulges  out  ;  the  right  outgrowth 
becomes  the  secreting  portion  of  the  permanent  kidney ;  the  left  degenerates,  but 
must  be  regarded  as  a  temporarily  appearing  rudiment  of  the  (original)  left  kidney. 
The  mantle  cavity,  which  lies  beneath  the  pericardium,  presses  into  it  to  the  right 
and  left  in  the  form  of  two  projections.  The  right  projection,  continuing  to  grow, 
becomes  connected  with  the  rudiment  of  the  right  kidney  and  forms  its  efferent  duct. 
The  left  projection  does  not  grow  further,  nor  does  it  become  connected  with  the 
rudiment  of  the  left  kidney. 

A  further  stage  is  depicted  from  the  right  side  in  Fig.  210  B.  The  following  are 
the  most  important  alterations.  The  optic  pit  has  become  constricted  into  an  optic 
vesicle.  The  mantle  fold  has  grown  further  forward,  and  has  become  deeper  to  the 
right.  The  undivided  pericardium  has  shifted  altogether  to  the  right  of  the 
stomach,  and  lies  above  the  rectum,  which  bends  forward  and  downward.  The 
body  is  already  asymmetrical. 

At  the  following  stage  (Fig.  210,  C)  the  posterior  and  dorsal  region  of  the  body 
rises  distinctly  from  the  rest  as  a  visceral  dome  ;  the  shell  covering  this  part  of  the 
body  has  increased  considerably  in  size.  The  mantle  fold  has  become  much  broader, 
and  the  mantle  cavity  much  deeper  ;  the  latter  now  lies  chiefly  on  the  right  side  of 
the  body.  The  looping  of  the  intestine  is  far  more  marked.  On  the  posterior  and 
dorsal  side  of  the  pericardium,  the  pericardial  wall  sinks  in  the  form  of  a  channel, 
which  soon  closes  and  forms  a  tube  ;  this  is  the  rudiment  of  the  heart.  The  two 
apertures  of  the  tube,  where  the  wall  of  the  heart  passes  into  that  of  the  pericardium, 


254 


COMPARATIVE  ANATOMY 


CHAP. 


communicate  with  the  body  cavity.  The  heart  tube  becomes  constricted  in  the 
middle,  the  anterior  division  forming  the  auricle  and  the  beginning  of  the  branchial 
vein,  the  posterior,  the  ventricle  and  the  rudiment  of  the  body  aorta. 


FIG.  210.— Development  of  Paludina  vivipara  (after  v.  Erlanger)  A,  Right  aspect  of  an 
embryo,  in  which  the  pericardium  is  divided  into  two  parts  by  a  septum.  B,  The  same  of  a  some- 
what older  embryo,  with  an  undivided  pericardium.  C,  The  same  of  an  older  embryo,  in  which  the 
first  rudiment  of  the  heart  has  appeared.  D,  Ventral  aspect  of  the  posterior  end  of  an  embryo,  in 
which  the  asymmetry  of  the  visceral  dome  begins  to  appear.  The  anus  is  still  median,  but  the 
mantle  cavity  is  already  deeper  on  the  right  (the  left  in  the  figure).  1,  Velum  ;  2,  mid-gut ;  3, 
digestive  gland  (liver) ;  4,  pericardium  ;  4a  and  45,  divisions  of  the  same  formed  by  a  septum  ;  5, 
free  edge  of  the  shell ;  6,  shell  groove  ;  7,  anus  ;  8,  mantle  cavity  ;  Sb,  base  of  the  mantle  cavity  = 
base  of  the  mantle  fold  ;  9,  free  edge  of  the  mantle  ;  10,  foot ;  11,  auditory  organ  ;  12,  oesophagus  ; 
13,  cephalic  tentacle ;  14,  eye ;  15,  efferent  duct  of  the  (originally)  right  nepliridium ;  155,  rudi- 
mentary efferent  duct  of  the  (originally)  left  nephridium ;  16,  primitive  kidney  ;  17,  rudiment  of 
the  heart ;  18a,  right  nephridium  ;  18&,  rudimentary  left  nephridium. 

Fig.  211  A  shows  a  somewhat  older  embryo  which  already  resembles  in  form  the 
adult  animal.  The  velum  is  reduced,  and  a  ventral  bulging  of  the  anterior  division 
of  the  oesophagus  represents  the  rudiment  of  the  radular  sac.  The  ventricle  and 


VII 


MOLL  USCA— ONTOGENY 


255 


the  auricle  are  distinct.  An  ectodermal  depression  on  the  foot  forms  the  operculum. 
The  mantle  cavity  which  lies  on  the  right  side,  and  into  which  the  rectum  opens, 
now  also  stretches  to  the  left  on  the  anterior  and  dorsal  side  of  the  sharply 
demarcated  visceral  dome.  The  gill  appears  in  the  form  of  a  protuberance  on  the 


FIG.  211.— Development  of  Paludina  vivipara  (after  v.  Erlanger).  A,  An  embryo  in  which 
the  first  rudiment  of  the  gill  has  appeared.  B,  A  nearly  mature  embryo.  Both  are  seen  from  the 
left  side.  Lettering  as  in  Fig.  210.  In  addition,  Via,  Auricle  ;  176,  ventricle  ;  18,  nephridium  ;  19, 
rectum ;  20,  rudiment  of  the  radular  sac ;  21,  rudiment  of  the  gill ;  22,  osphradium  (Spengel'S 
organ) ;  23,  rudiment  of  the  genital  duct ;  24,  rudiment  of  the  gonad  ;  25,  operculum. 

inner  surface  of  the  mantle  cavity,  and  the  osphradium  at  the  left  of  the  gill  as  an 
ectodermal  protuberance. 

Fig.  211  B  finally  shows  us  an  embryo  in  which  the  mantle  cavity  has  assumed 
the  anterior  position  on  the  visceral  dome.  The  ctenidium  and  osphradium  have 
developed  further.  The  velum  is  very  much  reduced  and  can  only  be  seen  in 
sections.  This  stage  is  important  on  account  of  the  appearance  of  the  rudiment  of 
the  genital  organs,  which  is  identical  in  the  two  sexes.  A  depression  of  the  (meso- 


UNIVERSITY 


256 


COMPARATIVE  ANATOMY 


CHAP. 


dermal)  pericardial  wall,  which  becomes  separated  from  the  pericardium,  forms  the 
rudiment  of  the  gonad,  while  an  ingrowth  from  the  base  of  the  mantle  cavity  runs 
towards  this,  and  is  the  (ectodermal)  rudiment  of  the  genital  .duct.  The  latter 
arises  on  one  side  of  the  anus,  just  as  the  efferent  duct  of  the  permanent  kidney 
rises  on  its  other  side  ;  this  ontogenetic  fact  confirms  what  was  stated  above  (p.  219) 
that  the  genital  duct  of  the  Monotocardia  corresponds  with  a  part  of  the  right  (which 
originally,  and  in  the  young  embryo,  is  the  left)  kidney  of  the  Diotocardia  (apparently 
wanting  in  the  Monotocardia}. 

The  vascular  system  arises  very  early  in  the  form  of  spaces  between  the  mesoderm 
and  ectoderm  or  entoderm,  round  which  the  mesoderm  cells  grow,  and  which  become 
secondarily  connected  with  the  heart. 

All  the  ganglia  of  the  nervous  system,  the  cerebral,  pleural,  pedal,  parietal,  and 
visceral  ganglia  arise  separately  as  ectodermal  thickenings,  which  become  constricted 
off  from  the  ectoderm  by  delamination.  They  only  secondarily  become  connected 
with  one  another  through  the  growing  out  of  the  nerve  fibres.  The  parietal  ganglia 
arise  to  right  and  left  in  the  middle  region  of  the  body,  but  soon  become  shifted  by 
the  displacement  of  the  organs  of  the  visceral  dome,  one  above  the  intestine  and  the 
other  below  it.  The  rudiment  of  the  visceral  ganglion  is  said  to  appear  dorsally  to 
the  hind-gut  and  to  move  later  to  its  position  beneath  the  same. 

The  observations  on  the  development  of  Paludina  vivipara,  here  briefly  described, 
are  in  many  ways  of  great  importance,  and  confirm  in  the  most  unmistakable 
manner  the  results  arrived  at  by  comparative  anatomy.  The  following  are  specially 
noteworthy. 

1.  The  manner  in  which  the  pericardium  originates  favours  the  opinion  that  it 
is  a  secondary  body  cavity.  It  is  important  to  note  that  the  pericardium  is  at 

first  paired,  being  divided  into 
two  lateral  halves  by  a  septum, 
which  afterwards  disappears. 

2.  The  fact  that  the  gonad 
arises  as  an  outgrowth  of  the 
pericardium,  confirms  the  view 
arrived     at     by     comparative 
anatomy,  that  the  genital  cavity 
also  is  a  secondary  body  cavity. 

3.  The  anus  and  the  mantle 
cavity  originally  lie  symmetri- 
cally at   the   posterior   end    of 
the  body,  but,  through  asym- 
metrical growth,    come   to   lie 
first  on  the  right  side  of  the 
visceral   dome,   and  finally  on 
its  anterior  side. 

FIG.  212.— Larva  of  Oncidium  celticum,  from  the  left  The  development  of  other 
side  (after  Joyeux  Laffuie).  1,  Cerebral  ganglion ;  2,  edge  Gastropods  cannot  here  be  de- 
of  the  mantle;  3,  rudiment  of  the  gonad;  4,  larval  shell-  scribed  in  dptail  Wp  rpfpr  t>lp 
muscle;  5,  hiud-gut ;  6,  rudiment  of  the  digestive  gland;  C  , 

7,  auditory  organ ;  8,  pedal  ganglion ;  9,  foot ;  10,  oesophagus ;  rea(ler  to  the  bibliography  at 
11,  eye  ;  12,  branched  muscle  cells  of  the  velum  ;  13,  velum,  the  end.  As  a  rule,  nutritive 

yolk     is     present     in      larger 

quantities  than  in  the  viviparous  Paludina,  in  which  the  small  provision  of 
yolk  is  evidently  connected  with  the  favourable  conditions  of  nutrition  of  the 
embryo. 

The  blastopore  generally  corresponds  in  position  with  the  future  mouth  ;  it  often, 


vii  MOLLUSGA— ONTOGENY  257 

perhaps  usually,  remains  open  ;  notwithstanding  this,  the  oesophagus  arises  by  the 
sinking  in  of  ectoderm  cells. 

Paludina  is,  as  far  as  is  known,  the  only  Mollusc  in  which  the  mesoderm 
originates  as  an  outgrowth  of  the  archenteron.  This  fact  is  no  doubt  connected 
with  its  poverty  in  nutritive  yolk.  In  other  Gastropods,  the  mesoderm  arises  in  the 
manner  already  described  for  other  Molluscs,  as  two  large  symmetrical  primitive 
cells,  at  the  posterior  edge  of  the  blastopore  ;  these  cells  look  more  like  endodermal 
than  ectodermal  cells,  and  soon  pass  into  the  segmentation  cavity. 

A  Veliger  larva,  i.e.  a  Trochophora  with  Molluscan  characteristics,  always  forms 
(1)  a  dorsal  shell  gland  with  the  embryonic  shell, 
and  (2)  a  ventral  rudiment  of  the  foot. 

The  outward  appearance  of  the  Yeliger  larva, 
however,  varies  much  in  different  groups,  the 
variations  being  connected  with  the  manner  of 
life  and  of  feeding  of  the  embryo. 

In  the  marine  Gastropods,  i.e.  in  the  majority 
of  the  Prosobranchia  (including  the  Hetcropoda). 
the  Pulmonate  genus  Oncidium,  and  all  Opistho- 
branchia,  the  embryo  leaves  the  egg  envelope  early, 
as  a  free-swimming  Yeliger  larva.  In  all  these 
forms,  the  preoral  ciliated  ring  is  well  developed. 
The  ectodermal  floor  of  the  ciliated  ring  usually  FIG.  213.  —  Larva  of  Cymbulia 
bulges  out  anteriorly,  so  that  the  cilia  appear  to  (Pteropod),  from  the  left  side  (after 
be  carried  by  a  distinct  circular  ridge.  This  ridge  Gegenbaur)  i,  velum  ;  2,  shell ;  3, 
J  .  parapodia  (fins);  4,  foot  with  oper- 

even  grows  out  laterally  to  form  a  lobe  of  varying    ^lu{^5) 

size,  which  carries  at  its  edge  long  and  strong 

cilia,  and  is  occasionally  itself  produced  into  an  upper  and  a  lower  'lobe.  This  is 
the  true  velum  of  the  free-swimming  Gastropod  larva,  and  is  its  only  organ  of 
locomotion.  It  is  internally  traversed  from  wall  to  wall  by  contractile  mesoderm 
cells  (muscle  cells),  which  make  it  highly  contractile.  In  the  older  larvae,  the  head 
with  the  velum  can  be  withdrawn  into  the  shell. 

It  is  probable  that  the  velum  of  the  larva  also  serves  for  respiration,  and  perhaps 
for  bringing  about  a  circulation  of  the  body  fluid  by  means  of  its  contractility. 

The  embryos  of  fresh-water  and  terrestrial  Gastropods,  where  these  animals  are 
not  viviparous,  remain  longer  in  the  egg,  and  leave  it  only  after  their  transformation 
into  young  Gastropods,  the  larval  organs  (the  velum,  the  primitive  kidney,  the 
cephalic  vesicle,  and  the  pedal  vesicle  or  podocyst)  having  degenerated  within  the 
egg  envelope.  Even  in  these  forms,  the  mass  of  nutritive  yolk  contained  in  the  egg 
is  not  very  great,  but  there  is  a  large  quantity  of  albumen  stored  up  within  the  egg 
capsule,  which  serves  as  food  for  the  developing  embryo ;  this  is  either  absorbed 
through  the  body  wall  or  swallowed.  .  The  egg  capsules  are  always  large,  in  some 
cases  (in  tropical  terrestrial  Gastropods)  as  large  as  the  egg  of  a  small  bird ;  but 
their  size  is  not,  as  in  the  Cephalopoda,  determined  by  that  of  the  egg  contained, 
but  by  the  quantity  of  albumen  in  which  the  small  egg  is  embedded.  The  mature 
egg  capsule  contains  a  young  Gastropod  of  considerable  size  with  a  well-developed 
shell. 

In  terrestrial  and  fresh-water  forms,  the  velum  is  not  needed  as  a  locomotory 
organ,  and  is  therefore  reduced  to  a  single  ring  of  cilia  or  to  two  lateral  ciliated 
streaks.  It  is  entirely  wanting  in  the  embryos  of  a  few  terrestrial  Gastropod  snails. 
The  respiratory  and  circulatory  functions,  which  were  originally  merely  accessory 
functions  of  the  velum,  here  become  of  greater  importance.  The  nuchal  region 
becomes  much  bulged  forward,  and  forms  a  cephalic  vesicle  (Fig.  214),  which  is 
sometimes  very  large,  and  undergoes  regular  pulsations.  The  posterior  division 
VOL.  II  S 


258 


COMPARATIVE  ANATOMY 


CHAP. 


of  the  foot,  in  the  same  way,  is  often 


FIG.  214.— Embryo  of  Helix  Waltoni  (4  mm. 
long),  from  the  right  side  (after  P.  and  F. 
Sarasin).  1,  Cephalic  vesicle  ;  2,  upper  (optic) 
tentacle ;  3,  eye ;  4,  lower  tentacle ;  5,  oral 
lobe ;  6,  sensory  plate  ;  7,  podocyst. 

In  the  larva  of  the  gymnosomaton 
rings  are  developed  on  the  body. 


widened  into  a  pulsating  pedal  vesicle  or 
podocyst.  Towards  the  end  of  larval  life 
the  cephalic  and  pedal  vesicles  and  other 
similar  "larval  hearts"  degenerate. 

The  embryonic  shell  is  either  retained 
throughout  life  or  is  thrown  off  at  an  early 
stage,  and  replaced  by  the  rudiment  of  the 
definitive  shell.  Even  a  second  temporary 
shell  occasionally  attains  development. 

It  must  again  be  noted  that  shell-less 
Gastropods,  to  whatever  natural  division 
they  belong,  pass  through  a  typical  Veliger 
stage,  and  at  the  older  Veliger  stage  have 
a  distinctly  demarcated  coiled  visceral 
dome,  with  a  corresponding  shell,  and 
usually  an  operculum  on  the  metapodium. 

s  Ptcropoda  three  postoral  accessory  ciliated 


C.  Scaphopoda. 

Ontogeny  of  Dentalium. — Segmentation,  in  these  animals,  leads  to  the  formation 
of  a  coeloblastula,  from  which  a  coelogastrula  arises  by  invagination.  The  blastopore 
at  first  lies  posteriorly  on  the  ventral  side,  but 
gradually  shifts,  as  in  Chiton,  more  and  more 
forward  along  the  ventral  side.  The  stomodieum 
arises  as  an  ingrowth  of  the  ectoderm,  the  blasto- 
pore nevertheless  remaining  open.  A  typical 
Molluscan  Trocophora  is  developed,  although  no 
primitive  kidney  has  been  found.  The  velum  is 
a  thick  ridge  round  the  body  of  the  long  oviform 
larva.  This  ridge  consists  of  three  rings  of  very 
large  ectoderm  cells,  each  row  carrying  a  circle 
of  long  cilia.  The  shell  gland  spreads  out  at  an 
early  stage,  its  lateral  edge  soon  growing  out 
ventrally  and  posteriorly  as  the  mantle  fold. 
The  free  edges  of  the  two  folds  fuse  at  a  later 
stage  below  the  body.  The  anus  forms  very  late. 
The  development  of  the  cerebral  and  pedal 
ganglia  and  of  the  auditory  organ  has  been 
specially  carefully  observed.  On  the  ventral 
side  of  the  pretrochal  area,  in  front  of  the  velum 


FIG.  215.— Larva  of  Dentalium,  37 
hours  old,  posterior  and  lower  aspect 
and  behind   the  tuft  of  cilia,  two  symmetrical    (after  Kowalevsky).    1,  Cephalic  tuft ; 

• •  J_"  ^  *!»       J_1  it  f*  ,1  1T« 


of  three  rows  of  cilia  ;  4,  mouth,  hidden 
under  the  ridge  of  the  velum  ;  5,  mantle 
fold. 


invaginations  of  the  ectoderm  form  the  cephalic  2,  rudiments  of  the  cerebral  ganglion 
sacs  or  tubes.  These  become  constricted  from  the  (cephalic  tubes);  3,  velum,  consisting 
ectoderm  at  a  later  stage,  their  lumen  gradually 
narrows  and  finally  disappears,  while  their  walls 
become  thick  and  multilaminar  by  the  con- 
tinuous growth  of  the  cells.  The  two  cell  masses  which  thus  arise  become  connected 
in  the  middle  line  above  and  below  the  oesophagus,  and  form  the  cerebral  ganglion. 
The  otocysts  arise  at  the  base  of  the  pedal  rudiment  on  each  side  as  ectodermal 
epithelial  pits,  which  soon  become  detached  from  the  ectoderm  in  the  form  of 
epithelial  vesicles.  Immediately  beneath  these  auditory  vesicles,  certain  ectoderm 


MOLL  USC A— ONTOGENY 


259 


cells  sink  below  the  surface,  and  form  on  each  side  an  ectodermal  cell  mass,  which 
becomes  detached  from  the  rest  of  the  ectoderm,  sinks  into  the  mesoderm  of  the 
foot,  and  fuses  with  the  similar  mass  on  the  other  side  to  form  the  pedal  ganglion. 


D.  Lamellibranchia. 

1.  Development  of  Teredo  (Figs.  216  and  217).     Segmentation  is  here  total  and 
unequal.     The  gastrula,  formed  by  epibole  (Fig.   216  A,   B)  consists  of  (1)  two 
large  endoderm  cells  (macromeres),   a   thick  cap  of  ectoderm   cells  (micromeres) 
closely   covering   these,    and 
two    symmetrical     primitive  A 

mesoderm  cells  of  medium 
size  at  the  posterior  edge  of 
the  blastopore.  The  blasto- 
pore  closes  from  behind  for- 
ward, the  ectoderm  cells  by 
continual  division  growing 
entirely  round  the  endoderm 
cells  ;  during  this  process  the 
two  mesoderm  cells  become 
covered  by  the  ectoderm  and 
come  to  lie  between  the  latter 
and  the  endoderm  (Fig.  216 
C).  Somewhat  anteriorly  on 
the  ventral  side,  a  depression 
of  the  ectoderm  forms  a  pit, 
the  stomodseum  (D).  The 
ectoderm  separates  off  from 
the  two  -  celled  mesoderm, 
thus  giving  rise  to  a  seg- 
mentation cavity,  or  primary 
body  cavity.  A  double 
preoral  ciliated  band  is 
formed  (D,  E).  The  two 
large  endoderm  cells,  by 
fission,  produce  other  smaller 
cells.  Cilia  appear  over  the 
whole  surface  of  the  germ, 
with  the  exception  of  the 
posterior  dorsal  surface,  where 
the  ectoderm  cells,  which 
have  become  cylindrical,  sink 
in  to  form  the  shell  gland  (F). 
The  latter  secretes  the  first 
rudiment  of  the  shell  in  the 
form  of  a  simple  cuticular 
membrane.  The  endoderm 
cells  begin  to  collect  to  form 
the  intestinal  wall.  After  the  formation  of  the  first  rudiment  of  the  shell,  the  shell 
gland  flattens  and  spreads  out ;  its  edge  can  still  be  found  as  a  ridge  running  under 
the  edge  of  the  shell.  The  endoderm  now  forms  a  large  globular  hollow  mid-gut, 
into  which  the  oesophagus  breaks  through.  Each  of  the  primitive  mesoderm  cells 


FIG.  216.— A-G,  Stages  in  the  development  of  Teredo  (after 
Hatschek).  A,  C,  D,  E,  F,  G,  from  the  right  side,  B  in  optical 
horizontal  section.  1,  Ectoderm;  2,  macromeres  =  endoderm 
cells ;  3,  primitive  mesoderm  cells ;  4,  segmentation  cavity ; 
5,  stomodseum  (oesophagus);  6,  mouth;  7,  preoral  ciliated 
band ;  8,  shell  gland ;  9,  shell  ;  10,  larval  muscle  cells ;  11, 
cephalic  plate  with  tuft ;  12,  anal  invagination,  anus ;  13, 
endodermal  mid-gut. 


260 


COMPARATIVE  ANATOMY 


CHAP. 


has  given  rise  to  two  or  three  smaller  cells.  The  thin  cuticular  shell  becomes 
bivalvular  by  the  appearance  of  a  mediodorsal  boundary  line. 

A  further  stage  is  distinguished  first  by  the  appearance  of  a  small  posterior 
ectodermal  invagination,  the  proctodseum,  which  produces  the  rectum  and  anus. 
An  ectodermal  thickening,  the  neural  plate,  appears  in  the  pretrochal  area,  carrying 
three  flagella.  Some  of  the  mesoderm  cells  become  muscle  cells  (Fig.  216  G). 

The  next  stage  may  be  called  that  of  the  Trochophora  larva.  This  larva  differs 
from  a  typical  Annelidan  Trochophora  only  by  possessing  a  shell,  which  now  covers 
the  greater  part  of  the  body,  and  by  a  mantle  which  appears,  at  first,  posteriorly,  and 


15 


FIG.  217.— Older  Larva  of  Teredo,  from  the  right  side  (after  Hatschek).  Lettering  as  in  Fig. 
216.  In  addition,  14,  rudiment  of  the  digestive  gland  (liver) ;  15,  preoral  ciliated  band  (velum) ; 
16,  postoral  ciliated  band ;  17,  primitive  kidney ;  18,  auditory  vesicle ;  19,  rudiment  of  the  pedal 
ganglion  ;  20,  rudiment  of  the  gill ;  21,  mesodermal  streak. 

then  at  the  sides,  as  a  fold,  and  continues  to  grow  from  behind  forward.  The  region  of 
the  body  which  lies  behind  the  cephalic  area  has  spread  out  on  each  side  to  form  a 
broad  fold,  which  becomes  outwardly  applied  to  the  shell.  The  neural  plate  has 
become  multilaminar,  and  the  proctodseum  has  broken  through  into  the  mid-gut. 
The  primitive  mesoderm  cells  have  given  rise  to  a  short  mesoderm  streak  on  each 
side.  At  the  anterior  end  of  each  mesoderm  streak  a  somewhat  long  body,  the 
primitive  kidney,  has  formed  ;  this  contains  a  channel  which  opens  externally,  and 
whose  lumen  *s  ciliated  at  a  later  stage.  The  rudiment  of  the  digestive  gland 
appears  in  the  mid-gut  as  a  paired  semi-spherical  outgrowth.  The  body  is  no  longer 
ciliated  all  over  ;  cilia  are  retained  only  on  the  neural  plate  and  in  the  anal  region. 
The  double  preoral  ciliated  band  now  becomes  very  distinct,  and  a  postoral  band  is 


vii  MOLLUSC  A— ONTOGENY  261 

now  added.  The  region  between  the  two  ciliated  bands  also  carries  cilia  and  forms 
an  adoral  ciliated  zone. 

A  further  stage  of  development  is  depicted  in  Fig.  217.  The  rudiment  of  the 
pedal  ganglion  can  be  recognised  as  an  ectodermal  thickening  on  the  ventral  side, 
and  that  of  the  gill  as  a  thick  epithelial  ridge.  The  stomach  has  formed  a  caecum 
posteriorly,  and  the  narrow  mid-gut  has  formed  a  loop.  The  two  auditory  vesicles 
containing  otoliths  have  arisen  between  the  mouth  and  anus  as  ingrowths  of  the 
ectoderm  which  have  become  detached.  The  mesoderm  consists  of  branched  muscle 
cells,  branched  cells  of  connective  tissue,  the  primitive  kidney  and  the  still 
undifferentiated  cells  of  the  mesoderm  streaks. 

The  ectodermal  thickening,  which  represents  the  rudiment  of  the  pedal  ganglion 
at  a  later  stage,  becomes  rounded  off  and  detached  from  the  ectoderm,  at  the  same 
time  becoming  surrounded  by  the  cells  of  the  mesoderm  streak,  which  have  rapidly 
multiplied,  and  which  unite  in  front  of  it  to  form  a  median  mass  of  cells.  This 
median  mass  of  mesoderm  cells  increases  greatly  by  rapid  division,  bulging  forward 
the  ectoderm  in  the  anterior  ventral  region,  and  thus  forming  the  rudiment  of  the 
foot.  In  the  growing  branchial  fold  slits  occur,  a  single  slit  appearing  first,  and 
another  soon  following  in  front  of  the  first.  The  further  development  of  this 
larva  is  unknown. 

The  development  of  other  marine  bivalves  runs  very  much  the  same  course  as 
that  of  Teredo,  the  same  larva  being  formed.  The  ciliated  band  is  very  strongly 
developed  in  all  marine  bivalves  (Teredo,  Ostrea,  Modiolaria,  Cardium,  Montaciria, 
etc. ),  and  is  generally  carried  by  a  collar-like  expansion  of  the  integument,  or  velum, 
which  is  often  divided  into  two  lateral  lobes.  The  velum,  which  on  account  of  the 
band  of  strong  cilia  it  carries  is  the  locomotory  organ  of  the  free-swimming  larvae  of 
these  Lamellibranchs,  can  be  protruded  out  of  and  withdrawn  into  the  shell. 

Among  fresh-water  Lamellibranchs  there  is  one  form,  Dreissensia  polymorpha, 
whose  larva  is  free-swimming  and  carries  a  well-developed  velum.  This  form  is 
said  to  have  migrated  from  salt  to  fresh  water  in  (geologically  speaking)  recent 
times. 

Special  arrangements  are  found  among  the  other  fresh-water  forms.  The  eggs  of 
Pisidium  and  Cyclas,  for  instance,  develop  in  special  brood  capsules  in  the  gills  of 
the  mother  animal,  and  leave  these  as  young  bivalves.  The  Trochophora  stage  is, 
nevertheless,  passed  through,  but  the  velum,  not  being  used  for  locomotion,  remains 
rudimentary. 

2.  Ontogeny  of  Cyclas  cornea  (Figs.  218  and  219). — We  shall  here  only  mention 
the  points  in  which  the  development  of  Cyclas  differs  from  that  of  Teredo,  and 
describe  such  observations  as  complete  those  made  on  the  latter.  The  blastula 
consists  of  a  cap  of  small  cells  (ectoderm  cells)  and  a  floor  made  of  three  large  cells, 
one  very  large  primitive  endoderm  cell  and  two  symmetrical  primitive  mesoderm 
cells.  The  primitive  endoderm  cell  yields  through  fission  a  disc  of  endodermal  cells. 
The  two  primitive  mesoderm  cells  become  overgrown  by  the  ectoderm  cells,  and 
thus  reach  the  segmentation  cavity.  The  endoderm  invaginates  in  such  a  way  that 
a  slit-like  blastopore  arises,  which  reaches  from  the  region  of  the  future  mouth  to 
that  of  the  future  anus.  This  blastopore  closes  completely.  The  oasophagus  arises 
as  an  ingrowth  of  the  ectoderm.  A  Molluscan  Trochophora  is  formed  with  a 
shell  gland,  a  rudimentary  foot,  a  mid-gut,  a  stomach,  anus,  primitive  kidney,  and  a 
neural  plate.  The  velum  is  reduced  to  a  ciliated  area  lying  at  the  sides  of  the 
mouth  (Fig.  218) ;  this  reduction  is  connected  with  the  fact  that  the  Trocho- 
phora of  Cyclas  is  not  a  free-swimming  larva,  for  the  eggs  of  Cyclas  pass  through 
the  whole  course  of  their  development  within  •  the  gills  of  the  mother  animal. 
Above  the  neural  plate  the  ectoderm  cells  are  large  and  flat,  and  form  a  projecting 
cephalic  vesicle.  The  mesoderm  consists  of  (1)  scattered  cells,  which  lie  under 


262 


COMPARATIVE  ANATOMY 


CHAP. 


the  ectoderm  of  the  cephalic  cavity,  in  the  foot  and  on  the  intestine  (especially  on 
the  oesophagus,  where  they  are  already  changed  into  muscle  cells) ;  and  (2)  two 
mesoderm  streaks  lying  at  the  sides  of  the  intestine.  The  pedal  ganglia  arise 
together  with  the  paired  rudiment  of  the  byssus  gland,  as  thickenings  of  the 
ectoderm  at  the  posterior  end  of  the  foot.  The  auditory  vesicles  originate  as 
ingrowths  of  the  ectoderm.  The  mantle  forms  by  degrees  from  behind  forward  as  a 
ridge,  which  grows  more  and  more  ventrally  downwards.  At  the  same  time  the 


FIG.  218.— A-D,  Four  stages  in  the  development  of  Cyclas  cornea,  from  the  right  side  (after 
Ziegler).  1,  Membranous  shell ;  2,  rectum  ;  3,  anus ;  4,  free  edge  of  the  mantle  ridge  or  fold  ; 
5,  rudimentary  byssus  cavity  with  gland  ;  0,  rudiment  of  the  pedal  ganglion  ;  7,  foot ;  8,  velar 
region;  9,  oesophagus ;  10,  stomach;  11,  calcareous  shell;  12,  pericardium;  13,  kidney;  14,  rudi- 
ment of  the  gonad  ;  15,  edge  of  the  membranous  shell ;  16,  edge  of  the  calcareous  shell ;  17,  rudi- 
ment of  the  gill ;  18,  byssus  thread ;  19,  visceral  ganglion ;  20,  posterior  adductor ;  21,  glandular 
part  of  the  kidney ;  22,  lateral  wall  of  the  pericardial  vesicle  ;  24,  median  wall  of  the  same ; 
25,  digestive  gland  (liver) ;  26,  cerebral  ganglion  ;  27,  mouth  ;  28,  auditory  vesicle. 

shell  gland,  which  at  its  edge  secretes  the  delicate  shell  membrane,  spreads  out  and 
becomes  flattened.  Beneath  the  shell-membrane  the  rudiments  of  the  permanent 
shell  valves  are  produced  from  two  small  round  areas  lying  to  the  right  and  left  of 
the  dorsal  middle  line  (B).  The  digestive  gland  (liver)  develops  from  two  lateral 
globular  outgrowths  of  the  wall  of  the  stomach.  The  gonads  arise  from  cells  of  the 
mesoderm  streaks,  which  are  larger  than  the  rest  and  also  in  other  ways  differen- 
tiated, so  that  they  can  very  early  be  distinguished.  In  the  anterior  and  dorsal 


VII 


MOLL  USC A— ONTOGENY 


263 


part  of  each  mesoderm  streak  a  group  of  cells  surrounds  a  cavity,  which  at  first  is 
very  small,  but  becomes  continually  larger.  The  two  vesicles  thus  formed,  the 
cavities  of  which  represent  the  secondary  body  cavity,  form  the  pericardium. 
Behind  these  the  mesoderm  cells  collect  in  such  a  way  as  to  form  on  each  side  a 
strand,  which  becomes  hollow  ;  this  is  the  rudiment  of  the  nephridium,  which  at 
once  becomes  connected  with  the  pericardial  vesicle,  and,  growing  further  towards 
the  ectoderm,  soon  opens  outward.  The  two  pericardial  vesicles  lengthen  posteriorly 
and  upward,  each  becoming  divided  into  two  parts,  one  lying  behind  the  other,  by  a 
constriction,  the  parts  still  communicating  dorsally  with  one  another  (Fig.  219  A). 
The  two  double  vesicles  grow  towards  one  another  above  the  rectum,  and  finally 
fuse  in  the  dorsal  middle  line  (B).  In  a  similar  manner  they  fuse  below  the 
rectum.  The  inner  wall  of  the  pericardial  vesicle  becomes  the  wall  of  the  ventricle 
(C),  and  its  lateral  wall  becomes 
that  of  the  auricle.  At  the  points  A 

where  the  lateral  vesicles  were  con-  3 

stricted  lie  the  slits  through  which 
the  auricles  communicate  with  the 
ventricle,  and  the  atrioventricular 
valves. 

The  visceral  ganglion  arises  at 
the  posterior  end  of  the  mantle 
furrow  from  an  ectodermal  thicken- 
ing. The  pleurovisceral  connectives  FIG.  219.— A-C,  Diagrams  illustrating  the  develop- 
form,  in  all  probability,  throughout  ment  of  the  pericardium  and  heart  of  Cyclas  cornea 
their  whole  length,  through  con-  <after  Ziegler).  1  and  2,  The  two  lateral  pericardial 


striction  from  the  ectoderm.     The 
gill  arises  on  each  side  as  a  fold  on 


vesicles ;  3,  rectum ;  4,  pericardial  cavity ;  5  and  6, 
imaginations  of  the  lateral  walls  of  the  pericardium  = 
rudiments  of  the  two  lateral  auricles ;  7  and  8,  median 
the  dorsal  edge  of  the  inner  surface  walls  of  the  two  lateral  pericardial  vesicles,  in  B  partly 
of  the  mantle.  It  develops  from  fused  to  form  a  median  septum  (above  and  below  the 
behind  forward.  In  the  contrary  "Destine),  which  in  C  has  disappeared ;  9,  rudiment  of 

J     the  ventricle, 
direction     furrows     form    on    the 

branchial  fold,  commencing  from  below  upwards  ;  these  are  found  on  the  inner  as 
well  as  the  outer  surface,  and  exactly  correspond.  The  inner  furrows  join  the  outer 
right  through  the  gill,  and  thus  give  rise  to  the  branchial  slits. 

3,  The  development  of  the  Unionidse  (Anodonta,  Unio)  is  much  influenced  by 
the  parasitic  manner  of  life  of  the  larva.  The  fertilised  eggs  reach  the  outer  leaf 
of  the  gill  of  the  female,  and  there  run  through  the  first  stages  of  their  develop- 
ment. Segmentation  leads  to  the  formation  of  a  coeloblastula,  in  which  the  rudi- 
ment of  the  shell  gland  appears  very  early  as  an  incurved  plate  of  large  and 
high  cells  of  the  blastula  wall.  The  archenteron  forms  by  invagination  at  a  very 
late  stage  ;  this  is  no  doubt  connected  with  the  later  parasitism  of  the  larva. 
Before  this  invagination  occurs  the  mesoderm  has  begun  to  form  ;  its  two  primi- 
tive cells  lie  in  the  blastoccel  at  the  part  where,  later,  the  enteric  invagination 
appears. 

The  embryo  known  as  Glochidium  parasiticum  has,  in  the  last  stage  of  its 
development,  which  is  passed  through  in  the  gills  of  the  mother  animal  but  within 
the  egg-shell,  the  following  structure  (Fig.  220).  It  is  bilaterally  symmetrical,  and 
has  a  bivalve  shell.  Each  valve  has,  at  its  ventral  edge,  a  triangular  process, 
the  exterior  of  which  is  beset  with  short  spines  and  thorns.  Between  the  two  valves, 
which  are  markedly  concave,  lies  the  soft  body,  which  lines  the  shell  internally  in 
such  a  way  that  its  ventral  epithelial  layer  might,  incorrectly,  be  called  a  mantle. 
It  may  be  called  the  false  mantle.  If  this  false  mantle  is  examined  from  below, 
when  the  shell  is  open,  it  is  seen  to  have  on  each  side  four  sensory  cells  furnished 


264 


COMPARATIVE  ANATOMY 


CHAP. 


•, 2 


B 


with  long  sensory  hairs  ;  three  of  these  cells  lie  near  the  shell  process,  and  the  fourth 
near  the  middle  line.  Between  the  two  more  median  sensory  cells  a  long  adhesive 
filament  projects  from  the  opening  of  the  gland  which  secretes  it.  Behind  this 
gland  are  found — (1)  the  oral  sinus;  (2)  a  small  prominence,  the  pedal  swelling ; 

(3)  the  ciliated  lateral  pits, 

K.  one  on  each  side  ;   and  (4) 

furthest  back  of  all,  the 
ciliated  shield  or  patch. 
Between  the  mantle  and 
shell  the  embryonic  adduc- 
tor runs  across  from  the  one 
valve  to  the  other.  Besides 
these  are  only  found  a  few 
isolated  muscle  fibres,  and 
the  rudiment  'of  the  mid- 
gut,  the  latter  as  an  epi- 
thelial vesicle,  which  be- 
comes entirely  separated 
from  the  ectoderm,  and  in 
no  way  communicates  with 
the  exterior. 

The  embryo  at  this  stage 
leaves  the  gills,  at  the  same 
time  emerging  from  the  egg 
shell.  Its  adhesive  filament 
floats  in  the  water.  If  a 

/^B  passing  fish  comes  in  contact 

.    'L  '    /  '  "<\  %  with  such  an  embryo,   the 

\  latter   can,    by   closing   its 

shell,  attach  itself  by  means 
of  the  triangular  processes 
mentioned  above,  to  its  in- 
tegument, into  which  the 
spines  on  these  processes 
•5  penetrate.  The  embryo  of 

Fio.  220.-Glochidium  larva  of  Anodonta,  from  the  outer  leaf  ^iwdanta  attaches  itself 
of  the  gill  of  a  female.  A,  from  below,  the  shell  being  open  chiefly  to  the  fins,  that  of 
(after  Schierholz).  B,  in  optical  transverse  section  (after  Unio  to  the  gills  of  the  fish. 
Flamming).  1,  Sensory  setae ;  2,  adhesive  filament ;  3,  shell-  The  epithelium  of  the  part 
process  ;  4,  false  mantle  ;  5,  lateral  pits  ;  6,  oral  sinus  ;  7,  pedal  f  th  fi  i  attacted  ffrows 
swelling ;  8,  ciliated  patch  ;  9,  embryonic  adductor ;  10,  shell. 

very  rapidly  in  such  a  way 

as  in  a  few  hours  to  surround  the  parasite  completely.  The  embryonic  false  mantle 
grows  out  from  each  valve  of  the  shell  as  a  fungus-like  body  to  penetrate  the  tissues 
of  the  host,  and  probably  serves  for  nourishing  the  embryo.  During  this  endo- 
parasitic  life,  which  lasts  for  several  weeks,  the  transformation  of  the  embryo  into 
the  young  Mussel  is  completed.  In  the  course  of  this  process  of  transformation 
some  larval  organs  are  resorbed,  and  also  serve  for  nutrition  ;  first  the  sensory  cells 
disappear  in  this  way,  then  the  gland  of  the  adhesive  filament  with  the  remains  of 
the  filament  itself,  then  the  adductor,  and  finally  the  false  mantle.  The  rudiment 
of  the  definitive  mantle  and  shell  then  appear.  The  vesicular  mid-gut  joins  the  oral 
sinus ;  the  pedal  swelling  grows  into  the  linguiform  foot,  and,  in  this,  the  rudi- 
mentary byssus  gland  appears  as  an  ingrowth  of  the  epithelium.  The  rudiments 
of  the  inner  branchial  leaves,  the  digestive  gland,  the  nephridium,  the  heart,  the 


//  -  \  —^r 

;          •/ 


vii  MOLLUSC  A— ONTOGENY  265 

cerebral,  pedal,  and  visceral  ganglia,  and  the  auditory  vesicle  appear  during  the 
parasitic  stage,  in  the  same  way  as  in  other  Lamellibranchs. 

During  the  last  week  of  parasitic  life  the  capsule  formed  by  a  growth  of  the 
tissue  of  the  host  which  surrounds  the  embryo  becomes  thinner  ;  the  parasite  breaks 
through  it,  and  falls  to  the  bottom  of  the  water  as  a  young  Mussel.  The  only 
organs  still  wanting  are  the  genital  organs,  the  outer  leaves  of  the  gills,  and  the  oral 
lobes. 

E.  Cephalopoda. 

Tetrabranchia. — Xothiug  is  known  of  the  development  of  NanMlus. 

Dibranchia. — The  egg  is  usually  very  large,  and  contains,  like  that  of  the  sharks, 
reptiles,  and  birds,  a  great  quantity  of  nutritive  yolk.  It  belongs  to  the  telolecithal 
meroblastic  type,  and  is  enclosed  in  a  capsule.  A  number  of  such  capsules  may 
become  cemented  together  to  form  strings.  The  partial  segmentation  takes  place 
at  the  animal  pole  of  the  egg,  and  leads  to  the  formation  at  that  point  of  a  germinal 
disc  (blastoderm). 

Ontogeny  of  Sepia. — The  blastoderm  grows  so  very  slowly  round  the  yolk,  that 
long  after  all  the  outer  organs  of  the  embryo  are  quite  recognisable  in  the  region 
of  the  original  germinal  disc,  the  opposite  pole  is  still  occupied  by  the  yolk.  The 
germ  lies  in  such  a  way  that  the  centre  of  the  germinal  disc  or  animal  pole  is  placed 
dorsally,  and  corresponds  with  the  uppermost  point  of  the  visceral  dome  of  the  adult 
animal,  while  the  mass  of  nutritive  yolk  lies  ventrally. 

1st  Stage  (Fig.  221  A). — In  the  centre  of  the  germinal  disc  there  appears  an 
oval-rhombic  bulging  ;  this  is  the  rudiment  of  the  visceral  dome  and  the  mantle. 
On  each  side  of  this  there  arises  a  bean-shaped  prominence,  the  rudiment  of  the  eye. 
Behind  the  eye,  on  each  side,  a  long  narrow  ridge  runs  backward  in  a  <;urve  ;  about 
half  way  down  this  ridge  a  small  prominence,  the  rudiment  of  the  funnel  cartilage, 
forms  close  to  its  outer  side.  The  part  of  the  ridge  lying  in  front  of  this  prominence 
becomes  the  muscle  which  runs  from  the  funnel  to  the  nuchal  cartilage;  the 
posterior  part  (which  lies  behind  the  rudiment  of  the  visceral  dome  and  mantle) 
forms  the  paired  rudiment  of  the  funnel  itself.  Between  the  two  rudiments  of 
the  funnel  two  other  prominences  rise  symmetrically — the  rudiments  of  the  gills. 
A  pit  in  the  centre  of  the  rudiment  of  the  visceral  dome  has  been  indicated  as  the 
rudiment  of  a  shell  gland  (?). 

2nd  Stage  (Figs.  221  B  and  222  A). — The  rudiments  just  described  stand  out 
more  prominently.  On  the  outer  and  posterior  sides  of  the  rudiments  of  the  funnel 
the  rudiments  of  the  two  posterior  pairs  of  arms  first  appear  as  prominences,  then 
those  of  the  third  and  fourth  pairs.  The  first  indications  of  the  head  are  seen  in 
the  form  of  a  large  double  swelling  on  each  side,  the  outer  and  anterior  part  of 
which  carries  on  each  side  the  rudiment  of  the  eye.  The  embryo  becomes  covered 
with  cilia.  At  the  extreme  anterior  end  the  mouth  appears  in  the  middle  line, 
forming  the  opening  of  the  oesophagus,  which  begins  to  sink  inwards. 

3rd  Stage  (Fig.  221  C). — The  whole  embryo  has  become  more  arched  dorsally, 
and  more  marked  off  from  the  yolk.  On  the  latter,  the  blastoderm,  which  consists 
of  two  layers,  an  external  ectoderm  and  an  internal  yolk  membrane,  has  spread  out 
further  towards  the  ventral  (vegetative)  pole  of  the  egg.  At  the  posterior  edge  of 
the  rudiment  of  the  visceral  dome,  the  mantle  fold  has  grown  out  forward  in  such  a 
way  as  to  form  a  small  mantle  cavity,  which  already  partly  covers  the  rudiments  of 
the  gills.  In  the  space  between  the  rudiments  of  the  funnel  and  the  gills  the 
proctodseum  has  formed  by  iuvagination,  and  its  aperture,  the  anus,  can  be  seen. 
The  rudiment  of  the  fifth  pair  of  arms  appears. 

4th  Stage  (Figs.  221  D  and  222  F,  G).— The  visceral  dome  projects  further, 


266 


COMPARATIVE  ANATOMY 


CHAP. 


and  has  a  free  mantle  edge  all  round  its  base.  The  gills  have  shifted  further 
into  the  mantle  cavity,  which  is  now  larger,  and  lies  posteriorly.  The  rudiments 
of  the  funnel  also  now  lie  close  to  the  mantle,  and  are  so  approximated  pos- 
teriorly as  nearly  to  touch.  The  rudiments  of  the  arms  have  shifted  from 
behind  further  forward  round  the  rudiments  of  the  head.  As  the  whole  embryo 
projects  more  distinctly  from  the  yolk,  the  rudiments  of  the  arms  shift  nearer  to 


FIG.  221.— Ontogeny  of  Sepia  (after  Koelliker).  A-E,  Five  stages  of  development.  The  free 
surface  of  the  germinal  disc  which  lies  on  the  yolk  is  seen,  its  centre  corresponding  with  the  dorsal 
point  of  the  visceral  dome  of  the  adult  Sepia.  The  anterior  side  of  the  embryo  lies  lowest  in  the 
figures,  a,  Visceral  dome  with  mantle  ;  b,  rudiment  of  eye  ;  c,  rudiment  of  gill ;  d,  halves  of  the 
funnel ;  e,  rudiment  of  the  funnel  cartilage  belonging  to  the  apparatus  for  closing  the  mantle  ; 
/,  peripheral  part  of  the  blastoderm,  which,  growing  all  round  the  yolk,  forms  the  yolk-sac  ; 
g,  mouth  ;  It,,  posterior  cephalic  lobe  ;  i,  anterior  cephalic  lobe ;  A',  anus ;  5,  anterior  or  first  pair 
of  arms  ;  4,  3,  2,  1,  second,  third,  fourth,  and  posterior  pairs  of  arms. 

one  another  and  under  the  rudiments  of  the  head.  The  anus  is  already  covered  by 
the  mantle  fold. 

5th  Stage  (Figs.  221  E  and  222  B,  H).— The  arms  shift  still  nearer  to  one 
another  (i.e.  towards  the  axis  of  the  embryo),  grouping  below  the  rudiments  of  the 
head  (which  have  become  fused),  and  form  a  somewhat  narrow  circle  on  the  ventral 
side  in  such  a  way  that,  when  the  embryo  is  seen  from  the  dorsal  side,  some  of  them 
are  hidden  by  the  head.  As  a  consequence  of  this  the  embryo,  which  is  already 
recognisable  as  a  young  Sepia,  now  becomes  sharply  constricted  from  the  yolk 
beneath  it.  The  free  edges  of  the  rudiments  of  the  funnel  fuse  and  move  to  a  position 
within  the  mantle  cavity. 

6th  Stage  (Fig.  222  C). — The  rudiments  of  the  head  and  arms  have  now 
assumed  the  typical  position  to  form  the  "head"  (Kopffuss).  The  embryo  is  now 
altogether  distinct  from  the  yolk,  to  which  it  merely  hangs  instead  of,  as  before, 
lying  upon  it.  The  blastoderm  finally  grows  round  the  yolk  and  so  forms  a  yolk 
sac.  At  first  this  sac  is  four  or  five  times  the  size  of  the  embryo,  but  in  proportion 


VII 


MOLL  USO A— ONTOGENY 


267 


FIG.  222.— Various  stages  in  the  development  of  Sepia  (after  Koelliker).  A,  B,  C,  D, 
Anterior  view ;  E  and  F,  from  the  left  side  ;  G  and  H,  from  behind.  Lettering  as  in  Fig.  221.  In 
addition :  d,  rudiment  of  the  funnel-nuchal  muscle  (collaris) ;  d],  paired  rudiment  of  the  funnel 
proper ;  p,  yolk  ;  ai,  edge  of  the  mantle  ;  t ,  optic  invagination  (?) ;  «,  region  of  the  shell ;  q,  edges 
of  the  two  rudiments  of  the  funnel  bent  round  ;  r,  fins.  In  G  the  mantle  fold  is  raised  up  in  H 
cut  off. 


268  COMPARATIVE  ANATOMY  CHAP. 

as  the  latter  grows  at  the  expense  of  the  yolk  and  develops  further,  the  sac  becomes 
smaller,  so  that  when  the  embryo  is  hatched  the  size  of  the  yolk-sac  is  only  one  third  of 
that  of  the  young  animal  (Fig.  222  D).  It  must  further  be  mentioned  with  regard  to 
the  yolk  sac  that  it  is  at  no  time  in  communication  with  the  intestine.  As  the 
embryo  becomes  constricted  from  the  yolk  the  latter  divides  into  two  parts — an  inner 
part,  lying  inside  the  embryo,  and  an  outer  part,  filling  the  sac.  These  two  parts 
are  connected  by  means  of  the  stalk  of  the  yolk  sac,  which  projects  downward  from 
the  "head."  The  yolk  within  the  embryo  is  divided  into  three  unequal  parts,  the 
largest  of  which  fills  the  visceral  dome  ;  another  mass  of  considerable  size  fills  the 
"head,"  and  these  two  masses  are  connected  with  a  smaller  portion  lying  in  the 
nuchal  region. 

Loligo  and  Argonauta  have  a  smaller  yolk  sac,  round  which  the  blastopore 
grows  at  an  earlier  stage  than  in  Sepia.  The  yolk  sac  of  Argonauta  is  entirely 
taken  into  the  body  before  the  latter  has  completely  closed  ventrally. 

The  quantity  of  nutritive  yolk  is  still  less  in  a  Cephalopod  (Ommatostrephes .?), 
the  spawn  of  which  floats  in  the  sea.  Segmentation  is  in  this  case  also  partial  and 
discoidal,  but  the  blastoderm  almost  completely  encloses  the  yolk  before  any  organ 
develops  on  the  germ,  and  no  external  yolk  sac  is  formed. 

The  results  of  the  investigations  hitherto  made  with  regard  to  the  germinal 
layers,  the  development  of  the  inner  organs,  and  the  inner  differentiations  of  the 
outwardly  visible  organs  are  so  contradictory  and  in  many  cases  so  incomplete, 
that  no  description  of  them  is  here  attempted.  Further  investigation  is  much 
needed.  The  development  of  the  eye  has  already  been  described  (p.  171),  and  that 
of  the  hind-gut  and  ink-bag  was  illustrated  (p.  197). 

Two  important  facts  in  the  ontogeny  of  the  Dibranchia  should  be  noted.  (1) 
In  considering  the  arms  as  parts  of  the  foot,  it  is  important  to  notice  that  they 
arise  behind  the  rudiments  of  the  head,  and  only  secondarily  come  to  lie  round  and 
below  the  latter.  The  mouth,  at  quite  a  late  stage,  lies  at  the  anterior  end  of  the 
circle  of  arms  (Fig.  222  C).  (2)  The  funnel  consists  of  two  separate  lateral  rudi- 
ments, the  free  edges  of  which  fuse  secondarily.  This  point  is  important  in  connec- 
tion with  the  separation  of  the  two  lobes  of  the  funnel,  which  lasts  throughout  life 
in  Nautilus.  For  the  view  of  the  funnel  as  epipodium,  cf.  p.  116. 

The  fact  that  the  velum  is  wanting  in  the  Cephalopod  embryo  must  also  be 
noted.  The  absence  of  this  organ  is  explained  by  the  direct  development  of  the 
Cephalopoda  within  the  egg  capsule  at  the  expense  of  a  large  quantity  of  nutritive 
yolk. 

Investigations  as  to  the  development  of  the  shell,  and  as  to  the  nature  of  the 
organ  which  has  been  called  the  shell  gland,  are  much  needed. 


XXIV.  Phylogeny. 

No  actual  points  of  connection  between  the  Molluscan  phylum  and  any  other 
division  of  the  animal  kingdom  have  as  yet  been  found  ;  the  origin  of  the  Mollusca 
is  therefore  merely  a  matter  of  speculation.  The  present  writer  favours  the  view 
that  the  Mollusca  descended  from  animals  like  the  Turbellaria,  which  had  become 
differentiated  from  the  modern  Platodes  by  the  acquisition  of  a  hind-gut  and  a  heart, 
and  the  (at  least  partial)  transformation  of  the  genital  cavity  into  a  secondary 
and  primitively  paired  body  cavity.  There  is  a  striking  agreement  in  the  nervous 
system  of  the  lower  Molluscs  (Chiton,  Solenogastres,  and  in  some  respects  the  Dioto- 
cardia)  and  that  of  the  Platodes ;  in  both  there  is  a  ladder-like  nervous  system 
with  the  principal  trunks  beset  along  their  whole  length  with  ganglion  cells  ;  the 


vii  MOLLUSCA— LITERATURE  269 

pleurovisceral  cords  answer  to  the  lateral  trunks  of  the  Platodes,  and  the  pedal 
cords  to  the  ventral  longitudinal  trunks  of  the  latter.  If  such  a  hypothetical  racial 
form  were  to  secrete  a  dorsal  shell,  perhaps  at  first  in  the  form  of  a  thick  cuticle 
containing  calcareous  particles,  a  typical  Molluscan  organisation  would  be  produced. 
The  development  of  a  shell  would  deprive  the  greater  part  of  the  surface  of  the  body 
of  its  original  respiratory  function,  and  would  lead  to  the  formation  of  localised 
gills.  By  means  of  the  development  of  a  mantle  fold  these  delicate-skinned  organs 
could  be  brought  under  the  protection  of  the  shell.  The  musculature  on  the  dorsal 
side,  which  the  shell  covered,  would  disappear,  and  with  it  the  dorsal  longitudinal 
nerve  trunks.  The  musculature  on  the  ventral  side,  which  was  already  strongly 
developed  in  the  Planaria,  would  become  strengthened  in  the  development  of  the 
foot  with  its  sole  for  creeping.  A  part  of  the  dorsoventral  musculature  would  be 
changed  into  the  shell  muscle. 

In  this  derivation  of  the  Mollusca  their  characteristic  larval  form  might  be 
explained,  without  any  need  for  tracing  it  to  the  Annelidan  Trocophora,  in  the 
following  way.  It  would  correspond  to  a  Turbellarian  larva  (Miiller's  Polyclade 
larva,  etc. ),  on  to  which  certain  Molluscan  characteristics  such  as  the  shell  gland, 
the  shell,  the  anus,  and  the  foot  had  been  shifted  back.  The  preoral  ciliated  band 
(the  velum)  of  the  Molluscan  larva  would  correspond  with  the  same  structure  in 
the  Turbellarian  larva.  The  primitive  kidney  of  the  former  would  answer  to  a 
simplified  water  vascular  system,  while  the  permanent  nephridia  as  ovarial  and 
seminal  ducts  might  be  homologised  morphologically  with  the  ducts  of  the 
genital  products  in  the  Turbellaria. 


Review  of  the  most  important  Literature. 

Comprehensive  Works.     Text-Books.     General  jWorks.     Investigations 
treating  of  all  or  several  Classes. 

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1862-1866.     (New  edition  now  appearing,  v.  Simroth.) 
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1817. 
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Histoirc  ii.<:itiirclle  des  Mollusques  (Exploration  de  I'Algerie).     1848. 

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toire  naturelle  des  Mollusques  vivants  etfossiles.     2  vols.     Paris,  1887. 
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Mollusken.     Leipzig,  1877. 

Keber.     Beitrdge  zur  Anatomie  und  Physiologic  der  Weichthiere.     Kb'nigsberg,  1851. 
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1883. 

R.  Leuckart.     Zoologische  Untersuchungen.     Heft  3.     Giessen,  1854. 
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H.  Simroth.  Ueber  einige  Tagesfrayen  der  Malacozoologie,  hauptsdchlich  Convcfgen- 
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The  early  parts  of  the  new  edition  of  vol.  iii.  of  Bronn's  Klassen  und 

Ordnungen  des  Thierreichs. 

J.  Thiele.  Die  Stammesverwandschaft  der  Mollusken.  Em  bcitrag  zur  Phylogenie  der 
Thiere.  Jenaische  Zeitschr.  f.  Naturwissensch.  25  Bd.  1891. 

S.  P.  Woodward.     A  Manual  of  the  Mollusca.     4th  edit.     1880. 


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Axel  Wiren.     Studien  uber  die  Solenogastres.     I.  Monographic  des  Chcctoderma  niti- 
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golfe  de  Marseilles.     Part  I.   Tectibranches.     Ann.  Mus.  Hist.   N.  Marseilles. 
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Nicolas  Wagner.     Die  wirbellosen  Thiere  des  weissen  Meeres.     1  Bd.     Leipzig.     Fol. 

1885. 
H.  Wegmann.     Contribution  a  I'histoire  naturelle  des  Haliotides.     Arch.  Zool.  cxpfr. 

(2).     Tome  II.     1884. 

Note  sur  I 'organisation  de  la  Patella  vulgata  L.     Recueil.  Z.  Suisse.     Tome 

IV.     1887. 

Emile  Yung.  Contributions  a  I'histoire  physiologiqiLc  de  Vescargot  (Helix  pomatia}. 
M6m.  Cour.  Acad.  Belg.  Tome  XLIX.  1887. 

Scaphopoda. 

Herm.  Fol.  Sur  r anatomic  microscopique  du  Dentale.  4  PI.  Arch.  Zool.  expir.  (2). 
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vn  MOLLUSC  A— LITERATURE  273 

H.  de  Lacaze-Duthiers.     Histoire  de  V organisation  et  du  dtveloppement  du  Dentale. 

Ann.  des  Sciences  Nat.  (4).     Tomes  VI.,  VII.,  and  VIII.     1856-57-58. 
L.  Plate.     Bemerkungen  zur  Organisation  der  Dentalien.     Z.  Anzeiger.     11  Jahrg. 

1888.      Ueber  das  Herz  der  Dentalien.     Ibid.     14  Jahrg.     1891. 
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Lamellibranchia. 

Ernst  Egger.      Jouannetia  Cumingi  Son.      Eine  morphol.    Untersuchung.      Arbeit. 

Zool.  Instit.   Wurzburg.         8  Bd.     1887. 
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London.     Vol.  II.     1841. 
H.  de  Lacaze-Duthiers.     Mtmoire  sur  V organisation  de  I'Anomie.     Ann.  des  Sciences 

Nat.  (4).     Tome  II.     1854. 

Morphologic  des  Acephales.     1  Mem.     Anatomie  de  VArrosoir  (Aspergillum 

dichotomum).     Arch.  Zool.  exper.  (2).     Tome  1.     1883. 

Leydig.     Anatomie  und  Entwickelung  von  Cyclas.     Mutter's  Archiv.     1835. 

H.  A.  Meyer  and  Moebius.     Fauna  der  Kieler  Bucht.     Leipzig,  1865. 

Paul  Pelseneer.     Report  on  the  anatomy  of  the  deep-sea  Mollusca  collected  by  H.M.S. 

"Challenger."     Report  Chall.  Zool.     Part  74.     1888. 
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1891. 
A.  de  Quatrefages.     Memoire  sur  le  genre  Taret.     Ann.  des  Sciences  Nat.  (3).     Tome 

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Cephalopoda. 

A.  G.  Bourne.     The  differences  between  the  males  and  females  of  the  pearly  Nautilus. 

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J.  Brock.     Studien  uber  die  Verwandtschaftsverhdltnisse  der  dibranchiaten  Cephalo- 
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Versuch  einer  Phylogenie  der  dibranchiaten  Cephalopoden.     Morph.  Jahrbuch. 

6  Bd.     1880. 

Zur  Anatomie  und  Systematik  der  Cephalopoden.     Zeitschr.  f.  wiss.  Zool.     36 

Bd.     1882. 

Delle  Chiaje.     Memorie  su'  Cefalopodi.     Memorie  sulla  storia  e  notomia  degli  ani- 

mali  senza  vertebre  del  regno  di  Napoli.     Napoli,  1829. 
Ferussac  and  d'Orbigny.     Histoire  naturelle  gtnerale  et  particuliere  des  Cephalopodes 

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Leon  Fredericq.     Recherches  sur  la  physiologic  du  Poulpe  commun  (Octopus  wdgaris}. 

Arch.  Zool.  exper.     Tome  VII.     1879. 
Carl  Grobben.     Zur  Kenntniss  der  Morphologic  und  Verwandtschaftsverhdltnisse  der 

Cephalopoden.     Arb.  Z.  Inst.  Wien.     7  Bd.     1886. 

B.  Haller.     Beitrdge  zur  Kenntniss  der  Morphologic  Nautilus  Pompilius.     Zool.  d. 

Semoiis  Forschungsreise  in  Australian.     Vol.  V.     1895. 
H.  von  Jhering.     Ueber  die  Verwandtschaftsbeziehungen  der  Cephalopoden.     Zeitschr. 

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Van  der  Hoeven.     Beitrag  zur  Kenntniss  von  Nautilus.     Amsterdam,  1856. 
Will.  E.  Hoyle.     Observations  on  the  anatomy  of  a  rare  Ceplialopod  (Gonatus  Fabricii). 

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H.  Muller.     Ueber  das  Mdnnchen  von  Argonauta  argo  und  die  Hectocotylen.     Zeitschr. 

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VOL.  II  T 


274  COMPARATIVE  ANATOMY  CHAP. 

R.  Owen.    Description  of  some  new  and  rare  Cephalopoda.     Trans.  Zool.  Soc.  London. 

Vol.  II.     1841. 

Cephalopoda.     Todd's  Cyclopcedia,  etc.     Vol.  I.     London,  1836. 

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E.  Ehrenbaum.     Untersuchungen  uber  die  Structur  und  Bildung  der  Schale  der  in 

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G.   Steinmann.      Vorlaufige  Mittheilung   uber   die   Organisation  der  Ammonitcn. 

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G.  Steinmann  and  L.  Doderlein.     Elemente  der  Paldontologie.     Leipzig,  1890. 
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vii  MOLLUSGA— LITERATURE  275 


Musculature,  Foot,  Pedal  Glands,  the  Taking  in  of  Water. 

Th.  Barrois.     Les  glandes  du  pied  et  les  pores  aquiferes  des  Lamellibranches.     Lille, 

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J.  Carriere.     Die  Drilsen  im  Fusse  der  Lamellibranchiaten.     Arbeit,  aits  d.  zool. 

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7  Bd.     1887. 
A.  Fleischmann.    Die  Bewegung  des  Fusses  der  Lamellibranchiaten.    Zeitschr.  f.  wiss. 

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J.  H.  List.    Zur  Kenntniss  der  Drusen  im  Fusse  von  Tethys  fimbriata  L.     Zeitschr.  f. 

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Paul  Pelseneer.     Sur  la  valeur  morphologique  des  bras  et  la  composition  du  systeme 

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L.   Bohmig.     Beitrdge  zur  Kenntniss  des  Centralnervensystems  einiger  pulmonaten 

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276  COMPARATIVE  ANATOMY  CHAP. 

B.  Haller.     Zur  Kenntniss  der  Muriciden.      Eine  vergl.-anat.  Studie.     I.    Theil. 

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H.  von  Jhering.      Vergleichende  Anatomie  des  Nervensy  stems  und  Phylogenie  der 

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C.  Semper.      Ueber  Sehorgane  vom  Typus  der  Wirbelthieraugen.     Wiesbaden,  1877. 
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1881. 


278  COMPARATIVE  ANATOMY  CHAP. 

Respiratory  Organs,  Circulatory  System. 

Felix  Bernard.     Recherches  sur  les  organes  palUaux  des  Gastropodes  prosdbranches. 

These.     Paris,  1890. 
Bojanus.     Ueber  die  Athem-  und  Kreislaufswerkszeuge  der  zweischaligen  Muscheln. 

Isis,  1817,  1820,  1827. 
L.  Cuenot.     Etudes  sur  le  sang  et  les  glandes  lymphatiques  dans  la  s6rie  animate. 

2  Partie.     Invertebres.     Arch,  de  Zool.  experim  (2).     Vol.  IX.     189]. 
Carl  Grobben.     Ueber  den  bulbus  arteriosus  und  die  Aortenklappen  der  Lamelli- 

branchiaten.     Arbeiten  a.  d.  Zoologischen  Institute  der  Universitdt  Wien.     9  Bd. 

1891. 
W.  A.  Herdmann.     On  the  structure  and  function  of  the  cerata  or  dorsal  papillae  in 

some  Nudibranchiate  Mollusca.     Quart.  Journ.  Microsc.  Science.     Vol.  XXXI. 

Part  I.     1891. 
L.  Joubin.     Structure  et  dtveloppement  de  la  branchie  de  quelques  Ctphalopodes  des 

cdtes  de  France.     Arch,  de  Zool  experim.  (2).     Vol.  III.     1885. 
Langer.     Ueber  das  Gef ass-system  der  Teichmuschel.     Denkschriften  der  Wiener  Aka 

demie.     1855  and  1856. 
A.  Menagaux.     Recherches  sur  la  circulation  des  Lamellibranches  marins.     Besanqon, 

1890. 
K.  Mitsukuri.     On  the  structure  and  significance  of  some  aberrant  forms  of  Lamelli- 

branchiate  gills.     Quart.  Journ.  Microsc.  Science.     N.  S.  21.     1881. 
H.  L.  Osborn.     On  the  gill  in  some  forms  of  prosobranchiate  Mollusca,.     Stud.  biol. 

labor.  J.  Hopkins  Univ.     Vol.  III.     1884. 
R.  Holman  Peck.     The  structure  of  the  Lamcllibranchiate  gill.     Quart.  Journ.  Micr. 

Science.     Vol.  XVII,     1877. 
C.  Posner.     Ueber  der  Bau  der  Najadenkieme.      Arch.  f.  mikrosk.  Anat.      Bd. 

XI.     1875. 


Secondary  Body  Cavity,  Nephridia,  Genital  Organs. 

Baudelot.     Recherches  sur  I'appareil  gener.  des  Mollusques  gastropodes.     Ann.  Sci. 

Nat.  (4).     Tome  XIX.     1862. 
Th.  Behme.     Beitrdge  zur  Anatomie  und  Entivickelungsgeschichte  des  Harnapparates 

der  Lungenschnecken.     Arch.  Naturg.  Jahrg.  55.     1889. 
J.  Brock.     Ueber  die  Geschlechtsorgane  der  Cepho.lopoden.     Erster  Beitrag.  Zeitschr. 

f.  wiss.  Zool.     32  Bd.     1879. 
J.  T.  Cuningham.     The  renal  organs  (nephridia}  of  Patella.     Quart.  Journ.  Micr. 

Science.     Vol.  XXIII.     1883. 

Note  on  the  structure  and  relations  of  the  kidney  in  Aplysia.     Mith.  Zool. 

Station  in  Neapel.     4  Bd.     1883. 

R.  von  Erlanger.  On  the  paired  Nephridia  of  the  Prosobranchs,  the  homologies  of 
the  only  remaining  nephridium  of  most  Prosobranchs,  and  the  relations  of  the 
nephridia  to  the  gonad  and  genital  duct.  Quart.  Journ.  Micr.  Science.  Vol. 
XXXIII.  1892. 

C.  Grobben.  Morpholog.  Studien  uber  den  Harn-  und  Geschlechtsapparat,  sowie  die 
Leibeshohle  der  Cephalopoden.  Arb.  Zool.  Inst.  Wien.  5  Bd.  1884. 

Ueber  die  pericardialdruse  der  Lamellibranchiaten.     Ein  Beitrag  zur  Kennt- 

niss  der  Anatomie  dieser  Molluskenklassen.     Arb.    Zool.    Inst.    Wien.     7  Bd. 
1888. 

Die  pericardialdriisen  der  Gastropoden.     Arbeit.  Zool.  Inst.  der  Univ.   Wien. 

9  Bd.     1890. 


MOLL  USCA—LITERA  TURE 


279 


A.  C.  Haddon.     On  the  generative  and  urinary  ducts  in  Chiton.     Proceed.  Royal 

Dublin  Soc.  (2).     Vol.  IV.     1885. 

B.  Haller.     Beitrage  zur  Kenntniss  der  Niere  der  Prosobranchier.      Morph.  Jahrb. 

11  Bd.     1885. 
A.  Hancock.     On  the  structure  and  homologies  of  the  renal  organ  in  the  Molluscs. 

Trans,  of  the  Linn.  Soc.     Vol.  XXIV. 
P.  P.  C.  Hoek.     Les  organes  de  la  generation  de  I'huttre.     Tijdschr.  Nederl.  Dierk. 

Vereen.     Suppl.  D.  1.     1883. 
H.  von  Jhering.     Ueber  den  uropneustischen  Apparat  der  Heliceen.     Zeitschr.  f.  uriss. 

Zool.     41  Bd.     1884. 
J.   Kollmann.      Ueber    Verbindungen  zwischen   Colom  und    Nephridien.      Baseler 

Festschrift  zum  Wurtzburger  Jubildum.     1882. 
A.  Kowalevsky.    Ein  Beitrag  zur  Kenntniss  der  Excretionsorgane.    Biol.  Centralblatt. 

9  Bd.     1889. 
E.  Ray  Lankester.     On  the  originally  bilateral  character  of  the  renal  organs  of 

Prosobranchia,  and  on  the  homologies  of  the  yolk  sac  of  Cephalopoda.     Ann.  of 

Nat.  Hist.  (5).     Vol.  VII.     1881. 
-     Observations  on  the  Pondsnail,  etc.     Quart.  Journ.  Micr.  Science.     Vol.  XIV. 

1874. 
E.  Ray  Lankester  and  A.  G.  Bourne.     On  the  existence  of  SpengeFs  olfactory  organ 

and   of  'paired  genital  ducts  in  the  pearly  Nautilus.     Quart.   Journ.   Micr. 

Science.     Vol.  XXIII.     1883. 
G.  F.  Mazarelli.     Intorno  all'  anatomia  delT  apparato  riproduttore  delle  Aplysice  del 

golfo  di  Napoli.     Z.  Anz.     12  Bd.     1889. 
—    Intorno  air  apparato  riproduttore  di  alcuni  Tectibranchi  (Pleurobranchcea, 

Oscanius,  Accra).     Zool.  Anz.     14  Jahrg.     1891. 
0.  Niisslin.     Beitrage  zur  Anatomie  und  Physiologic  der  Pulmonaten.      Habilita- 

tivnsschrift  (Carlsruhe}.     Tubingen,  1879. 
R.  Owen.     On  the  external  and  structural  characters  of  the  male  Spirula  australis. 

Proceed.  Zool.  Soc.  London.     1880. 
Remy  Perrier.     Rccherchts  sur  I'anatomie  et  Vhistologie  du  rein  des   Gastropodes 

prosobranches.     Annales  des  Sciences  Nat.  (7).     Tome  VIII.     1890. 
Walter  Rankin.     Ueber  das  Bojanus'sche  Organ  der  Teichmuschel  (Anodonta  cygnea 

Lam.}    Jenaische  Zeitschr.  fur  Naturwissensch.     24  Bd.     1890. 
A.  Schmidt.     Der  Geschlcctitsapparat  der  Stylommatophoren,   etc.      Abh.  des  Nat. 

Vcreinsfur  Sachsen  und  Thuringen.     1  Bd.     1885. 
P.  Stepanoff.     Ueber  Geschlechtsorgane  und  Entwickelung  von  Ancylus  jluviatilis. 

St.  Petersburg,  1886. 
W.  J.  Vigelius.     Bijdrage  tot  de  Kennis  van  het  excretorisch  Systeem  der  Cephalopoden. 

Acarl.  Proefschrift.     Leiden,  1879. 
--     Ueber  das  excretionssystem  der  Cephalopoden.      Niederl.  Arch.  f.  Zool.     5  Bd. 

1880. 

Parasitic  Gastropoda. 

Albert  Bauer.     Beitrage  zur  Naturgeschichte  der  Synapta. 

schnccke   in  der  Leibeshohle  der  Synapta  digitata. 

Lcop-Carol.     Tome  XXXI.     1864. 
Max  Braun.     Ueber  parasitische  Schnecken.     Zusammenfassender  Bericht 

tmlbl.  f.  Bakteriologie  u.  Parasitenkunde.     5  Bd.     1889. 
Johannes  Miiller.     Ueber  Synapta  digitata  und  die  Erzeugung  von  Schnecken  in 

Holothuricn.     Berlin,  1852. 
Paul  and  Fritz  Sarasin.    Ueber  zwei  parasitische  Schnecken.    Ergebn.  Naturw.  Forsch. 

auf  Ceylon  in  1884-1886.     1  Bd.     Wiesbaden,  1887. 


III.  Die  Eingeweide- 
Nova  Ada  Academ,  Cces. 

Cen- 


280  COMPARATIVE  ANATOMY  CHAP- 

P.  Schiemenz.      Parasitische  Schnecken.      Kritisches  Re/erat.      Biol.    Centralblatt. 

9  Bd.     1889-1890. 

Walter  Voigt.  Entocolax  Ludwigii,  ein  neuer  seltsamer  Parasit  aus  einer  Holothurie. 
Zeitschr.f.  wiss.  Zool.  47  Bd.  1888. 

Ontogeny. 

F.  Blochmann.     Ueber  die  Entwickelung  von  Neritina  Jluviatilis,  Mull.     Zeitschr. 

f.  wiss.  Zool.     36  Bd.     1881. 

Beitrage  zur  Kenntniss  der  Entwickelung  der  Gastropoden.     Zeitschr.  f.  wiss. 

Zool.     38  Bd.     1883. 

W.  K.  Brooks.     The  development  of  the  Squid  (Loligo  Pealii,  Lesueur).     Annivers. 

Mem.  Boston  Soc.  Nat.  Hist.     Boston,  1880. 
R.  von  Erlanger.     Zur  Entwickelung  von  Paludina  vivipara.     I.  and  II.     Mor- 

phologisches  Jahrbuch  von  Gegenbauer.     17  Bd.     1891. 
Hermann  Fol.     Etudes  sur  le  developpement  des  Mollusques.     I.  Sur  le  developpement 

des  Pteropodes.    Archives  de  Zool.    exptrim.     Tome   IV.     1875.     //.    Sur  le 

developpement  embryonnaire  et  larvaire  des  Heteropodes.     Tome  V.     1876.     ///. 

Sur  le  developpement  des  Gastropodes  pulmone.     Tome  VIII.     1879-1880. 
H.  Grenacher.     Zur  Entwickelungsgeschichte  der  Cephalopoden,  zugleich  ein  Beitrag 

zur  Morphologic  der  hdheren  Mollusken.     Zeitschr.f.  wiss.  Zool.     24  Bd.     1874. 

A.  C.  Haddon.     Notes  on  the  development  of  Mollusca.     Quart.  Journ.  Micr.  Science. 

Vol.  XXII.     1882. 

B.  Hatschek.     Ueber  Entwickelungsgeschichte  von  Teredo.     Arb.  a.  d.  Zool.  Instil. 

Universitdt  Wien.     Tome  III.     Heft  1.     1880. 

E.  Horst.  Embryogenie  de  Vhuitre.  Tijdschr.  Nederl.  Dierk.  Ver.  Suppl.  Decl.  1. 
1884. 

Development  of  the   European   Oyster.     Quart.   Journ.  Micr.  Science.     Vol. 

XXII.     1882. 

A.  Kolliker.     Entwickelungsgeschichte  der  Cephalopoden.     Zurich,  1884. 

A.  Kowalevsky.     Etude  sur  I' embryogenie  du  Dentale.     Annales  du  Musee  d'histoire 

naturelle  de  Marseilles.     Zoologie.     Tome  I.     1883. 
Embryogenie  du   Chiton    Polii    (Philippi)   avec    quelques  remarques  sur  le 

developpement  des  autres  Chitons.     Ann.  Mus.  N.  H.  Marseille.    Tome  I.    No.  5. 
A.  Krohn.     Beitrage  zur  Entwickelungsgeschichte  der  Pteropoden  und  Heteropoden. 

Leipzig,  1860. 
E.  Ray  Lankester.     On  the  developmental  history  of  the  Mollusca.     Philos.  Transact. 

London.     1875. 

Observations  on  the  development  of  the  Cephalopoda.     Quart.  Jour.  Micr.  Science. 

Vol.  XV.     N.S.     1875. 

S.  Loven.  Beitrage  zur  Kenntniss  der  Mollusca  acephala  lamellibranchiata.  Stock- 
holm, 1879. 

J.  Playfair  MacMurrich.  A  contribution  to  the  embryology  of  the  prosobranch, 
Gastropods.  Stud.  Biol.  Lab.  J.  Hopkins  Univ.  Vol.  III.  1886. 

William  Patten.  The  embryology  of  Patella.  Arbeit.  Zool.  Inst.  Wien.  6  Bd. 
1885. 

G.  Pruvot.     Sur  le  developpement  d'un  Solenogastre.     Comptes  rend.    Paris.     Tome 

CXI.     1890. 

CarlRabl.     Ueber  die  Entwickelung  der  Teller schnecke.    Morph.Jahrb.    5  Bd.    1879. 
—    Die   Ontngenie  der  Susswasserpulmonaten.      Jenaische    Zeitschrift.      9    Bd. 
1875. 

Ueber  die  Entwickelungsgeschichte  der  Malermuschel.     Jenaische  Zeitschrift. 

10  Bd.     1876. 


vii  RHODOPE  VERANII  281 

W.  Salensky.     fitudes  sur  le  developpement  du  Vemnet.     Arch.  Biol.     Tome  VI. 

1887. 
Beitrage  zur  Entwiekelungsgeschichte  der  Prosobranchier.      Zeitschr.  f.  wiss. 

ZooL     22  Bd.     1872. 
P.  B.  Sarasin.     Entivickelungsgeschichte  der  BUhynia  tentaculata.     Arb.  Zool-Zoot. 

Instit.   Wurtzburg.     6  Bd.     1882. 
Paul  and  Fritz  Sarasin.      Aus  der  Entwiekelungsgeschichte  von  Helix    Waltonii. 

Ergtbn.  Nat.  Forsch.  Ceylon,  1884-1886.     1  Bd.     Wiesbaden,  1888. 
P.    Schiemenz.      Zusammenfassende    Darstellung   der   Beobachtungen    von    Eisig, 

Rouzaud,  Jourdain,  Brock,  etc.,  uber  die  Entwickelung  der  Genitalorgane  der 

Gastropoden.     Biol.  Centralblatt.     7  Bd.     1888. 
C.  Schierholz.     Ueber  Entwickelung  der  Unioniden.     Denkschr.  Akad.  Wien.     55 

Bd.     1888. 
F.  Schmidt.     Beit  rag  zur  Kenntniss  der  postenibryonalen  Entwickelung  der  Najaden. 

A  rch  i v.  fit  r  Nat urgesch ichte.    51  Jahrg.     1885. 
M.  Ussow.     Untersuchungen  uber  die  Entwickelung  der  Cephalopoden.     Arch.  Biol. 

Tome  II.     1881. 
L.  Vialleton.     Recherches  sur  les  premiere  phases  du  developpement  de  la  Seiche. 

Annal.  Sc.  Nat.  (7).     Tome  VI.     1888. 
Wladimir  Wolfson.     Die  embryonale  Entwickelung  des  Lymnceus  stagnalis.     Bullet. 

Acad.  Imp.  Sc.  St.  Petersbourg.     26  Jahrg.     1880. 
H.  E.  Ziegler.     Die  Entwickelung  von  Cyclas  cornea,  Lam.  Zeitschr.  f.  wiss.  Zool. 

41  Bd.     1885. 

Appendage. 

Rhodope  Veranii. 

This  small  animal  (circ.  4  mm.  in  length)  is  long  and  spindle-shaped,  and  out- 
wardly bilaterally  symmetrical.  The  body  epithelium  is  ciliated  all  over.  There 
is  a  dermo-muscular  tube,  inside  which,  embedded  in  the  connective  tissue  (paren- 
chyma), are  found  numerous  irregularly  shaped  calcareous  particles. 

Alimentary  Canal. — The  mouth  lies  at  the  anterior  end  of  this  canal,  and  leads 
into  a  wide  buccal  or  oesophageal  cavity,  into  the  first  part  of  which  two  acinose 
salivary  glands  open.  A  radula  and  jaws  are  wanting.  A  narrow  oesophagus  con- 
nects the  cesophageal  cavity  with  the  tube-like  mid-gut,  which  runs  through  the 
whole  length  of  the  body.  The  midgut  possesses  a  well-developed  muscular  wall,  and 
is  continued  anteriorly,  above  the  point  where  the  oesophagus  enters  it,  in  the  form 
of  a  diverticulum,  which  runs  forward  over  the  brain.  There  is  no  separate  digestive 
gland.  The  right  side  of  the  mid-gut  gives  rise  to  a  short,  thin,  ciliated  rectum, 
which  runs  through  the  posterior  third  of  the  body,  and  opens  through  the  anus  to 
the  right. 

The  nervous  system  consists  of  two  pairs  of  ganglia  lying  so  close  together  above 
the  oesophagus  as  almost  to  form  one  mass,  and  of  one  infra-cesophageal  ganglion, 
which  lies  somewhat  asymmetrically  to  the  left.  The  two  ganglia  of  each  of  the 
upper  pairs  are  connected  by  transverse  commissures,  and  the  posterior  dorsal  pair 
with  the  lower  ganglion  by  two  connectives  which  embrace  the  oesophagus.  Two 
lateral  nerves  which  run  backward  are  the  most  strongly  developed.  They  arise  out 
of  the  posterior  upper  pair  of  ganglia,  close  to  which  lie  a  pair  of  eyes  and  a  pair  of 
ciliated  auditory  vesicles,  each  of  the  latter  containing  an  otolith. 

Genital  Organs. — Rhodope  is  hermaphrodite.  The  gonads  consist  of  about  20 
follicles  which  lie  ventrally  in  the  median  and  posterior  thirds  of  the  body ;  the 
anterior  follicles  produce  eggs  and  the  posterior  spermatozoa.  The  ducts  of  all  the 


282  COMPARATIVE  ANATOMY  CHAP. 

follicles  are  said  to  unite  to  form  a  common  duct.  If  this  is  really  the  case,  then 
the  gonadial  follicles  together  form  a  hermaphrodite  gland.  The  hermaphrodite 
duct,  which  runs  forward,  is  said  to  divide  into  an  oviduct  and  a  vas  deferens. 
The  latter  leads  to  the  muscular  penis,  which  can  be  protruded  from  the  male  genital 
aperture  on  the  right  anteriorly.  With  the  oviduct  are  connected  a  receptacu- 
lum  seminis  and  a  gland  (albuminous  or  nidamental  gland).  The  female  genital 
aperture  is  said  to  lie  on  the  right  side,  behind,  and  distinct  from,  the  male 
aperture. 

A  differentiated  blood  vascular  system  has  not  been  found.  A  well-developed 
body  cavity  is,  however,  present,  filled  with  colourless  nutritive  fluid,  in  which  blood 
corpuscles  are  suspended. 

Special  respiratory  organs  are  wanting. 

The  nephridial  system  has  been  described  as  follows.  To  the  right,  in  front  of 
the  anus,  between  the  latter  and  the  genital  aperture,  lies  the  outer  nephridial 
aperture.  It  leads  through  a  short  ciliated  canal  into  a  spacious  renal  chamber, 
which  is  a  widening  of  a  longitudinal  canal.  The  renal  chamber  bulges  out  at 
several  points  to  form  short  caeca.  Into  this  chamber  nine  or  ten  small  flask-like 
organs  open  ;  these  resemble  the  excretory  ciliated  cells  of  the  Platodes,  inasmuch 
as  "flames"1  arise  at  the  base  of  each  flask,  the  neck  of  which  opens  into  the 
chamber. 

Development  is  direct.  At  no  stage  are  there  any  indications  of  a  shell  gland,  a 
shell,  or  a  foot. 

Systematic  Position. — Rhodope  is  by  some  classified  among  the  Turbellaria 
(near  the  Rhabdoccslidce),  by  others  among  the  Mollusca  (near  the  Nudibranchia], 
while  others  again  are  inclined  to  see  in  it  a  transition  form  between  these  two 
phyla. 

There  is  apparently  only  one  single  point  to  support  the  theory  of  the  relation  of 
Rhodope  to  the  Turbellaria,  viz.  the  presence  of  the  ciliated  excretory  cells  in  the 
nephridial  system.  On  the  other  hand,  the  derivation  of  the  nephridial  system  of 
Rhodope,  with  its  renal  chamber  and  aperture  to  the  right,  from  that  of  the  Nudi- 
branchia appears  far  more  probable  than  its  derivation  from  the  water  vascular 
system  of  the  Platodes.  The  presence  of  a  rectum  and  anus,  and  of  an  infra-ceso- 
phageal  ganglion  (pedal  ganglion),  is  difficult  to  reconcile  with  a  relationship  to  the 
Turbellaria.  The  occurrence  of  an  infra-oesophageal  commissure  in  one  isolated  case, 
that  of  Microstoma  lineare  (cf.  vol.  i.  p.  166),  is  hardly  a  convincing  argument.  The 
genital  apparatus  of  Rhodope  is  much  nearer  to  the  Nudibranchiate  than  to  the 
Turbellarian  type. 

There  are,  no  doubt,  serious  obstacles  in  the  way  of  those  who  seek  to  establish 
the  relationship  of  these  animals  with  the  Mollusca.  The  chief  of  these  is  the  want 
of  a  heart  and  the  entire  absence  of  a  shell  and  a  foot,  even  in  the  embryo.  The 
question  to  be  decided  is  whether  it  would  be  possible  for  a  Mollusc  which  had  lost 
foot,  gills,  and  shell  (e.g.  Phyllirhoe)  by  the  further  loss  of  the  heart,  so  far  to  depart 
from  the  typical  organisation  of  the  Mollusca,  that  these  organs  would  not  appear, 
even  temporarily,  in  the  course  of  development.  If  this  question  is  answered  in  the 
affirmative,  then  the  asymmetry  of  Rhodope,  and  especially  the  position  of  the 
genital,  nephridial,  and  anal  apertures  on  the  right  side,  which  entirely  agrees 
with  their  position  in  the  Nudibranchia,  affords  strong  support  to  its  claim  to  be 
related  with  tbe  Mollusca. 

The  view  that  Rhodope  is  a  transition  form  between  the  Turbellaria  and  the 
Mollusca  need  hardly  be  treated  seriously. 


Cf.  vol.  i.  p.  152,  where  flame  cells  are  described. 


VII 


RHODOPE   VERANII— LITERATURE 


Utfi 


Literature. 

L.  von  Graff.  Ueber  Rhodope  Veranii.  Koell.  (  =  Sidonia  elegans,  M.  Sehulze). 
Morph.  Jahrbuck.  8  Bd.  1883. 

A.  Koelliker.  Rhodope,  nuovo  genere  di  Gastropodi.  Giomdle  dell1  Istltuto  R.  Lom- 
bard o  di  scienze  e.c.  Tome  16.  Milano,  1847. 

S.  Trinchese.  Nuovo  osservazione  sulla  Rhodope  Veranii.  Koell.  Rendic.  delV 
A>:-:ad.  di  Napoli.  1887. 


CHAPTEE   VIII 
SEVENTH  EACE  OE  PHYLUM  OF  THE  ANIMAL  KINGDOM 

ECHINODERMATA. 

THE  Echinodermata  are,  as  a  rule,  essentially  radiate  in  structure. 
They,  however,  always  deviate  from  strict  radial  symmetry  in  minor 
points,  both  in  the  skeletal  system  and  in  the  arrangement  of 
the  inner  organs ;  sometimes  they  may  become  almost  bilaterally 
symmetrical.  The  Echinodermata  possess  a  skeleton  of  calcare- 
ous matter  deposited  in  the  deeper  connective  tissue  layers  of 
the  integument.  This  skeleton  is  in  texture  a  fine  rigid  sponge- 
work.  It  consists  either  of  microscopically  small  isolated  calcareous 
bodies  (Holothurioidea)  or  of  larger  plates  which  often  carry  spines,  and 
are  connected  together  either  movably  or  immovably  (other  Echino- 
derms).  The  ccelom  is  spacious.  There  is  a  blood  vascular  system. 
The  intestine,  which  is  provided  with  a  mouth  and  anus,  is  completely 
separated  from  the  coelom.  The  Echinodermata  possess  a  peculiar  sys- 
tem of  canals  or  tubes — the  water  vascular  system.  This  system,  on 
the  one  hand,  takes  in  water  from  the  exterior  through  a  stone  canal 
(sometimes  several  such  canals  are  present),  which  primitively  opens 
outwards,  and,  on  the  other  hand,  sends  out  terminal  canals  to  ex- 
ternal extensible  appendages  arranged  in  the  radii  or  ambulacra. 
These  are  the  ambulacral  feet  or  tentacles,  which  in  free  forms  serve 
principally  for  locomotion,  but  also  for  respiration ;  in  attached  forms, 
for  respiration,  and  also  perhaps  for  conducting  food.  The  sexes  are 
almost  always  separate.  [JDevelopment  is  accompanied  by  metamor- 
phosis. The  larvae  are  free-swimming  and  pelagic ;  they  are  bilater- 
ally  symmetrical,  with  ciliated  bands,  generally  produced  on  processes. 
The  Echinodermata  are  exclusively  marine^and  contain  a  great  number 
of  fossil  forms ;  certain  extinct  types  attained  a  great  development 
during  the  palaeozoic  age. 

The  race  of  the  Echinodermata  is  divided  into  five  classes — Holo- 
thurioidea, Eehinoidea,  Asteroidea,  Ophiuroidea,  and 


CHAP,  vin      ECHINODEEMATA—  SYSTEMATIC  REVIEW 


285 


Systematic  Review. 

CLASS  I.  Holothurioidea. 

The  body  is  elongated  along  its  principal  axis  ;  it  is  cylindrical  or  vermiform.  It 
shows  more  or  less  distinct  bilateral  symmetry.  The  integument  is  soft  or  leathery, 
and  contains  irregularly  arranged,  generally  microscopically  small,  calcareous  bodies. 
The  mouth  lies  at  the  oral  (anterior)  end  of  the  principal  axis  of  the  body,  and  is 
surrounded  by  feelers.  The  anus  lies  at  the  apical  (posterior)  end  of  the  principal 
axis.  Ambulacral  or  tube-feet  are  either  present  or  wanting.  An  external  madre- 
porite  is  usually  not  found. 

ORDER  1.  Actinopoda. 

All  the  outer  appendages  of  the  water  vascular  system  arise  from  the  radial 
canals,  and  take  the  form  of  feelers  round  the  mouth 
and  of  tube-feet  (and  ambulacral  papillae)  in  other 

parts  of  the  body  ;  such  feelers  are  always  present,  Arl  \      \v 

the  feet  and  papillae,  however,  may  be  wanting. 


Family  1.  Aspidochirotse. 

Tube  -feet  present.  Mouth  often  more  or  less 
ventral  in  position.  Body  usually  shows  distinct 
flattening  of  the  ventral  surface.  18-30  peltate 
tentacles.  Tentacular  ampullae  well  developed, 
Stone  canals  often  numerous.  Retractor  muscles 
wanting.  Respiratory  trees  present.  Cuvier's 
organs  often  present.  Mulleria,  Holothuria, 
Stichopus. 

Family  2.  Elasipoda. 

Tube-feet  present.  Mouth  more  or  less  ventral 
in  position.  Body  almost  always  distinctly  flattened 
on  the  ventral  surface.  10,  15,  or  20  tentacles,  more 
or  less  peltate  in  shape.  Stone  canal  always  single, 
and  not  infrequently  in  direct  communication  with 
the  exterior  through  the  integument.  Retractor 
muscles  wanting.  Respiratory  trees  wanting  or 
quite  rudimentary.  Cuvier's  organs  wanting. 
Sub-fam.  Psychropotidse  :  Psychropotes  (Fig.  223), 
Benthodytes.  Sub-fam.  Deimatidae  :  Deima,  Pan- 
nychia,  Laetmogone.  Sub-fam.  Elpidiidae  :  Elpidia, 
Kolga,  Peniagone. 


Family  3.  Pelagothuriidse. 

Tube-feet  wanting.     Mouth  and  anus  terminal. 
Body  cylindrical;  round  the  crown  of  tentacles  it 
widens  out  into  a  thin  disc,  the  edge  of  which  is    tacle  ;  2,  mouth 
produced  into  long  rays.       13-16    tentacles.      Re- 
tractor  muscles  wanting.     Neither  respiratory  trees, 
nor  ciliated   organs,   nor   Cuvier's    organs    present. 
Calcareous  bodies  altogether  wanting.      Pelagic,   swimming  by  means  of  the  disc. 
Single  genus  and  species  :  Pelagothuria  natatrix  (Figs.  224  and  225). 


OwltS- 

3,  4,  8,  ambulacral 
appendages  of  the  (ventral)  triviuin  ; 

5'  anus  ;  6'  dorsal  aPPendaSe  ™*& 
its  tw«  P«^erior  processes  (7). 


286 


COMPARATIVE  ANATOMY 


CHAP. 


FIG.  224.— Pelagothuria  natatrix  (after  Ludwig),  completed  ;  from  above.    1,  Body ;  2,  anus. 


FIG.  225.— Pelagothuria  natatrix  (after  Ludwig) ;  front  view,  i.e.  from  the  oral  pole.     1,  Mouth 
2,  oral  tentacles  :  3,  disc  ;  4,  canals  of  the  disc. 


VIII 


ECHINODERMATA— SYSTEMATIC  REVIEW 


287 


Family  4.  Dendrochirotse. 

Tube-feet  present.     Mouth  dorsal  or  terminal.     Anus  also  often  dorsal.     Body 
cylindrical,  or  pentagonal,  or  with  a  distinctly  marked  creeping  sole.     10-30  arbor- 


FIG.  2-27.  —  Psolus  epliippifer, 
young  female,  from  the  dorsal  side 
(after  The'el).  1,  Oral  valves ;  2, 
anus. 


FIG.  226.— Cucumaria  planci  (original).  1,  The 
two  smaller  ventral  oral  tentacles ;  2,  mouth ; 
3,  anus. 

escent  tentacles,  often  of  unequal  size.  Tentacular 
ampullae  not  distinct.  Not  infrequently  more  than 
one  stone  canal.  Retractor  muscles  well  developed. 
Respiratory  trees  present  ;  Cuvier's  organs  only 
occasionally  found.  Cucumaria  (Fig.  226),  Thyone, 
Phyllophorus,  Colochirus,  Theelia,  Psolus  (Figs.  227 
and  228),  Rhopalodina. 

Family  5.  Molpadiidse.  FIG.  228.  — Psolus  epliippifer, 

female,  dorsal  aspect  (after  Theel). 

Tube-feet  wanting.     Mouth  terminal.     The  pos-    i,  Oral  valves,  opened ;  2,  anus ; 
terior  end  of  the  cylindrical  body  often  narrowed  to    3,  oral   tentacles ;   4,  dorsal  cal- 
a  shorter  or  longer  tail-like  piece,  which  is  more  or    careous  scales, 
less  distinct  from  the  trunk.     15  tubular  or  digitate 

tentacles  normally  present.  Tentacular  ampullae  present.  A  single  stone  canal. 
Retractor  muscles  distinct  only  in  the  genus  Molpadia.  Respiratory  trees  present. 
Cuvier's  organs  almost  always  absent.  Molpadia,  Caudina,  Trochostoma,  An- 
kyroderma. 


288 


COMPARATIVE  ANATOMY 


CHAP. 


ORDER  2.  Paractinopoda. 

Only  some  of  the  outer  appendages  of  the  water  vascular  system  arise  from  the 
radial  canals,  the  rest  from  the  circular  canal,  and  the  only  form  taken  by  them  is 
that  of  tentacles  round  the  mouth. 

Family  1.  Synaptidae. 

Tube-feet  wanting.  Mouth  terminal.  Body  cylindrical,  more  or  less  elongated 
and  vermiform.  10-27  feathered  or  digitate  tentacles.  Stone 
canals  occasionally  numerous.  Retractor  muscles  sometimes 
present.  Respiratory  trees  and  Cuvier's  organs  wanting.  Sexual 
glands  often  hermaphrodite.  Synapta  (Fig.  229),  Ckirodota, 
Myriotrochus.1 

CLASS  II.  Echinoidea  (Sea-urchins). 

The  body  of  these  Echinoderms  is  covered  by  a  usually  firm 
but  sometimes  flexible  test,  which  contains  the  ccelomic  cavity 
and  the  viscera.  The  test  varies  in  shape,  from  spherical  to  a 
form  which  is  flatly  compressed  in  the  direction  of  the  principal 
axis.  It  consists  of  numerous  pentagonal  or  hexagonal  closely 
contiguous  plates,  which,  arranged  in  meridional  rows,  form  five 
ambulacral  and  five  interambulacral  areas.  It  is  covered  by  the 
outer  layer  of  the  integument,  and  carries  spines  articulating 
with  it.  .  At  the  apical  pole  there  is  a  system  of  plates,  consisting 
of  five  basal  plates,  five  radials,  and  the  anal  plate.  The  mouth 
is  usually  in  the  middle  of  the  oral  surface,  less  frequently  shifted 
towards  the  edge  in  what  is  called  the  anterior  direction.  An 
anus  is  always  present,  either  at  the  apical  pole  or  at  some  part 
of  the  posterior  interambulacral  area.  The  apertures  of  the 
madreporite  lie  in  the  apical  system,  generally  in  one  of  the  basal 
plates  ;  they  are  connected  not  only  with  the  stone  canal  but 
with  the  so-called  dorsal  organ.  The  ambulacral  vascular  system 
has  outer  appendages  developed  as  tube-feet  and  gills.  Mouth 
with  or  without  teeth.  In  the  former  case  a  complicated 
masticatory  apparatus  is  developed  within  the  test  for  the  move- 
ment of  the  teeth ;  the  muscles  moving  this  apparatus  are 
attached  to  a  perignathous  apophysial  ring  developed  at  the 
edge  of  the  oral  aperture  of  the  test  (i.e.  round  the  peristome). 
Sexually  separate  or  hermaphrodite.  The  genital  ducts  open 
externally  through  pores  in  the  basal  plates  or  outside  these 
latter.  Development  direct  (with  care  of  the  brood),  or  with 
metamorphosis  (free-swimming  larvse). 

SUB-CLASS  1.  Palseechinoidea. 

Either  only  one  row  or  more  than  two  rows  of  plates  in  each 
dlgltata  (orig^f)P  a    interambulacral  area.     Two  or  more  meridional  rows  of  plates  in 
each  ambulacral  area.     Plates  of  the  test  do  or  do  not  imbricate. 
Oral  aperture  of  the  test  (with  peristome)  in  the  middle  of  the  oral  surface.     Jaws 

1  The  arrangement  of  the  classes  and  families  of  the  Holothurioidea  by  Ludwig  in 
Bronn's  Klassen  und  Ordnungen  des  Thierreidis,  1892,  is  here  followed. 


VIII 


ECHINODERMATA— SYSTEMATIC  REVIEW 


289 


present.     Anal  area  either  within  the  apical  system,  or  outside  it,  in  the  posterior 
interambulacral  area.     Palaeozoic  forms. 


Order  1.  Bothriocidaroida. 

Regular  Echinoidea,  with  a  more  or  less  spherical,  firm  test.  In  each  inter- 
radius  there  is  only  one  meridional  row  of  plates  ;  in  each  ambulacral  area  there  are 
two.  Anal  area,  with  anus  within  the  apical  system.  Mouth  in  the  centre  of  the 
oral  surface.  Boihriocidaris. 

Order  2.  Perischoechinoida. 

Regular  Echinoidea.  More  than  two  meridional  rows  of  plates  in  each  inter- 
radius.  Two  or  many  meridional  rows  in  each  radius.  Test  thick  and  rigid,  or 

/          Z 


FIG.  230.— Palaeechinus  elegans  M'Coy 
(after  Baily). 


FIG.  231.— Tiar echinus  princeps  Laube  (after  Lovfen). 
1,  Genital  aperture  ;  2,  anus ;  3,  basal ;  4,  radial ;  5,  ambu- 
lacrum ;  6,  the  3  upper  plates  of  an  interambulacrum. 


thin  ;  in  this  latter  case  more  or  less  imbricated.  Jaws  present.  Fam,  Archseo- 
cidaridae :  Lepidocentrus,  Archceocidaris  (  =  Echinocrinus),  Palceechinus  (Fig.  230) 
Fam.  Melonitidae  :  Melonites. 

Order  3.  Plesiocidaroida. 

Test  small  and  rigid,  almost  hemispherical.  Apical  system  very  large,  with 
large  united  basal  plates  and  central  anal  area.  Ambulacra  narrow,  with  two  meri- 
dional or  vertical  rows  of  plates.  Interambulacra  with  one  single  peristome  plate, 
followed  by  three  plates  separated  by  vertical  sutures.  Tiarechinus  (Fig.  231). 


Order  4.  Cystocidaroida. 

Test  irregular  (exocyclic),  spherical  or  ovoid,  thin  and  flexible.      Madreporite 
central.     Ambulacral  areas  narrow,  with  two  vertical  rows  of  plates.     Interambu- 
lacral areas  broad,  with  numerous  vertical  rows  of  scale-like  movable  plates.     Anus 
in  the  posterior  interambulacrum  above  the  ambitus.    Echiuocystis  ( =  Cystocidaris). 
VOL.  II  U 


290  COMPARATIVE  ANATOMY  CHAP. 


SUB-CLASS  2.  Euechinoidea. 

Echinoidea  with  two  vertical  rows  of  plates  in  each  anibulacral  and  in  each 
interambulacral  area.  Mouth  on  the  oral  side,  rarely  shifted  towards  the  edge 
(anteriorly).  Teeth  and  jaws  present  or  wanting.  Anus  either  within  the  apical 
system,  or  outside  it,  i.e.  somewhere  in  the  posterior  interradius. 


Order  1.  Cidaroida. 

Mouth  central,  anus  within  the  apical  system.  No  external  gills.  With  jaws 
and  almost  perpendicularly  placed  teeth.  Perignathous  apophysial  ring  interrupted. 
Both  the  anibulacral  and  the  interambulacral  plates  are  continued  over  the  peri- 
stome  on  to  the  oral  area  as  far  as  the  mouth.  On  the  oral  area  they  are  imbricated. 
Ambulacra  narrow.  Large  principal  and  small  accessory  spines.  Sphseridia  want- 
ing. Cidaris. 

Order  2.  Diadematoida. 

Mouth  central,  anus  within  the  apical  system.  So-called  internal  gills  well 
developed,  or  rudimentary,  or  wanting.  With  external  gills,  and  incisions  in  the 
peristome.  With  jaws  and  teeth.  Perignathous  circular  apophysial  ring  closed. 
Only  the  ambulacral  plates  are  continued  over  the  peristome  on  to  the  oral  area, 
where  they  often  appear  as  separate  buccal  plates.  Sphreridia  present. 

Sub- Order  1.  Streptosomata. 

Test  more  or  less  flexible,  with  inner  dorso-ventral  longitudinal  muscles.  Both 
external  and  internal  gills  present.  The  anibulacral  plates  (and  only  these)  are 
continued  over  the  peristome  on  to  the  oral  area.  Fam.  Echinothuridse  :  Pdan- 
echinus,  Echinothuria,  Phormosoma,  Asthenosoma. 

Sub-Order  2.  Stereosomata. 

Test  rigid,  without  internal  longitudinal  muscles.  External  gills  present,  in- 
ternal gills  rudimentary  or  wanting.  The  ambulacral  plates  on  the  oral  area  are 
replaced  by  isolated  buccal  plates.  Fam.  1.  Saleniidse  :  Peltastes,  Salenia  (almost 
exclusively  fossil).  Fam.  2.  Hemicidaridse  :  Hemicidaris,  Acroddaris,  Gonio- 
pygus,  etc.  (fossil).  Fam.  3.  Aspidodiadematidse :  Aspidodiadema.  Fam.  4. 
Diadematidse  :  Diadema,  Diplopodia,  Pedina,  Echinothrix,  Astropyga,  Codechinus, 
Orthopsis,  Peronia,  Echinopsis,  etc.  (fossil  and  extant).  Fam.  5.  Cyphosomatidse, 
Cyphosoma,  etc.  (almost  exclusively  fossil).  Fam.  6.  Arbaciidse  :  Arbacia,  Echi- 
nocidaris  (Fig.  232),  Coslopleurus,  Podocidaris  (extant  and  fossil).  Fam.  7.  Tem- 
nopleuridse :  Glyphocyphus,  Temnopleurus,  etc.  (extant  and  fossil).  Fam.  8. 
Echinometridae  :  Echinometra,  Parasalenia,  etc. ,  Spongy  locentrotus,  SpJicercchinus 
(mostly  extant).  Fam.  9.  Echinidse  :  Echinus,  Tox&pncustcs,  Tripneustes  (extant 
and  fossil). 

Order  3.  Holectypoida. 

Mouth  central.  Anus  outside  of  the  apical  system  in  the  posterior  interradius 
(exocyclic).  With  external  gills.  Only  one  pair  of  pores  or  a  single  pore  on  each 
ambulaoral  plate.  Jaws  weak  ;  teeth  perpendicular  ;  both  jaws  and  teeth  may  be 
wanting.  Sphseridia  present,  (a)  Ambulacral  apophyses  present :  Holedypus, 


ECHINODERMA  TA  —S  YSTEMA  TIC  RE  VIE  W 


291 


/•,  etc.  (principally  fossil),     (b)  Ambulacral  apophyses  rudimentary  or  want- 
ing :  Discoidea,  C&noclypeus  (fossil). 


Order  4.  Clypeastroida. 

Mouth  central  or  sub-central.     Anus  outside  of  the  apical  system  in  the  posterior 
interambulacrum.     With  external  gills.     With   tentacle   pores   in   the   interradii. 


FIG.  232.— EcMnocidaris  (Arbacia)  pustulosa,  from  the  apical  side  (original).    The  spines  have 
been  removed  from  part  of  the  shell.     1,  Interambulacrum  ;  2,  ambulacrum. 

More  than  one  pair  of  pores  on  each  ambulacral  plate.  Tentacles  differ  in  one  and 
the  same  animal.  Teeth  usually  almost  horizontal,  rarely  vertical.  The  jaws  lie 
above  the  apophysial  ring,  which  is  interrupted.  Sphseridia  present. 

The  test  is  seldom  much  arched  ;  it  is  usually  more  or  less  flattened,  and  often 
even  disc-like.  It  often  has  many  incisions  and  perforations,  and  is  usually  bilater- 
ally symmetrical.  Its  dorsal  wall  is  connected  internally  with  its  ventral  wall  by 
means  of  pillars,  needles,  septa,  etc.  Basal  plates  of  the  apical  system  fused.  The 
ambulacra  form  petaloids  in  the  apical  region. 

Fam.  1.  Fibulariidae  :  Echinoeiiamus,  FibvJaria,  etc.  (extant  and  fossil).  Fam. 
2.  Clypeastridse  :  Clypeaster  (Fig.  233),  etc.  (extant  and  fossil).  Fam.  3.  Laganidse  : 
Lagan  urn,  (extant  and  fossil).  Fam.  4.  Scutellidse.  In  all  the  genera  of  this  family 


292 


COMPARATIVE  ANATOMY 


CHAP. 


FIG.  233. — Clypeaster  sp.,  test  from  the  apical  side  (original). 


FIG.  2^4.— Scutella  sexforis,  test  from  the  apical  side  (original). 


VIII 


ECHINODERMATA  -SYSTEMATIC  REVIEW 


293 


the  shell  is  very  flat :  Scutella  (Fig.  234),  EchinodAscus,  Encope,  MellUa  (Fig.  235), 
t,  Arachnoid's,  etc.  (extant  and  fossil). 


FIG.  235.— Mellita  testudinata?  from  the  oral  side  (original). 


Order  5.  Spatangoida. 

Mouth  central,  sub-central,  or  on  the  anterior  edge  of  the  oral  surface  of  the  test 
Anus  outside  the  apical  system,  in  the  posterior  interradius.  External  gills,  jaws, 
teeth,  and  perignathous  apophysial  ring  wanting.  Sphseridia  present.  The  ambu- 
lacra generally  form  apical  petaloids.  The  test  is  bilaterally  symmetrical,  arched, 
often  heart-shaped. 

Sub-Order  1.  Cassiduloidea. 

Fam.  1.  Echinoneidse  :  Echinoconus,  Echinoncus,  Oligopygus,  Echinobrissus,  etc. 
(extant  and  fossil).  Fam.  2.  Cassidulidse  :  Cassi'lv.lus.  Catopygus,  Clypetis,  Pygurus, 
Echinolampas,  etc.  (mostly  fossil).  Fam.  3.  Collyritidae :  Collyrites,  Disaster,  etc. 
'fossil).  Fam.  4.  Plesiospatangidae  :  EnJanipcis,  Archiacia,  etc.  (fossil). 


Sub-Order  2.  Spatangoidea. 

Fam.  1.  Anan-chytidae  :  Eclnnocorys,  Holaster,  Hemipmustes,  Cardiaster,  Ure- 
si.  Cijstechiiius,  Calijmne,  etc.  (the  last  three  genera  extant,  the  rest  fossil). 
Fam.  2.  Spatangidae  —  Group  1,  Adetes  :  Isastcr,  Eehinospatagus,  Heterolampas, 
H'--,nipof.agus,  etc.  (almost  exclusively  fossil);  Group  2,  Prymnadetes  :  Hemiaster. 
F'">ri:ia.  Linthia,  8chi~astcr  (Fig.  236),  Agassizia  (extant  and  fossil)  ;  Group  3. 
Prymnodesmia :  Micmste.r,  Bi-issus.  Spaianffomorpha,  Brissopsis,  Spatangus,  Palceop- 
,,  votes  (Fig.  237),  Echiiiocardiu,,i.  Lwiiia,  etc.  (extant  and  fossil)  ;  Group  4. 
Apetala :  Cfenieopcttoffut^  Pdlrrob'riw*.  Ac?stc,  Aerope,  etc.  (extant  and  fossil). 


294 


COMPARATIVE  ANATOMY 


CHAP. 


FIG.  L'36.  —  Schizaster  lacu- 
nosus  ?  from  the  apical  side 
(original).  The  spines,  and  the 
protuberances  on  which  they 
stand,  are  not  depicted.  1,  The 
anterior  unpaired  ambulacrum  ; 

2,  the  right  anterior  ambulacrum  ; 

3,  fascicle  ;  4,  the  right  posterior 
interambulacrum ;    5,   the    right 
posterior    ambulacrum  ;     6,    the 
unpaired  posterior  interambula- 
crum ;  7,  anal  region. 


Fio.  237.— Palaeopneustes 
Murray!  (after  Agassiz), 
from  the  oral  side.  1,  The 
anterior  ambulacrum  ;  2,  3. 
the  anterior  right  and  the 
posterior  right  ambulacra : 
4,  peristome  ;  5,  anal  region. 


viii  ECHINODERMATA—  SYSTEMATIC  REVIEW  295 

Fain.   3.    Leskiidse  :    Palccostoma  (extant).      Fam.    4.   Pourtalesiidae :   Pourtalesia 
(Fig.  238),  Spatagocystis,  Echinocrcpis  (extant).1 


IV 

0* 

FIG.  238.— Pourtalesia  Jeffreys!,  from  the  side  (after  Loven).  The  smaller  tubercles  are  not 
depicted.  r<p,  Apex  ;  os,  oral  pole  ;  an,  anal  region.  The  numbers  are  explained  in  the  text,  in  the 
section  on  the  perisomatic  skeleton  of  the  Echinoidea,  p.  342. 


CLASS  III.  Asteroidea  (Stelleridea),  Star-fish. 

Echinodermata,  with  body  flattened  in  the  direction  of  the  principal  axis,  the 
radii  being  produced  laterally  into  longer  or  shorter  arms.  The  arms  are  usually  five 
in  number,  but  their  number  may  be  increased  to  forty  or  more.  They  are  not  dis- 
tinctly marked  off  from  the  central  part  of  the  body  (the  disc) ;  and  besides  the  radial 
blood  vessels,  nerves,  and  ambulacral  vessels,  diverticula  of  the  intestine  and  con- 
tinuations of  the  genital  organs  run  into  the  ccelomic  cavities  of  the  arms.  The 
body  is  usually  covered  with  calcareous  plates,  but  is  flexible.  The  calcareous 
plates  carry  spines,  and  often  pedicellarire  as  well.  Along  each  arm  runs  a  ventral 
furrow,  within  which  there  is  a  longitudinal  row  of  paired  ambulacral  plates.  The 
consecutive  pairs  are  movably  articulated  with  one  another.  Besides  these,  there 
are,  on  the  arms,  adambulacral,  inframarginal,  supramarginal,  and  dorsal  plates. 
The  ambulacral  grooves  run  from  the  central  mouth  on  to  the  arms,  and  along  these 
on  their  oral  (ventral)  side,  below  the  ambulacral  plates,  to  their  tips.  The  tube- 
feet  (tentacles)  rise  from. the  base  of  this  groove,  to  which  they  are  limited.  Anus 
apical  (i.e.  in  the  centre  of  the  upper  side),  rarely  wanting.  Madreporite  also  on  the 
apical  side  of  the  disc.  The  sexes  are  separate.  Development  is  in  most  cases  with 
metamorphosis  (free-swimming  pelagic  larvae) ;  when  the  brood  is  protected  develop- 
ment is  direct. 

SUB-CLASS  1.  Palaeasteroidea. 

Palaeozoic  Asteroidea,  in  which  the  ambulacral  plates  in  the  two  longitudinal 
rows  in  each  arm,  at  least  in  the  middle  of  the  arm,  are  arranged  alternately  (not 
opposite  or  in  pairs).  Aspidosoma,  Palccastcr,  Palceocorna,  etc.  (all  Paleeozoic  forms). 

1  The  classification  of  the  Echinoidea  here  given  is  after  Martin  Duncan,  A  Revision 
of  (he  Genera  and  Great  Groups  of  the  Echinoidea.  London,  1889. 


296 


COMPARATIVE  ANATOMY 


CHAP. 


SUB-CLASS  2.  Euasteroidea. 
Asteroidea  with  paired,  i.e.  opposite  ambnlacral  plates  or  "  vertebrse." 


Order  1.  Phanerozonia. 

Asteroidea  with  large,  strongly  developed,  marginal  plates.  The  inframarginal 
and  supramarginal  plates  are  closely  fitted  together.  Papulae  (branchial  vesicles) 
only  occur  on  that  surface  of  the  body  which  is  surrounded  by  the  supramarginal 


FIG.  239.— Ctenodiscus  procurator  (after  Sladen),  from  the  oral  side.    A  Gastropod  in  the 
stomach  is  visible  through  the  mouth. 

plates,  i.e.  on  the  apical  or  upper  side.  Ambulacral  plates  broad.  In  each  ambu- 
lacral  furrow  there  are  two  longitudinal  rows  of  tube-feet.  The  adambulacral  plates 
are  prominent  in  the  oral  skeleton.  Where  pedicellarise  occur  they  are  sessile. 

Fam.  1.  Archasteridse :  Parar chaster,  Dy  taster,  Plutonaster,  Pseudar chaster, 
Archaster,  etc.  Fam.  2.  Porcellanasteridse,  the  centre  of  the  apical  system  pro- 
duced into  a  more  or  less  long  outgrowth  :  Porcellaviaster,  Hyplmlaster ,  Ctenodiscus 
(Fig.  239),  etc.  Fam.  3.  Astropectinidse,  without  anus  and  usually  without  pedicel- 
larise  :  Astropecten,  BathyUaster,  lly aster,  Luidia,  etc.  Fam.  4.  Pentagonasteridae  : 
Pentagonaster,  Astrogonium,  Nectria,  Calliaster,  Stellaster,  Goniodiscus,  Mimaster,  etc. 
Fam.  5.  Antheneidae  :  Anthenea  (Fig.  240),  Goniaster,  etc.  Fam.  6.  Pentacerotidae  : 


VIII 


ECHINODERMATA—  SYSTEMATIC  REVIEW 


29' 


Pentaceros,  Amphiaster,  Culcita,  Asterodiscus,  etc. 
nasteria,     Tylaster,    Astevopsis, 
Marginaster,  etc.     Fam.  8.  As- 
terinidae :    Gaiieria,    Asterina, 
r<tl,nipes,  etc. 

Order  2.   Cryptozonia. 

Asteroidea,  in  which  the  mar- 
ginal plates  are  indistinct  and 
more  or  less  rudimentary  in  the 
adult.  The  supramarginal  plates 
are  often  separated  from  the  in- 
framarginal  by  intermediate  plates.     The 
papulae  are  not  limited  to  the  apical  sur- 
face,   but  often   occur  also    between    the 
marginal  plates  and  on  the  oral  (lower) 


Fam.  7.  Gymnasteriidae 


FIG.  240.  —  Anthenea  tuberculosa,  Gray? 
juv.  (after  Sladen).  1,  Supramarginal  plates  ; 
2,  pedicellarise  ;  3,  madreporite ;  4,  anus. 


FIG.  241.— Cnemidaster  Wyvillii  (after  Sladen).     ilc,  Dorsocentral ;  r,  radials ;  be,  basals  ; 
sm,  suprainarginals  ;  d,  dorsals  ;  t,  terminals. 

surface  of  the  body.  Ambulacral  plates  narrow,  closely  crowded.  Tube-feet  often 
in  four  rows.  In  the  oral  skeleton  the  ambulacral  or  interambulacral  plates  are 
prominent.  Pedicellarire  sessile  or  pedunculate. 


298  COMPARATIVE  ANATOMY  CHAP. 

Fam.  1.  Linckiidse :  Chaetaster,  OpMdiaster,  Linekia,  Metrodira,  etc.     Fam.  2. 


FIG.  242.— Hymenaster  caelatus  (after  Sladen),  with  arms  bent  back. 

Zoroasteridse :   Zoroaster,    Cnemidaster   (Fig.   241).     Fara.   3.    Stichasteridse :   Sti- 
chastcr,  etc.     Fam.  4.  Solasteridse  :  Sol-aster,  Crossastcr,  Corethraster,  etc.     Fam.  5. 


FIG.  <243.-  -Hymenaster  nobilis  (after  Sladen),  from  the  oral  side,  f  natural  size. 

Pterasteridse,  with  brood  cavity  on  the  apical  side  of  the  disc  :  Pteraster,  Retaster, 
Hymenaster  (Figs.  242  and  243),  Myxaster,  Benthaster,  Pythonaster,  etc.     Fam.  6. 


VIII 


ECHINODERMATA— SYSTEMATIC  REVIEW 


299 


Echinasteridse :  Acanthaster  (numerous  arras),  Mitkrodia,  Cribrefaa,  Echinaster, 
rut  vaster,  etc.  Fam.  7.  Heliasteridae.  with  numerous  short  arms  :,  Hcliaster.  Fam. 
*.  Pedicellasteridae  :  P>:  die  ell  aster.  Fam.  9.  Asteriidae,  tube -feet  in  four  rows: 
Aatffias,  Uniophora,  Coronaster,  etc.  Fam.  10.  Brisingidae,  with  numerous  very 
long  arms,  marked  off  from  the  small  disc  :  Brisinga,  Labidiasftr,  etc.1 


CLASS  IV.  Ophiuroidea. 

Echinodermata  flattened  in  the  direction  of  the  principal  axis  of  the  body,  the 
radii  of  which  are  produced  into  five  long,  round,  simple  or  much  branched  slender 
arms.  The  arms  are  sharply  marked  off  from  the  central  part  of  the  body,  and  do 
not  contain  either  ca?ca  of  the  intestine  or  extensions  of  the  genital  organs.  The 


Fio.  -244.— Ophiolepis  elegans,  Lutken  (after  Lyman).    ds,  Dorsal  shields  ;  as,  lateral  shields  ; 
dc,  dorsocentral ;  ib,  infrabasal ;  fac,  basal ;  rs,  radial  shields ;  r,  radial. 

axial  part  of  the  arms  is  occupied  by  a  longitudinal  row  of  vertebral  ossicles,  articu- 
lated together,  and  consisting  of  two  fused  lateral  ambulacral  plates  or  ossicles. 
The  body  is  usually  covered  with  calcareous  plates.  On  the  arms  we  can  distinguish 
a  longitudinal  row  of  ventral  shields  on  the  oral  side,  two  longitudinal  rows  of 

1  The  classification  of  the  two  orders  of  the  Euasteroidea  is  that  of  W.  Percy  Sladen, 
Report  on  the  Asteroidea  collected  by  H.M.S.  Challenger.     London,  1889. 


300 


COMPARATIVE  ANATOMY 


CHAP. 


lateral  spine-bearing  shields,  and  a  longitudinal  row  of  dorsal  shields.  On  the 
apical  surface  of  the  disc  larger  radial  shields  are  found  at  the  sides  of  the  bases  of 
the  arms  :  thus  ten  in  all.  On  the  oral  side  of  the  disc  there  are  five  interradial 
plates  which  are  distinguished  by  their  great  size  ;  these  are  the  buccal  shields.  One 
of  these  plates  is  at  the  same  time  the  madreporitic  plate.  Mouth  at  the  centre  of 
the  lower  side.  Anus  wanting.  The  ambulacral  tube-feet  appear  on  each  side  on 
the  arms  between  the  ventral  and  lateral  shields.  On  the  lower  side  of  the  disc,  close 
to  the  bases  of  the  arms  laterally,  there  are  in  all  ten  or  twenty  slit-like  apertures — 
the  bursal  apertures.  These  lead  into  blind  sacs  projecting  into  the  ccelom  ;  these 
are  the  bursae,  which  serve  for  respiration  and  for  the  reception  and  ejection  of  the 
genital  products.  Development  direct  (viviparous  and  with  care  of  the  brood),  or 
with  metamorphosis  (free-swimming  pelagic  larvae). 


Order  1.  Ophiurae. 

Arms  unbranched,  movable  in  the  horizontal  plane,  usually  distinctly  plated. 
Buccal  shields,  one  of  them  at  the  same  time  the  madreporitic  plate,  distinctly 
developed. 

Fain.  1.  Ophioglyphidse  :  Opliiura,  Pectinura,  Ophiolepis  (Fig.  244),  Ophiozona, 
Ophioglypha,  Opliioctcn,  Ophiomusium.  Fain.  2.  Amphiuridse  :  Ophiadis  (Fig.  245), 


FIG.  245.—  Ophiactis  poa,  Lym.  (after  Lyman).  Disc  and  basal  portions  of  t.he  arms  ;  from  the 
oral  side.  1,  Ventral  shields  ;  2,  spines  on  the  lateral  shields  (4)  ;  3,  tentacle  scales  ;  5,  lateral 
buccal  shields  ;  6,  bursal  apertures  ;  7,  buccal  shields  ;  8,  first  ventral  shield  of  the  arm  ;  9,  torus 
angularis  ;  10,  oral  papillae. 


Amphiura,  Ophiocnida,  Ophiocoma,  Ophiacantha,  Oirfiwthrix.  Fam.  3.  Ophio- 
myxidae,  disc  and  arms  covered  by  a  thick  naked  integument  :  Ophiomyxa,  Hemi- 
curyalc. 


VIII 


ECHINODERMATA— SYSTEMATIC  REVIEW 


301 


Order  2.  Euryalse. 

Arms  simple  or  branched,  can  be  rolled  up  vertically  towards  the  mouth.  Only 
rudimentary  shields  are  found  below  the  soft  but  thick  outer  integument.  Without 
spines.  In  forms  with  unbranched  arms  there  are  usually  5  buccal  shields,  one  of 
which  is  the  madreporitic  plate.  Most  of  the  forms  with  branched  arms  have  no 


FIG.  246.— Astrophyton  LincM  (Miiller  a 


}Ch.el),  from  the  oral  side  (original). 


distinct  bnccal  plates.     There  is  then  either  a  single  madreporite  in  an  oral  inter- 
brachial  area  or  else  there  are  5  interbrachial  madreporites. 

Single  Fam.  Astrophytidse  :  Astrophyton  (Fig.  246),  Gorgonocephalus,  Euryale, 
Trichaster  (arms  slightly  and  only  at  their  tips,  dichotomously  branched),  Astroclon 
(the  same),  Astrocnida  (the  same),  Astroporpa  (arms  undivided),  Astrogomphus  (the 
same),  Astrochcle  (the  same),  Astrotoma  (the  same),  Astroschema  (the  same),  Ophio- 
creas  (the  same),  etc.1 

1  For  a  more  recent  classification  of  Opliiuroidea,  see  F.  J.  Bell,  Proc.  Zool.  Soc. 
London,  1892,  pp.  175-183. 


302  COMPARATIVE  ANATOMY  CHAP. 


CLASS  V.  Pelmatozoa. 

Echinodermata  which  are  either  permanently  or  temporarily l  attached  by  the 
centre  of  the  apical  surface,  so  that  the  oral  surface  (with  the  mouth,  as  a  rule,  in  its 
centre)  looks  upward.  The  body  is  usually  raised  upon  a  jointed  stem  attached  to 
it  at  the  apex.  An  axial  canal,  in  which  are  blood  vessels  and  nerves,  runs  through 
the  stem.  This  stem  is  sometimes  found  only  in  the  young,  the  body  becoming 
detached  later,  and  further  in  a  few  attached  forms  no  stem  at  all  is  developed. 
The  apical  system  of  plates  consists  of  5  basals  and  5  radials,  to  which  5  infra- 
basals  and  a  varying  number  of  interradials  are  often  added.  The  plate  in  the 
embryo  Antcdon,  which  becomes  fixed  to  the  ground  and  is  subsequently  lost,  is 
called  "dorsocentral,"  and  is  supposed  to  belong  to  the  apical  system.  The  number 
of  the  principal  rays  is  rarely  4  or  6.  The  plates  just  mentioned  form  a  cup 
(dorsal  cup),  which  either  simply  carries  or  else  more  or  less  completely  encloses  the 
visceral  mass.  The  cup  carries  jointed  appendages, — arms  or  pinnulse  or  both. 

The  oral  side  (in  these  animals  turned  uppermost)  is  often  provided  with  5  oral 
plates,  which  surround  or  cover  the  central  mouth,  and  it  may  further  be  protected 
in  very  various  ways  by  radially  and  interradially  situated  plates  (ambulacrals, 
interambulacrals,  and  orals),  which  together  form  the  tegmen  calycis.  Or  again  this 
cover  of  the  calyx  may  be  either  naked  or  set  with  very  small  isolated  calcareous 
pieces.  The  anus  lies  usually  at  the  end  of  a  longer  or  shorter  tube,  excentrically  in 
an  interradius  of  the  tegmen,  occasionally,  however,  at  the  boundary  between  the 
cup  and  the  tegmen.  The  circumo3sophageal  canal  of  the  water  vascular  system  does 
not  communicate  direct  with  the  exterior.  The  radial  canals  of  this  system  run 
into  the  arms.  Each  of  the  latter  has  a  food  groove  on  its  oral  (uppermost)  side. 
The  tube-feet,  which  rise  from  the  edge  of  this  furrow,  are  tentacular,  and  do  not 
serve  for  locomotion,  but  for  respiration,  and  possibly  for  conducting  food. 
Development,  so  far  as  is  known,  with  metamorphosis. 

SUB-CLASS  1.  Crinoidea. 

Pelmatozoa  with  long  usually  branched  arms.  The  arms  are  jointed,  the  con- 
secutive ossicles  being  connected  by  muscles  and  bands.  The  arms  can  be  expanded, 
and  closed  up  together,  or  again  can  roll  up  orally.  They  may  carry  jointed, 
unbranched  appendages,  the  pinnulse,  which  are  probably  modified  branches.  The 
nervous  system  is  generally  said  to  be  "double,"  i.e.  there  is  an  abactinal  and  an 
oral  system.  The  abactinal  nervous  system  consists  of  a  central  portion  lying  in 
the  apex  of  the  dorsal  cup  and  of  radiating  strands  which  run  through  the  skeletons 
of  the  stem,  the  arms,  and  the  pinnulse.  The  oral  nervous  system  consists  of  a 
circumoral  nerve  ring,  and  radiating  strands  which  run  into  the  arms  through  the 
epithelium  at  the  base  of  the  food  grooves,  and  which  branch  with  the  arms.  The 
food  grooves  of  the  arms  pass  at  their  bases  on  to  the  tegmen,  running  in  it  to 
the  central  mouth.  Ambulacral  tentacles  may  be  wanting.  The  circular  canal  of 
the  water  vascular  system  is  connected  with  the  body  cavity  by  means  of  several 
stone  canals,  and  the  body  cavity  is  in  open  communication  with  the  exterior  by 
means  of  water  pores.  The  mouth  is  in  the  centre  of  the  tegmen  (exc.  Adinometra}. 
The  sexual  organs  extend  right  into  the  basal  parts  of  the  arms,  and  even  into 
their  pinnulse.  In  pinnulate  crinoids.  so  far  as  is  known,  however,  the  genital 
products  only  ripen  in  the  pinuulse. 

1  There  is,  however,  no  evidence  to  show  that  Marsupites  was  attached  even  in  the 
larval  stage ;  unlike  Antedonidse,  it  has  no  trace  of  a  stem. 


VIII 


EGHINODERMATA— SYSTEMATIC  REVIEW 


303 


The  old  division  into  Pakcocrinoidea  and  Neowinoidca  seems  artificial ;  that  here 
adopted  also  cannot  be  considered  as  definitive.1 


Order  1.  Inadunata. 

Calyx  comparatively  small ;  dorsal  cup  with  nionocyclic  or  dicyclic  base  ;  the ' 
basals  in  the  former,  and  infrabasals  in  the  latter  case  may  be  fused  to  4,  3,  2,  or 
1.  The  only  other  plates  in  the  apical  capsule  are  5  radials.  In  the  posterior 
interradius  there  are  very  often  1-3  asymmetrically  placed  anal  plates,  but  no  plates 
in  the  other  interradii. 

The  tegmen  calycis  varies.  In  some  Inadunata  (Larviformm)  there  are  5  large 
oral  plates,  which,  rising  at  the  edge  of  the  calyx  directly  above  the  radials,  form  a 
closed  pyramid  covering  the  food  grooves  of  the  disc,  and  the  mouth.  In  many 
other  forms  the  orals  (which  may  be  partly  resorbed)  lie  at  the  centre  of  the  tegmen 
calycis.  The  posterior  oral  plate  is  often  larger  than  the  others,  and  is  shifted  for- 
ward between  them.  The  ambulacra  appear  at  the  surface  of  the  tegmen  calycis  be- 
tween the  oral  plates  and  the  edge  ;  they  are  bordered  on  each  side  by  rows  of  small 
lateral  pieces,  the  ambulacral  groove  being  also  roofed  in  by  small  covering  pieces. 
Plates  of  various  shapes,  size,  and  arrangement  are  found  in  the  interambulacral 
regions.  In  the  posterior  ambulacral  region  the  tegmen  calycis  often  bulges  out  in 
the  form  of  a  plated  sac,  the  so-called  ventral  sac  (Fistulata)  ;  this  varies  in  form 
and  size  (sometimes  reaching  beyond  the  arms),  and  may  sometimes  have  contained, 
besides  the  rectum,  a  large  part  of  the  body  cavity.  The  anus  lies  at  its  tip  or  on  its 
anterior  side. 

Arms  free,  i.e.  not  included  in  the  dorsal  cup  (hence  the  name  Inadunata), 
simple  or  branched,  with  or  without  pinnulpe.  The  food  grooves  of  the  arms  are 
roofed  in  by  two  or  more  rows  of  alternating,  wedge-shaped,  interlocking,  ambulacral 
plates  ;  these  plates  could  probably  be  erected. 

Almost  exclusively  palaeozoic  forms. 

A.  Monocyclica. 

With  monocyclic  basis  (without  infrabasals  ;  several  radials  often  horizontally 


\ 


FIG.  247.— Haplocrinus  mespiliformis  (after  Wachsmuth  and  Springer).  A,  from  the  anal 
side  ;  B,  from  the  oral  side.  1,  Orals ;  2,  oral  pole  ;  3,  anus ;  4,  radials  ;  5,  right  posterior  infer- 
radial  or  radianal ;  6,  basals  ;  7,  first  brachial ;  8,  facet  for  attachment  of  the  arm. 

bisected).     Haplocrinus  (type  of  the  so-called  Larrifonnia,  without  anal  plate)  (Fig. 

1  Classification  chiefly  after  the  recent  works  of  Wachsmuth  and  Springer  and  Her- 
bert Carpenter.     See  Bibliography,  p.  551, 


304 


COMPARATIVE  ANATOMY 


CHAP. 


247).  Hcterocrinus,Hcrpctocrinus,  Calceocrinus,  Catillocrinus,  Pisocrinus,  Hybocrinus, 
locrinus,  Symbathocrinus,  Belemnocrinus,  Gastrocoma  (?),  Cupressocrinus. 


B.  Dicyclica. 

With  dicyclic  base  (with  infrabasals).      Fam.   Dendrocrinidse  :    Dendrocrinus, 
Homocrinus,    Poteriocrinus.       Fam.     Decadocrinidse :    Botryocrinus,    Barycrinus, 


m 


FIG.  248.— Encrinus  liliifonnis  (original). 
cl»  ca*  Costals  or  primibrachials  ;  r,  radials  ; 
co,  stem  ;  p,  pinnulae. 


FIG.  249.  — Cyathocrinus 
longimanus  (after  Angelin). 
pr,  Ventral  sac ;  !,  place  where  an 
arm-branch  has  been  removed ; 
r,  radials ;  ba,  basals ;  ib,  infra- 
basals ;  col,  stem  ;  x,  anal  plates  ; 
co,  costals  or  primibrachials. 


Atelestocrinus,  Decadocrinus,  Graphiocrinus,  Encrinus  (Fig.  248),  (without  anal 
plates,  ventral  sac  reduced  to  a  short  cone,  Trias),  Cromyocrinus,  Agassizo- 
crinus.  Fam.  Cyathocrinidse :  Cyathocrinus  (Fig.  249),  Gissocrinus,  Lecythocrinus. 
Hypocrinus. 

The  genus  Marsupites  from  the  Chalk,  and  the  following  extant  families  are 
perhaps  to  be  classed  near  the  Inadunata ;  in  these  latter  five  large  separate  orals 
occur,  the  ventral  sac  being  reduced  to  an  anal  tube,  and  no  anals  appearing  in  the 
dorsal  cup.  Holopidaz  (Fig.  250)  (Lias,  to  present  time),  Hyocrinidce  (Fig.  251)  (Lias, 
present  time),  Bathycrinidce  (extant). 


VIII 


EGHINODERMATA—  SYSTEMATIC  REVIEW 


305 


1 


.sll    ft.* 


FIG.  250.— Holopus  Rangi  d  Orbigny.  from  the  trivial 
side  (after  P.  H.  Carpenter). 


VOL.  II 


306  COMPARATIVE  ANATOMY  CHAP. 


Order  2.  Camerata. 

Plates  of  the  calyx  firmly  connected  by  means  of  sutures.  The  apical  capsule 
shows  a  tendency  to  develop  a  very  rich  system  of  plates,  incorporating  the 
proximal  brachials  to  a  greater  or  lesser  extent.  These  brachials  are  connected 
together  in  the  interradii  by  interradial  plates,  which  vary  in  number,  and  to  which, 
in  the  anal  interradius,  special  anal  plates  may  be  added.  In  those  cases  in  which 
the  arms  are  incorporated  in  the  calyx  to  such  an  extent  that  they  branch  in  the 
latter  before  they  become  free  from  it,  their  branches  may  be  connected  by  inter- 
calated plates.  Each  of  the  five  radials  is  usually  followed  by  two  brachial  plates, 
formerly  called  2nd  and  3rd  radials.  The  tegmen  calycis  is  richly  plated  with  firmly 
connected  pieces,  and  is  often  much  arched,  forming  a  so-called  vault.  The  mouth, 
which  lies  in  the  centre  of  the  tegmen,  is  covered  with  five  firmly  united  oral 
plates  ;  the  hindermost  of  these,  which  is  often  the  largest,  projects  in  between 
the  four  others.  The  ambulacra,  with  their  lateral  and  covering  plates,  are  mostly 
not  visible  from  outside,  as  the  interambulacral  plates  which  border  them  laterally, 
and  which  are  often  very  numerous,  close  together  over  them  by  means  of  processes, 
and  thus  cover  them  externally.  The  ambulacra,  in  their  course  on  to  the  bases  of 
the  free  arms,  divide  as  many  times  as  the  arms  have  already  divided  on  the 
dorsal  cup.  The  interradials  of  the  dorsal  cup  often  pass,  without  any  sharp 
boundary,  into  the  interradially  arranged  interambulacrals  of  the  tegmen  calycis. 
The  subcentral  (less  frequently  central)  anus,  which  is  surrounded  by  firm  anal 
plates,  is  either  sessile  or  else  comes  to  lie  at  the  tip  of  a  chimney -like  prolongation 
of  the  tegmen  ;  this  anal  tube,  formerly  thought  to  be  a  proboscis,  may  project 
beyond  the  arms.  Arms  branched ;  in  adults,  almost  without  exception,  the 
brachials  become  arranged  in  a  double  row  with  primitive  articulation,  and  pinnules 
closely  folded  together.  Dorsal  canals  (in  the  brachials)  have  never  been  observed. 
Exclusively  palaeozoic  forms. 

Family  1.  Reteocrinidse. 

Apical  capsule,  with  monocyclic  or  dicyclic  base.  Four  or  five  basals.  Inter- 
radial  and  interaxillary  regions  deeply  sunk,  plated  with  a  large  number  of  irregular 
immovable  pieces,  which  are  continued  on  to  the  interambulacral  areas  of  the  tegmen 
calycis.  Posterior  interradial  region  broader,  and  divided  by  a  perpendicular  row 
of  somewhat  large  anal  plates.  Anus  subcentral.  Arms  composed  of  a  single  row  of 
calcareous  joints.  Pinnules  strong.  Reteocrinus.  Xenocrinus. 

Family  2.  Rhodocrinidse. 

Apical  capsule  with  dicyclic  base.  The  circle  of  the  five  radials  interrupted  by 
that  of  the  five  first  interradials,  which  are  in  direct  contact  Avith  the  basals. 
Interradial  area  plated  with  regular  definitely  arranged  pieces.  Posterior  interradial 
area  differs  but  slightly.  Tegmen  calycis  thickly  plated.  The  plating  of  the  apical 
interradial  region  passes  without  break  into  that  of  the  tegmen  calycis.  Ambulacra 
not  externally  visible.  Orals  often  indistinct.  Anus  subcentral.  Rhodocrinus, 
Gilbertsocrinus,  Rhipidocrinus. 

Family  3.  Glyptasteridae. 

Base  dicyclic.  With  the  exception  of  the  first  anal  plate,  which  is  in  contact 
with  the  posterior  basal,  the  interradials  do  not  touch  the  basals.  Interradial 
region  of  the  apical  capsule  and  tegmen  calycis  as  in  the  Rhodocrinidce.  Oral  plates 
distinct.  Anus  subcentral.  Glyptaster. 


VIII 


EGHINODERMATA— SYSTEMATIC  REVIEW 


307 


Family  4.  Melocrinidse. 

Base  monocyclic,  3-5  basals.  The  basals  in  contact  only  with  the  radials. 
Interradial  areas  of  the  apical  capsule  with  numerous  large  regularly  arranged  plates. 
Plates  of  the  tegmen  calycis  often  small  and  regular.  Orals  distinct.  Anus  sub- 
central.  Mdocriiias  (Fig.  252),  Mai-iacriiius,  <rl ^Anti-inns,  Stelidiocrinus. 


Family  5.  Actinocrinidae. 

Base  monocyclic,  3,  rarely  4,  basals.     The  first  anal  plate  rests  upon  the  circle  of 
basals  ;    the  first  interradials  otherwise  being  in  contact  only  with  the  circle  of 


FIG.  i'a-2.— Melocrinus  typus,  Br. 
/>,  Pinnuhe  ;  br,  arms;  (//',  distichals; 
t'l,  c-2,  first  and  second  costal ;  /•,  radial ; 
tin,  basal ;  co,  stein  ;  it-  and  id,  inter- 
radinls. 


Fi<;.  253.— Batocrinus  pyriformis. 
Shum.  (after  Meek  and  Worthen). 
>:k,  Ventral  capsule  ;  br,  arms  :  p.  pin- 
nulit:  ;  (U,  distichals ;  GI,  Co,  costals  ; 
/•,  radials;  /x?,  basals;  co.  stem;  ir, 
interradials ;  abr,  points  of  insertion 
ufthe  arms. 


~£Q^abr 


radials.  Tegmen  calycis  usually  much  arched,  consisting  of  numerous  firmly 
connected  plates,  some  of  which  at  least  are  large,  arranged  in  definite  order.  The 
ambulacra  of  the  tegmen  calycis  with  their  skeleton  hidden,  or  only  visible  in  forms 
with  flat  tegmina.  Anus  subcentral.  Orals  usually  distinct.  Carpocrinus,  Agarico- 
ff/iiiis,  Pcricchocrinus,  Jfogistoeriwus,  Adinocrinus,  Teleiocrinus,  Steganocrinus, 
Amphoracriniu,  Physetocrinus,  Strotocriiius,  Batocrinus  (Fig.  253),  Erctiuocriiius. 
Dorycrinax. 

Family  6.  Platycrinidse. 

Base  monocyclic,  3  basals,  which  are  unequal.     Anal  and  interradial  plates  not 
in  contact  with  the  basals.     The  very  large  radials  together  with  the  basals  form 


308 


COMPARATIVE  ANATOMY 


CHAP. 


almost  the  whole  of  the  apical  capsule.  Each  radial  is  connected  with  a  short  and 
small  costal  plate.  The  various  brachials  which  follow  (distichals,  palmars,  etc.) 

are  free,  i.e.  belong  to  the  freely  out- 
standing arms.  In  each  interradius 
there  are  at  least  three  interradials, 
which,  however,  appear  more  or  less 
shifted  on  to  the  oral  side.  In  the 
proximal  (apical)  interradial  ring  there 
are  no  special  anal  plates,  this  ring 
consisting  in  each  interradius  of  3-5 
transversely  placed  plates,  the  central 
one  being  the  largest.  Orals  large. 
Tegmen  calycis  mostly  much  arched. 
The  ambulacra  and  their  covering 
plates  often  appear  at  the  surface. 
Anus  subcentral.  Platycrinus  (Fig. 
254),  Marsupiocrinus,  Eudadocrinus. 

Family  7.  Crotalocrinidse.1 

Base  dicyclic.  The  apical  capsule 
consists  exclusively  of  the  typical 
plates  of  the  apical  system  (infrabasals, 
basals,  and  radials),  to  which  is  added 
an  anal  plate.  The  brachials  of  the 
separate  rays  (to  the  fourth  order) 
firmly  united  by  sutures.  Arms  very 
mobile,  uniserial,  long  and  much 
branched  ;  branches  free  or  connected 
together  in  such  a  way  as  to  form  a  net- 
work around  the  calyx  ;  this  network 
is  either  continuous  or  else  divided  into 

five  leaf-like  lobes  corresponding  with  the  rays.  Arms  and  their  branches  traversed 
by  large  axial  canals.  Tegmen  calycis  flat,  richly  plated  with  distinct  orals,  iuter- 
radials,  and  anals  ;  ambulacra  externally  visible,  with  large  rigid  covering  plates, 
which  combine  with  the  other  plates  to  form  the  solid  teginen.  Anus  subcentral. 

(This  family  is  distinguished  from  all  other  Camerata  by  the  presence  of  axial  canals, 
and  by  the  mobility  of  the  free  joints  of  the  arms.)     Crotalocrinus,  Eimllocrinus. 


CO 


FIG.  254.— Platycrinus  triacontadactylus  (after 
M'Coy).  di,  Distichals ;  c,  costals ;  r,  radial ;  ba, 
basal ;  co,  stem  ;  ir,  interradials ;  vk,  ventral  capsule. 


Family  8.   Hexacrinidae. 

Base  monocyclic.  2  or  3  basals.  The  first  anal  plate  rests  on  the  circle  of 
basals,  and  resembles  the  radials  in  shape.  In  other  respects  like  the  Platycrinida.'. 
Hexacrinus,  Talarocrinus,  Dichocrinus. 


Family  9.  Acrocrinidae. 

Base  monocyclic.  2  basals,  separated  from  the  radials  by  a  broad  zone  of 
small  plates  arranged  in  circles  round  the  basals  ;  these  form  the  largest  part  of 
the  apical  capsule.  Each  radial  is  followed  by  2  costals.  The  radials  and 

1  This  family,  originally  placed  near  Cyathocrinus,  was  referred  by  Wachsmuth  and 
Springer,  first  to  the  Ichthyocrinoidse  aud  then  to  the  Camerata  ;  Bather,  however, 
would  refer  it  to  its  original  position  in  the  Inadunata. 


viii  ECHINODERMATA— SYSTEMATIC  REVIEW  309 

costals  of  the  5  rays  laterally  distinct.  Interradials  in  two  circles  ;  in  the  first 
circle  there  are  two  plates  to  each  interradius,  and  in  the  second  circle  only  one, 
which,  however,  is  larger  than  the  former  two.  Posterior  interradius  much  larger, 
with  twice  as  many  interradials,  between  which  there  is,  further,  an  intercalated 
vertical  row  of  anal  plates.  Acrocrinus. 

» 

Family  10.  Barrandeocrinidae. 

Base  monocyclic.  3  basals.  The  first  anal  plate  rests  on  the  circle  of  basals. 
The  interradials  rest  upon  the  sloping  oral  ends  of  the  radials.  Arms  bent  back  on 
the  calyx,  fusing  laterally  with  one  another  by  means  of  their  pinnulse  in  such  a 
way  as  to  form  a  firm  envelope  around  the  calyx.  Barrandeocrinus. 

Family  11.  Eucalyptocrinidse. 

Base  monocyclic.  The  apical  capsule  consists  of  4  basals,  5  radials,  2x5 
costals,  2  x  10  distichals,  3x5  interradials,  and  1x5  interbrachials.  Xo  anal 
plates.  The  tegmen  calycis  consists  of  5  large  interradials,  5  large  and  10  small 
interbrachials,  the  oral  plates,  and  two  other  plates  lying  further  up  towards  the 
apex.  Anus  shifted  quite  to  the  centre.  The  plates  of  the  tegmen  form  10  niches  ; 
in  the  bases  of  these  niches  ambulacral  grooves  (two  in  each)  run  to  the  bases  of 
the  10  pairs  of  arm-branches,  which  are  received  into  the  niches.  Eucalyptocrinus. 
CktUierinus. 

Order  3.  Articulata  (Ichthyocrinidae). 

Skeleton  flexible.  Anal  plates  often  occur  in  the  posterior  interradius  of  the 
calyx.  Base  dicyclic.  Three  infrabasals  of  unequal  size,  which  are  usually  hidden 
by  the  uppermost  joint  of  the  stem.  Radials  perforated,  with  one  or  more 
primai'y  brachials.  The  circle  of  the  combined  radials  and  primary  brachials  is 
closed,  or  else  interrupted  by  one  or  more  plates  in  each  interradius.  The  brachials 
of  the  first,  second,  and  often  also  of  the  third  order  are  incorporated  in  the  calyx. 
The  radials  and  the  separate  brachials  are  articulated  together.  Arms  uniserial. 
Pinnule  appear  to  be  wanting.  Interradials  irregular  and  varying  in  shape,  size, 
and  arrangement,  inconstant  (may  be  either  present  or  wanting  in  one  and  the 
same  species).  In  the  posterior  interradius  there  is  often  one  asymmetrical  plate. 
Tegmen  calycis  only  known  in  a  few  forms,  soft  and  flexible,  the  plates  lying  in  it 
not  being  firmly  fused  together.  Five  separate  orals  of  unequal  size  grouped  round 
the  open  mouth,  the  posterior  oral  being  the  largest.  Ambulacra  with  their  cover- 
ing plates  appear  at  the  surface.  Between  them,  there  are  interambulacral  plates 
which  are  occasionally  distinguished  by  their  remarkable  size.  Interambulacral 
areas  often  sunk.  Food  grooves  of  the  arms  enclosed  by  movable  covering  plates. 
A  plated  process  (anal  tube  with  anus  ?)  is  found  excentrically  in  the  posterior 
interradius  of  the  tegmen. 

Fam.  Ichthyocrinidae  —  Palseozoic  forms  :  Ichthyocrimis,  Forbesiocrinus,  Gleio- 
cri  iiu.s,  Taxocrinus  (Fig.  255),  etc. 

The  unstalked  genus  Uintacriiius,  from  the  upper  Chalk,  and  the  extant  unstalked 
genus  Thaumatoci-inus  (Fig.  256),  ought  probably  to  be  classed  here.  In  the  latter 
the  uppermost  ossicle  of  the  stem  is  retained  as  centrodorsal.  The  dorsal  cup 
consists,  apart  from  the  centrodorsal,  of  5  basals,  5  radials,  and  5  interradials, 
which  last  rest  on  the  circle  of  basals,  and  alternate  with  the  radials.  Tegmen 
with  central  open  mouth,  which  is  protected  by  a  pyramid  of  5  large  separate 
orals.  Between  the  orals  and  the  edge  of  the  calyx  (or  the  oral  edge  of  the 
interradials  of  the  dorsal  cup)  the  tegmen  is  covered  with  small  irregular  plates 


310  COMPARATIVE  ANATOMY  CHAP. 

indistinctly  arranged  in  two  to  three  rows.      The  anal  interradial  carries  a  short 


ir 


CO 


FIG.  255.— Taxocrinus  multibrachiatus,  Ly. 
and  Cass.  ir,  ir^,  and  ir.2,  Interraclials ;  di,  dis- 
tichals ;  ba,  basals  ;  ib,  infrabasals  ;  co,  stem  ;  r, 
radials  ;  c\,  Co,  and  03,  primary  brachials. 


FIG.  256.—  Thaumatocrinus  renovatus. 
P.  H.  C.  (after  P.  H.  Carpenter).  Calyx 
from  the  anal  side,  cj,  c-2,  and  03,  Primary 
brachials  ;  r,  radials  ;  c«,  points  of  insertion 
of  the  cirri ;  cd,  centrodorsal ;  ir,  inter - 
radials  ;  ia,  interradialia  analia  ;  pa,  proces- 
ses analis  ;  ta,  tubus  analis  ;  p,  pimmlee. 


jointed  appendage, 
arms  with  pinnulre. 


Besides  this   there  is  a  short   anal   tube.     Five   unbranched 


Order  4.  Canaliculata. 


Calyx  symmetrically  five-rayed.  Base  dicyclic,  the  infrabasals  usually  not 
separate,  but  atrophied  or  fused  with  the  proximal  columnal  5  basals,  occasionally 
not  externally  visible.  Each  radial  is  followed  by  2  costals.  Anal  plates  always 
wanting  (hence  the  regularity  of  the  calyx).  Interradials  with  few  exceptions 
wanting.  Arms  simple  or  divided  (one  to  ten  times).  Tegmen  calycis  usually  flat, 
with  open  mouth  and  ambulacra  appearing  at  the  surface.  Orals  rarely  present. 
Tegmen  calycis  often  plated  with  small  loose-lying  plates.  Stem  present  either  only 
in  young  forms  or  also  in  adults.  Basals  and  radials  perforated  by  dorsal  canals. 
To  this  order  belong,  besides  Mesozoic  and  Tertiary  forms,  most  of  the  extant 
Crinoids. 

Family  1.  Apiocrinidae. 

Calyx  consists  of  5  basals  of  equal  size,  5  radials  and  2x5  primary  brachials. 
Distichals  may  also  take  part  in  its  formation.  Interbrachials  and  interdistichals 
may  occur.  Tegmen  flexible,  with  small  plates.  Arms  more  or  less  branched,  con- 
sisting of  a  single  row  of  joints.  Stem  without  cirri,  usually  expanding  in  its 
proximal  region  to  the  same  width  as  the  calyx,  but  not  containing  the  viscera. 
Jurassic,  to  present.  Apiocrinus,  Millericrinus,  and  the  extant  Calamocrinus. 


Family  2.  Bourgueticrinidse. 

Calyx  consists  of  5  basals  and  5  radials.  Brachials  connected  in  pairs  by 
syzygial  sutures.  Five  orals  in  the  tegmen  calycis.  Interambulacral  region  other- 
wise not  plated.  Ambulacra  with  covering  plates,  but  without  lateral  plates.  Stem, 


viii  ECHINODERMATA— SYSTEMATIC  REVIEW  311 

with  root-like  processes  at  its  base,  or  with  irregularly  arranged  cirri :  its  proximal 


FIG.  257.— Metacrinus  Murray!  (after  P.  H.  Carpenter).     Most  of  the  arms  and  the  larger  ];ait 
of  the  stem  broken  off.    p,  Pinnulse  ;  ci,  cirri :  ng,  node. 

ossicle  usually  enlarged.     Upper  Jurassic,  Chalk.  Tertiary,    Recent.     Ehizocrinus, 
Bourguetiarimu. 


312 


COMPARATIVE  ANATOMY 


CHAP. 


FIG.  259.— A,  Cystoblastus  Leuchtenbergi. 
1,  Interradial ;  2,  3,  radial ;  9,  basal ;  10,  infra- 
basal  ;  8,  anus  ;  6,  genital  aperture.  B,  From 
the  oral  side  (after  Volborth).  4,  Mouth;  5, 
ambulacrum.  Fig.  295,  p.  332,  shows  the  apical 
side. 


;.  -jr.*.— Ante  don  incisa  (after  P.  H. 
Carpenter).     1,  Anns;  2,  cirri. 


FIG.  260.— Protocrinus  oviformis,  Eicliwald 
(after  Volborth).  2,  Anns  ;  1,  tliird  aperture  ;  3. 
ambulacrum. 


VI II 


ECHINODERMATA— SYSTEMATIC  REVIEW 


313 


Family  3.  Pentacrinidse. 

Calyx  small  as  compared  with  the  stem  and  the  arms  ;  it  consists  of  5  basals 
and  5  radials.  (In  the  genus  Extracrirms  the  infrabasals  are  separate).  Rays 
divided  one  to  ten  times.  Stem  surrounded  at  intervals  by  whorls  of  cirri.  No 
root-like  processes  on  the  stem.  One  or  more  free  primary  brachials.  Orals  wanting 
in  the  adult.  Trias,  to  Recent.  Pcntacnnus,  Metacrinus  (Fig.  257),  Extracrinus, 
Balanocrinus. 

Family  4.  Comatulidse. 

Adult  free,  larva  stalked.  The  calyx  is  closed  apically  by  the  uppermost  ossicle 
of  the  larval  stem,  which  is  fused  with  the  larval  infrabasals  ;  this  ossicle  carries  cirri 
and  becomes  detached  from  the  rest  of  the  stem.  It  is  called  "centrodorsal." 
The  basals  are  externally  visible,  or  else  form  an  internal  hidden  rosette.  Five  or 
ten  simple  or  branched  rays.  The  radials  of  the  radial  circle  are  usually  followed, 
in  forms  with  divided  arms,  by  two  fixed  primary  brachials.  Interradials  wanting. 
Orals  wanting  in  the  adult.  Atchcrinus  (basals  externally  visible),  Eudiocrinus. 
Antcdon  (Fig.  258),  Promachocrinus,  Adinomctra  (the  only  Crinoid  genus  with 
excentric  mouth).  Since  Jurassic  times,  many  living  species. 


SUB-CLASS  2.  Cystidea. 

Body  (calyx)  oviform  or  spherical,  plated  with  numerous  very  variously  shaped 
pieces,  which  are  rarely  quite  regularly,  and  often  irregularly  arranged  ;  stalked,  sessile. 
or  (rarely)  free.  Arms  in  many  cases  unknown,  perhaps  wanting  in  many  forms  ;  when 
present,  weakly  developed,  resembling  pinnules,  and  rising  near  the  mouth.  Food 


* 


FK;.  2(51.— Orocystis  Helmhackeri. 

Baur  (after   Barrande).     1-3,   The  Fm. -j^.-Agelacrinus  cincinnatensis. 

three  apertures. 

grooves,  arranged  irregularly  on  the  calyx,  radiate  from  the  mouth.  At  some  dis- 
tance from  the  mouth  a  second  aperture  (anal  aperture),  and  between  the  two  a  third 
aperture  of  unknown  significance.  Double  pores  or  "  pectinated  rhombs  "  on  some  or 
all  of  the  plates.  Palaeozoic  Pelmatozoa,  whose  organisation  is  still  little  understood. 

Order  1.  Cystocrinoidea  (cf.  the  section  on  the  perisomatic  skeleton  of  the 
C instilled)  :  Pcrocrinus,  Canjocrinus,  Echinocncnnus,  CystoWastus  (Fig.  259  A  and  B). 

Order  2.  Eucystidea  :  Protocrinus  (Fig.  260),  WypfnspJuerites,  Orocystis  (Fig. 
261),  EchinoqpJuera,  Ari*t<icystis,  Ascocystis,  Mcsit*'.-*.  Afi>'I<i<>riinis  (Fig.  262). 


314 


COMPARATIVE  ANATOMY 


CHAP. 


SUB-CLASS  3.  Blastoidea. 

Armless   Pelmatozoa,   either  pear-shaped,   club-shaped,    oviform,   or  spherical. 
Body  usually  regularly  radiate.      Base  mouocyclic.     Three  basals,   one  small  and 


FIG.  263.— Pentremites,  from 
the  side,  without  pinnules.  1, 
Interradial  =  deltoid  ;  2,  3,  radials  ; 
4,  basal  ;  5,  ambulacrum  ;  6,  spir- 
acle. 


FIG.  265.— Codaster  bilobatus,  M'Coy,  from  the  oral 
side  (after  Etheridge  and  Carpenter).  1,  Hydrospiiv 
slits  ;  2,  lateral  plates  ;  3,  ambulacral  groove  ;  4,  mouth  ; 
5,  radial ;  6,  suture  between  two  radials;  7,  anus;  8,  inter- 
radial ;  9,  ridge  on  an  interradial. 


Fio.    264.  —  Granatocrinus   Norwood! 
(after  Etheridge  and    Carpenter);    from  FIG.  266.-Orophocrinus  stelliformis  (after  Ethe- 

the  apical  side,  with  stem.  ridge  and  Carpenter) ;  from  the  oral  side.    1,  Lateral 

plates  ;  2,  covering  plates  of  the  ambulacra  ;  3,  hydro- 
spire  slits  ;  4,  anus  ;  5.  ambulacral  groove  ;  6,  points 
of  attachment  of  the  pinnules. 

two  larger.     Five  radials,  more  or  less  deeply  cut  out  for  the  reception  of  the  five 
ambulacra.     Five  interradials  lying  above  the  five  radials,  and  surrounding  the 


VIII 


ECHIXODEKMA  TA  —S  YS  TEMA  TIC  RE  1  'IE  11 ' 


315 


peristome.  One  of  these  is  perforated  by  the  anus.  The  ambulacra  are  bordered 
along  each  side  by  a  single  or  double  longitudinal  row  of  jointed  pinnule-like 
appendages.  Ambulacra  with  lateral  and  accessory  lateral  plates.  In  each  ambu- 
lacrum, under  the  lateral  plates,  there  is  a  lancet-like  piece,  which  is  penetrated 
lengthwise  by  a  canal,  and  in  which  a  radial  ambulacral  vascular  trunk  probably 
ran.  Ten  groups  of  ' '  hydrospires "  on  the  radials  and  interradials.  Peristome 
covered  by  small  plates,  which  are  continued  into  the  covering  plates  of  the  ambu- 
lacra. For  details  cf.  the  section 
on  the  Skeletal  System,  p.  3:2 S. 
Pal.eozoic  forms. 


Order  1.  Regulares. 

Stalked    Blastoids    with    sym- 
metrical base.    The  radials  resemble 
one  another,  as  do  the  ambulacra. 
Fam.    1.    Pentremitidse :     Pen- 
<  (Fig.   263),  Pcnti-fiiiiti'l:".. 
Fam.     2.     Troosto- 
blastidae  :    Tr<x>*to<:rinn3.  M> /<//////>•- 
//'*.  etc.    Fam.  3.  Nucleoblastidae  : 
ttttS,    Xdir.ijllastux.     ' 
.    Fam.  4.  Granatoblastidse : 

"S   (Fig.    264  .    . 

iili.istiis.      Fam.     5.    Codasteridse : 
•  /•  (Fig.  265),  PJw:nox<:lii*in<i, 
Ci-ifptosch  isnia.    Oropho<:rin  vs   (Fig. 
266). 


FIG.  2i57.—  Astrocrinus  Benniei  (after  Etheridge 
and  Carpenter).  1,  4,  5,  Interradials  or  deltoid  plates  ; 
2,  radials  :  6,  the  modified  radial ;  3,  ambulacrum  :  ;». 
the  modified  ambulacrum  ;  7,  basal ;  S.  notch-like  sinus. 


Order  '2.  Irregulares. 

Unstalked   Blastoids,  in  which  one  ambulacrum  with  its  radial  is  differently 
developed  from  the  rest. 

Single   family,    Astrocrinidse  :    EJevth'  rocrinus,  Astrocrinus   (Fig.    267),   l'>nt>:- 


I.  General  Morphology  of  the  Eehinoderm  Body. 

The  body  of  most  Echinoderms,  superficially  observed,  appears  to 
be  of  strictly  radiate  structure,  but  more  careful  examination  reveals 
that  even  in  apparently  perfectly  radiate  forms,  e.g.  regular  Sea-urchins 
and  Star-fish,  strict  radiate  symmetry  is  not  found  either  in  the 
external  or  in  the  internal  organisation  ;  in  the  latter,  indeed,  the 
asymmetry  is  evident.  Nevertheless,  in  order  to  facilitate  a  simple 
description  of  the  position  and  arrangement  of  the  organs,  terms  are 
habitually  used  which  assume  a  strictly  radiate  structure.  For  the 
purposes  of  description  we  may  imagine  the  Eehinoderm  body  to 
be  spherical  or  egg-shaped.  Two  poles  may  be  distinguished  in  it. 
At  the  oral,  ^faetinal,  or  ventral  pole  there  lies,  in  most  Echinoderms. 
the  oral  aperture,  while  at  the  other  apical,  abaetinal.  or  dorsal 
pole  in  many  forms  is  found  the  anal  aperture.  The  line  which 
connects  the  oral  and  apical  poles  is  called  the  principal  axis. 


316 


COMPARATIVE  ANATOMY 


CHAP. 


Round  this  principal  axis  many  important  parts  of  the  body  are 
grouped  in  a  radiate  manner.  The  typical  number  of  the  rays  is, 
with  few  exceptions,  five.  In  the  Echinoderms,  as  in  the  radiate 
Coelenterates,  rays  of  the  first,  second,  and  third  order  may  be  distin- 
guished. The  radii  or  radial  regions  of  the  first  order,  in  which  the 
principal  organs  lie,  are  called  perradii,  ambulacral  radii,  or  simply 
radii.  The  five  radii  of  the  second  order,  which  regularly  alternate 
with  these  five  principal  radii,  are  the  interradii  or  interambulaeral 


FIGS.  2(38  and  209.— Representatives  of  the  principal  divisions  of  the  Echinodermata.  In 
Fig.  268,  in  the  morphological  position  ;  in  Fig.  269,  in  the  natural  position  with  regard  to  the 
sea-floor.  A,  Holothurian.  B,  Sea-urchin.  C,  Star-fish.  D,  Crinoid— a,  Apical  pole  ;  o,  oral 
pole ;  an,  anus. 

radii.  The  far  less  important  ten  radii  of  the  third  order,  each  of 
which  lies  between  a  perradius  and  an  interradius,  may  be  called 
adradii.  Between  the  two  poles,  at  right  angles  to  the  principal  axis, 
we  have  the  equator.  In  those  Echinoderms  which  are  provided  with 
large  skeletal  plates,  the  body  and  skeleton  is  further  divided  into  two 
zones,  separated  from  one  another  by  the  equator ;  these  are  the  oral, 
adactinal,  or  ventral  zone,  and  the  apical,  abaetinal,  or  dorsal  zone. 
In  the  centre  of  the  former  lies  the  mouth. 


vin         ECHIXODEHMATA  —  MORPHOLOGY  OF  SKELETON        317 

While  these  terms  facilitate  the  morphological  description  of  the 
body  they  do  not  take  into  account  its  position  in  the  water,  or 
with  regard  to  the  sea-floor,  which  is  assumed  to  be  horizontal. 
Thus  the  normal  position  of  the  Star-fish  and  Sea-urchin  is  such  that 
the  oral  zone  is  directed  downwards  and  the  apical  zone  upwards  ; 
while  the  very  reverse  is  the  case  in  the  Crinoids,  where  the  oral 
zone  faces  upwards  and  the  body  is  attached  to  the  substratum  by  a 
stem  which  is  inserted  at  the  apical  pole.  In  the  Holothurians,  again, 
the  principal  axis  of  the  body  lies  parallel  to  the  substratum,  and  the 
oral  pole  forms  its  anterior,  the  apical  pole  its  posterior  end. 

For  particulars  as  to  the  form  of  the  body  and  the  external 
organisation  of  the  various  classes  and  orders  of  the  Echinodermata, 
cf.  the  Systematic  Review,  and  also  specially  the  two  sections  which 
treat  of  the  skeletal  and  ambulacral  systems. 


II.  Morphology  of  the  Skeletal  System. 
Meaning  of  the  Most  Important  Lettering  of  the  Figures. 

"  Apical  pole.  ian  Anal  interradials  or  anals. 

a  in  Ambulacral  plates.  ib  Infra basals. 

a.  it  Anus  or  anal  area.  i>./  Interdistichals  or  intersecuudi- 

ap  Ambulacral  pores.  brachs. 

B  Buccal  plates.  ir  Interradials. 

IKI.  Basals.  m  Madreporite,    pore  -  openings   of 

In-  Brachials,  arms.  the  stone  canal, 

q  First  costal  or  primibrach.  n  Xodal  columnal. 

'•_,  Second  costal  or  primibrach.  o  Oral  pole,  mouth. 

ca  Points  of  insertion  of  the  cirri.  or  Orals,  or  mouth-plates. 

cd  Centrodorsal.  p  Pinnules. 

ce  or  c  Central  plate.  pa  Anal. 

ci  Cirri.  rs  Radial  shields. 

co  Column,  stem.  r  Radial s. 

i-p'.''  Covering  plates  of  the  ambulacral        ss  Lateral  shields. 

grooves.  t  Terminals. 

D  Dentes,  teeth.  ta  Anal  tube  or  ventral  sac. 

ii<:  Dorsocentral.  vfc  Tegmen  calycis. 

fU  Distichal  or  secundibrach.  1-5  Interradii     or     interambulacral 

ds  Dorsal  shields.  areas  of  the  Echinoidea. 

<j<j  Genital  aperture.  I-V  Radii  or  ambulacral  areas  of  the 

ia  Interambulacral  plates.  Echinoidea. 

(In  many  of  the  diagrams  of  the  apical  system  of  various  Echiuoderms  the 
infrabasals  are  dotted,  the  basals  shaded  with  concentric  lines,  and  the  radials 
marked  black.  The  brachials  of  the  Crinoids  are  shaded  with  radial  lines. ) 


Introduction. 

The  extensive  comparative  and  ontogenetic  researches  which  have 
been  made  on  the  Echinoderms  have  shown  that  it  is  to  some  degree 


318 


COMPARATIVE  ANATOMY 


CHAP. 


probable  that  certain  pieces  or  plates  of  the  skeleton  are  homologous 
in  all  the  divisions.  It  may  be  assumed  that  these  plates  composed 
the  primitive  Echinoderm  skeleton.  From  this  primitive  arrange- 
ment, the  skeletons  of  all  known  Echinoderms,  whether  extant  or 
extinct,  appear  to  be  derived,  on  the  one  hand,  through  the  loss  of 
certain  pieces  of  this  primitive  skeleton,  and,  on  the  other  hand,  by 
the  acquisition  of  new  or  secondary  pieces  of  varied  form,  number,  and 
arrangement. 

The  hypothetical  primitive  Echinoderm  skeleton  consists  of  two 
principal  groups  or  systems  of  plates  :  (1)  the  oral,  and  (2)  the 
apical. 

The  oral  system  has  five  interradially  placed  oral  plates, 
arranged  radially  round  the  oral  pole.  This*  oral  system  develops 

round  the  left  ccelomic  vesicle, 
out  of  which  the  oral  portion 
of  the  coelom  rises. 

In  the  apical  system  the 
following  plates  occur:  (1)  a 
central  plate  at  the  apical 
pole  ;  (2)  a  circle  of  five 
radially  placed  plates,  the 
infrabasals  ;  (3)  alternating 
with  these,  five  interradially 
placed  plates,  the  basals  ;  and 
(4)  around  these,  five  radially 
placed  plates,  the  radials. 
The  apical  system  develops 
on  the  right  coelomic  vesicle, 
from  which  the  apical  portion 
°^  tne  coelom  is  derived. 

The      Stalked       larva      Of 
A^to^y,        /T?,'n,         O7A\        I, 
Anted0n        (*  Jg-        2  '  0)        has 

retained  this  System  of    plates 

less  altered  than  in  any 
known  Echinoderm.  It  has  all  the  typical  pieces  of  the  oral  and 
apical  (or  aboral)  system  of  plates. 

All  the  skeletal  plates  of  the  Echinodermata  consist  of  carbonate 
of  lime.  Their  microscopical  structure  is  very  characteristic,  so  fhat 
small  fragments  can  at  all  times  be  recognised  and  distinguished 
from  similar  fragments  belonging  to  the  skeletons  of  other  animals. 
The  structure  is  a  sponge-work  ;  and  thin  sections  of  the  skeletal  plates 
or  of  the  microscopic  calcareous  bodies  appear  to  be  perforated  in  a 
lattice-like  manner.  The  finer  structure,  especially  of  the  spines  of 
Sea-urchins,  is  of  great  systematic  importance. 


Fi«;.  LTO.-  Diagram  of  the  apical  system  of  the 
Antedon  larva,  combined  from  various  stages.  Ex- 
planation  of  the  lettering  on  p.  317.  The  number  of 
infrabasals  is  here  shown  as  3,  but  these  are  produced 
by  fusion  of  5,  which  number  has  also  been  seen. 


vin         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        319 

A.  The  Apical  System  (Calyx). 
I.  Echinoidea. 

The  part  of  the  test  which  in  the  Sea-urchins  is  formed  by  the  apical 
system  varies  greatly  in  size.  In  the  older  and  more  primitive  forms, 
the  regular  Echinoidea,  it  is  still  somewhat  extensive  as  compared  with 
the  rest  of  the  test  (Fig.  271),  but  in  modern,  especially  irregular  forms 
(L'l^.-adridix  and  Spantangidce),  it  continually  diminishes  in  relative 
size  till  it  is  nothing  more  than  a  minute  region  at  the  apical  pole.  It 
is  possible  to  deduce  the  apical  system  of  the  Echinoidea  directly  from 
the  hypothetical  primitive  form  by  the  help  of  certain  Saleniidce  (Fig. 
272).  It  is  true  that  in  the  apical  system  of  this  family,  as  in  that 


"\^\^=Si&MQ0'  FIG.  272.— Salenia  sp.    Apical  sys- 

I  tern  (after  Loven).     For  lettering  see 

*  G       Q        &  $  P-  317. 

FIG.  271.— Tiar echinus  princeps,  Laube  (after  Loven). 
1,  Genital  aperture  ;  2,  anus  ;  3,  basal ;  4,  radial ;  5, 
ambulacrum ;  6,  the  three  upper  plates  of  an  inter- 
ambulacrum. 

of  all  other  Echinoidea,  the  infrabasals  are  entirely  wanting,  but  all 
the  other  typical  plates  are  present  :  i.e.  a  central  plate,  and  round  it 
five  basals,  and  outside  these,  alternating  with  them,  five  radials.  In 
the  right  posterior  interradius  each  of  the  three  plates,  the  central 
and  the  two  basals,  is  incomplete  at  the  point  where  they  meet.  A 
circular  region,  the  anal  region,  in  which  the  anus  lies,  is  thus  formed. 
The  anus,  therefore,  here  lies  asymmetrically  in  the  apical  system,  and 
this  is  the  case  in  most  Palceechinoidea  and  in  most  regular  Euechinoidea. 
According  to  the  universally  accepted  terminology,  it  lies  in  the  right 
posterior  interradius. 

The  typical  system  above  described  for  the  adult  Saleniidce  has 
been  found  to  be  repeated  in  very  young  specimens  of  other  Sea- 
urchins  examined  for  this  purpose  (Echinus,  Fig.  273  ;  Toxopneustes, 
Fig.  274). 

Apart  from  these  cases,  where  a  primitive  condition  is  shown  by 


320 


COMPARATIVE  ANATOMY 


CHAP. 


the  presence  of  the  central  plate,  most  regular  Echinoidea  show  the 
following  typical  composition  of  the  apical  system :   In  the  centre  of 


CJO 


cvm. 


FIG.  273. —  Echinus  sp.  (1  to  -2  mm.  long). 
Apical  system  (after  Loven).  For  lettering  see 
p.  317. 


FIG.  274.  —  Toxopneustes  drcebachiensis, 
O.F.M.  (10  mm.  long).  Apical  system  (after 
Loven).  For  lettering  see  p.  317.  sh,  Tubercles 
carrying  spines  ;  ap,  anal  plates. 


the  system  lies  the  anal  area,  with  a  few  large,  or  many  small,  calcareous 
plates.     A  central  plate  cannot   be  distinguished.      Within  the  anal 

area  lies  the  anal  aperture, 
usually  excentric,  less  fre- 
quently central.  Round  the 
anal  area  are  found  the 
circles  of  plates  present  in 
all  Echinoids,  viz.  the  proxi- 
mal circle  of  five  basal 
plates,  and  the  distal  circle 
of  five  radial  plates  (Fig. 
275).  One,  or  several,  or 
even  all  of  the  radials  may, 
however,  become  wedged  in 
between  the  basals  apically, 
and  finally  may  take  part 
in  the  limitation  of  the  anal 
area. 

The   ontogeny  of    Toxo- 

FIG.    275.  —  Toxopneustes   drcebachiensis,    O.F.M.    pneustes  shows   that   there  is 

Forlettering  at  first  in  the  anal  area  of 
very  young  Echinoidea  one 
large  central  plate  (Fig.  274).  Near  this  central  plate,  which  ceases 
to  grow  and  degenerates,  accessory  plates  appear.  Among  these 


vin       .EGHIXODERMATA— MORPHOLOGY  OF  SKELETON        321 

accessory  plates,  which  continually  increase  in  number,  the  anal 
aperture  then  forms,  somewhat  excentrically  (Fig.  274).  After  a  time 
the  central  plate  can  no  longer  be  distinguished  from  the  accessory 
plates. 

As  a  rule,  i.e.  in  the  greater  number  of  eases,  the  basals  and 
radials  attain,  in  Echinoidea,  the  following  special  significance : 

1.  Each   basal  is   perforated   by  a  large  pore  or   hole,  through 
which   one   of  the  five  genital  glands  opens   externally.     On  this 
account  the  basals  of  the  Echinoidea  have  long  and  almost  universally 
been  called  genital  plates. 

2.  Each  radial  is  also  perforated  by  a  narrow  canal,  which  opens 
at    its  surface  through  a  single   (rarely   double)   aperture.      In   this 
canal  lies  the  terminal  tentacle  of  the  water  vascular  system,   the 
frequently  pigmented  end  of  which  projects   somewhat  beyond  the 
aperture.     Since  these  collections  of  pigment  were  formerly  considered 
to  be  eyes,  the  plates  (radials)  carrying  them  were  called  the  ocular 
plates. 

3.  The  fine,  and  usually  very  numerous,  apertures  of  the  water- 
vascular  system  perforate  one  of  the  five  basals  (genital  plates),  which 
becomes  the  madreporite  (m).     This  is  the  right  anterior  plate. 

It  must,  however,  be  noted  that  (1)  the  genital  apertures  are  not 
necessarily  connected  with  the  basal  (genital)  plates. .  The  latter 
must  not  be  regarded  as  terminal  appendages  of  the  genital  ducts, 
but  as  independent  portions  of  the  test.  For  (a)  the  basals  are  solid 
when  first  developed,  and  are  only  perforated  by  the  genital  pores 
after  the  genital  ducts  have  completely  developed ;  (b)  the  genital 
apertures  in  some  Echinoidea  lie  outside  the  basals.  For  example, 
among  the  Clypeastroida,  in  some  species  of  the  genera  Laganum,  Encope, 
MeHita,  etc.,  the  genital  pores  lie  outside  the  apical  system,  between 
its  edge  and  the  first  two  interradial  plates  which  border  it ;  in 
L'liipe'.itter  rosaceus,  they  lie  in  the  five  sutures  between  the  interradial 
plates,  and  are  separated  from  the  apical  system  by  two  or  three  pairs 
of  interambulacral  plates  (Fig.  277) ;  and  further,  in  another  true 
Echinoid,  GoniopyguSj  the  genital  apertures  lie  interradially  quite  outside 
the  whole  apical  system.  (2)  The  madreporite,  through  which  water 
flows  into  the  stone  canal,  is  not  necessarily  exclusively  connected  with 
the  right  anterior  basal  (genital)  plate.  On  the  contrary,  the  neigh- 
bouring genital  plates,  indeed,  all  the  five  plates,  and  in  isolated  cases, 
even  the  neighbouring  interradial  plates  of  the  corona  may  be  perforated 
by  the  afferent  ducts  of  the  stone  canal.  In  Palceechinus  each  basal  plate 
is  perforated  by  three  pores,  which  are  perhaps  apertures  of  the  stone 
canal,  perhaps  genital  apertures,  or  else  partly  the  one  and  partly  the 
other.  In  no  case,  however,  do  the  madreporic  apertures  extend  to 
the  radials  (ocular  plates). 

In  the  Echinoidea  the  primitive  character  and  especially  the 
radiate  structure  of  the  apical  system  may  be  more  or  less  strongly 
modified.  The  original  cause  of  such  modification  is  principally  to 

VOL.  II  Y 


322 


COMPARATIVE  ANATOMY 


CHAP. 


be  sought  in  the  shifting  of  the  anus  and  anal  area  out  of  the 
apical  system  into  the  posterior  interradius  ;  by  this  shifting  the  anus 
may  come  to  lie  at  any  point  between  the  (aboral  or  dorsal)  apical 
system  and  the  oral  (or  ventral)  area.  In  its  posterior  and  downward 
shifting  the  anus  thus  does  not  carry  the  apical  system  with  it,  but 
the  latter  remains  on  the  dorsal  side,  although  it  is  often  shifted 
somewhat  excentrically  anteriorly,  rarely  posteriorly.  The  whole  body 
is  then  bilaterally  symmetrical,  and  when  seen  from  above  it  is  oval  or 
heart-shaped,  etc.,  in  outline.  The  line  connecting  the  mouth  with  the 
anus,  which  in  the  regular  endoeyelie  Echinoidea  altogether  or  nearly 


in 


FIG.  276.— Holectypus  depressus,  Cot- 
teau.  Apical  system  and  neighbouring 
parts  of  the  perisome  (after  Lov&n).  For 
lettering  see  p.  317. 


FIG.  277.— Clypeaster  rosaceus,  L.  Apical 
system  and  neighbouring  parts  of  the  perisome 
(after  Lov&n).  For  lettering  see  p.  317. 


coincides  with  the  vertical  (principal)  axis,  now  becomes  the  more  in- 
clined, i.e.  approaches  the  more  nearly  to  the  horizontal,  the  further  the 
anal  aperture  is  removed  from  the  apical  system  into  the  posterior 
interradius,  and  is  shifted  on  to  the  under  side  (into  the  oral  or  actinal 
region).  Those  Echinoidea  in  which  the  anal  aperture  has  been 
shifted  outside  the  apical  system  are  called  exoeyclie  or  irregular. 

Among  the  Palceechinoidea  the  genus  Ecliinocystis  (Cystocidaris) 
alone  is  exoeyclie.  It  appears  that  in  this  form  the  whole  apical 
system  consisted  merely  of  one  madreporic  plate. 

Among  the  Euecliinoidea  the  three  orders  Holectypoida,  Clypeastroida, 
and  Spatangoida  are  exoeyclie. 

a.  Holectypoida  (Fig.  276).  In  consequence  of  the  wandering  of  the  anus  out 
of  the  apical  system,  the  posterior  basal  plate  has  lost  its  genital  aperture,  probably 
in  connection  with  the  disappearance  of  the  related  genital  gland  (the  place  of  which 
has  been  taken  by  the  rectum)  ;  in  Conoclypeus  and  Galeropygus  this  plate  has  even 


viii         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        323 

altogether  disappeared.  In  some  more  recent  species  of  the  genus  Holectypus 
the  genital  pore  of  the  posterior  basal  plate  reappears  secondarily.  The  space  in 
the  apical  system,  vacated  by  the  anal  area,  is  occupied  by  the  madrepore  (the 
right  anterior  basal  plate),  which  greatly  increases  in  size,  or  else  all  the  five  basal 
plates  shift  together  towards  the  apical  pole,  the  pores  of  the  stone  canal  being  then 
distributed  over  several  or  all  of  them.  The  plates  of  the  apical  system  may  fuse 
to  a  greater  or  less  extent. 

b.  Clypeastroida  (Figs.  277  and  278).  The  whole  apical  system  is  here  extra- 
ordinarily reduced  in  extent ;  it  is,  indeed,  very  minute.  All  the  five  basals  are  fused 
together,  and  sometimes  also  fused  with  the  radials.  At  least  four  genital  pores  are 
retained.  Where  there  are  only  four,  it  is  always  the  posterior  which  is  wanting. 
The  pores  of  the  stone  canal  open  in  very  various  ways  in  the  region  of  the  fused 
basals.  Many  scattered  pores  are  sometimes  found,  or  one  single  large  pore,  or  the 
pores  open  into  irregular  pits  or  grooves.  In  the  family  of  the  Clypeastridat  the 


.  qo 


FIG.  .  27S.— Laganum  depressum.  Less.     Apical  FIG.  279.— Apical  system  and  neigh  - 

system  and  neighbouring  parts  of  the  perisome  (after  bouring  parts  of  the  perisome  of  Meoma 

Loven).    rp,  Ocellar  pores  in  the  fused  radials ;  mp,  ventricosa,  Lamk.  (after  Loven).     For 

pores  of  the  uiadreporite  in  a  branched  furrow.     For  lettering  see  p.  317. 
lettering  see  p.  317. 

genital  pores  have  wandered  out  of  the  apical  system  ;  they  lie  either  at  its  edge,  or 
further  removed  in  the  sutures  between  the  (paired)  rows  of  interradial  plates. 

c.  Spatangoida.  The  apical  system  of  these  exocyclic  Echinoids  is  much  reduced 
in  extent,  although  not  so  much  so  as  that  of  the  Clypeastroida.  It  varies  much  in 
detail,  and  in  a  few  extreme  forms  (e.g.  Pourtalesia)  the  primitive  condition  is  to  a 
very  great  extent  obliterated  and  destroyed. 

1.  In  many  geologically  ancient  forms  the  influence  of  the  wandering  of  the  anus 
out  of  the  apical  system  is  seen  in  the  disappearance  of  the  posterior  basal  plate 
(together  with  the  genital  pore  belonging  to  it),  and  in  the  absence  of  a  central  plate. 
The  other  basals  and  the  radials  (each  of  which  has  its    pore)  have  all  shifted, 
and  occupy  an  area  which  is  sometimes  circular  or  almost  regularly  pentagonal, 
sometimes  lengthened  in  the  direction  of  the  plane  of  symmetry.     In  the  latter  case 
(Fig.  283).  the  two  middle  radials  touch  in  the  median  line,  separating  the  two 
anterior  from  the  two  posterior  basals.     The  apertures  of  the  stone  canal  are  found 
in  the  right  anterior  basal,  which  is  occasionally  somewhat  enlarged. 

2.  In  most  recent  fossil  forms  and  in  the  great  majority  of  the  extant  Spatangoida, 


324 


COMPARATIVE  ANATOMY 


CHAP. 


when  their  development  is  also  taken  into  account,  we  find  the  following  condi- 
tions. The  posterior  basal  plate  again  appears,  but  never  has  a  genital  pore.  The 
central  plate  also  reappears.  The  apertures  of  the  stone  canal  spread  out  from  the 
right  anterior  basal  towards  the  centre,  i.e.  on  to  the  central  plate. 


From  this  they 


FIG.  280.— Apical  system 
of  Abatus  cavernosus, 
Phil,  (after  Loven).  For 
lettering  see  p.  31V. 


FIG.  281.  —Apical  system  of 
Spatangus  purpureus,  33  nun.  in 
size  (after  Loven).  For  lettering 
see  p.  317. 


pass  on  to  the  posterior  basal  plate,  and  the  three  plates  fuse  together.  The  sutures 
between  them  disappear,  and  so  a  large  central  madreporic  plate  is  formed,  which 
in  very  many  forms  shows  a  tendency  to  increase  in  size  and  to  spread  out  in  the 
direction  of  the  posterior  interradius,  and  thus  more  or  less  to  press  asunder  the  two 


nv 


FIG.  282.— Apical  system  and  neighbour- 
ing parts  of  the  perisome  of  Micraster  coran- 
guimun  (after  Loven).  For  lettering  see  p.  317. 


FIG.  283.— Apical  system  and  neigh- 
bouring parts  ot  the  perisome  of 
Holaster  suborbicularis,  Defr.  (after 
Loven).  For  lettering  see  p.  317. 


posterior  radials  (Figs.  279-281).  The  genital  aperture  on  the  right  anterior  basal 
may  disappear,  in  which  case  only  three  genital  pores  remain.  In  isolated  cases, 
the  left  anterior  basal  plate  may  also  lose  its  genital  pore. 

3.  A  method  of  dissolution  of  the  apical  system,  unique  among  the  Echinoidae, 
is  found  in  many  Collyritidce  (Fig.  284).  If  we  imagine  that  the  elongated  apical 
system,  described  under  heading  1  (Fig.  283),  becomes  very  much  more  elongated 
in  the  direction  of  the  plane  of  symmetry,  and  breaks  into  two  groups,  one  anterior 
and  the  other  posterior,  we  have  the  condition  in  these  animals.  The  anterior 
group  contains  the  four  basals,  of  which  the  right  anterior  is  the  madreporitic 
plate,  and  three  radials,  viz.  the  anterior  unpaired,  and  the  right  and  left  anterior. 


vin         ECHIXODERMATA—  MORPHOLOGY  OF  SKELETON        325 

The  posterior  group  consists  of  the  two  posterior  radial  plates;  the  posterior 
unpaired  (fifth  basal)  plate  is  wanting.  The  anterior  group  is  separated  from  the 
posterior  by  a  row  of  plates  which  belong  to  the  right  and  left  posteiior  interradii,  as 
can  be  seen  by  comparing  the  figures.  This  arrangement  is  found  in  no  other  Echi- 
noid.  As  in  all  Echinoids,  however,  the  radials  remain  connected  with  the  apical 
ends  of  the  five  double  rows  of  ambulacral  plates,  so  that  these  latter  divide  in  a 


Fio.  285.— Apical  system  and  neighbour- 
ing part  of  the  perisome  of  Pourtalesia 
Jeffreys!,  W.  Th.  (after  Loven).  For  letter- 
ing see  p.  317. 


FIG.  2S4.— Apical  system 
of  Collyrites  elliptica,  Lam. 
(after  Loven).  For  letterin.-' 
see  p.  317. 

remarkable  manner  into  three  anterior  ambulacra  (trivium),  the  anterior  unpaired 
and  the  anterior  right  and  left,  and  two  posterior  ambulacra  (bivium). 

4.  The  apical  system  is  most  of  all  reduced  and  obliterated  in  the  peculiar 
Spatangoid  family  of  the  Povrt<il's!id<:>'  (Fig.  285).  Let  us  take  as  an  example  P. 
The  whole  system,  which  is  irregularly  pentagonal  in  outline,  is  shifted 
forward,  and  separated  from  the  apical  ends  of  the  two  posterior  ambulacra  by 
the  uppermost  plates  of  the  posterior  unpaired  and  of  the  right  and  left  posterior 
interradii.  It,  almost  certainly,  consists  of  four  basal  plates,  each  perforated  by  a 
genital  pore,  but  fused  together  into  one  single  piece  in  which  no  suture  can  be  seen. 
In  the  central  and  anterior  portion  of  this  plate  lie  the  scattered  pores  of  the  stone 
canal.  No  radials  can  be  recognised. 


326  COMPARATIVE  ANATOMY  CHAP. 

Although  there  are  good  palseontological  reasons  for  the  generally  accepted  belief 
that  all  known  exocyclic  (irregular)  Echinoidea  are  descended  from  endocyclic 
(regular)  forms,  it  has  been  conjectured  that  these  latter  may  themselves  have  had 
exocyclic  ancestors  (which,  indeed,  are  unknown  to  us).  Thus  the  modern  Spatan- 
yoida  and  Clypcastroida,  for  example,  by  the  position  of  the  anus  in  the  posterior 
unpaired  interradius,  may  secondarily  have  attained  a  primitive  condition.  The 
anus  would  then  have  wandered  first  from  the  posterior  unpaired  interradius  to  the 
centre  of  the  apical  area,  and  then,  in  the  exocyclic  forms  known  to  us.  have  shifted 
back  again  in  the  same  direction.  This  suggestion,  which  is  of  special  significance 
with  reference  to  the  primitive  Pelmatozoa,  receives  some  (not  very  satisfactory) 
support  from  the  fact  that  in  the  very  old  family  of  the  Saleniidce  among  the  regular 
Echinoidea,  the  anus  lies  at  the  posterior  edge  of  the  apical  system  in  the  oldest 
forms,  but  during  geological  development  approaches  more  and  more  near  the  centre 
of  the  system,  near  which  it  is  found  asymmetrically  (posteriorly  to  the  right)  in 
the  modern  forms. 

II.  Asteroidea. 

The  typical  plates  of  the  apical  system  are  not  present  in  most 
adult  Star-fish,  or  at  any  rate  cannot  be  made  out  among  the  numerous 
calcareous  pieces  embedded  in  the  dorsal  area  of  the  disc.  There  are, 

however,  exceptions  to  this  rule.  For 
instance,  in  species  of  the  genera  Penta- 
gonaster,  Tosia,  Astrogonium,  Stellaster, 
Nectria,  Ferdina,  Pentaceros,  Gymnasteria, 
Set/taster,  Ophidiaster,  Zoroaster,  the  central 
plate,  the  five  basals  and  the  five  radials 
can  still  be  more  or  less  clearly  recognised 
in  the  adults.  Occasionally  (in  species 
of  Pentagonaster,  Gymnasteria,  Pentaceros, 
and  many  Goniasteridce)  there  are  even  to 
be  found  plates  which  in  position  corre- 
spond with  the  infrabasals.  The  whole 
apical  system  is  specially  well  developed 
in  young  specimens  of  the  deep-sea  Star- 
fish  Zoi'oaster  fulgens  (Fig.  286).  The 

FIG.  286.-APicarsystem  of  plates  aPerture  of  fche  stone  canal  lies  in  the 
in  a  young  specimen  of  Zoroaster  right  anterior  interradius,  outside  the 
sSgen3Si7after  Slad6n)'  Forlettering  basal;  the  anus  in  the  right  posterior 

interradius,    inside    the    basal.       In    all 

Asteroids,  the  madreporic  plate  and  anus  lie  in  these  interradii  of  the 
apical  region  (cf.  the  Echinoidea,  Figs.  272-275). 

The  typical  apical  system  can  also  be  proved  ontogenetically  in 
Star-fishes,  even  in  forms  in  which  it  is  absent  or  unrecognisable  in 
the  adult.  Five  basals,  a  central  plate  and  five  radials  are  actually 
among  the  first  plates  formed  in  the  embryo  Star-fish,  in  the  very 
order  in  which  they  are  here  named,  though  always  after  the  terminals, 
presently  to  be  described,  which  appear  first  of  all.  Small  plates, 
appearing  radially  within  the  circle  of  basals,  have  been  considered  to 


viii         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        327 

be  infrabasals.  This  view  is,  however,  not  certain,  because  other 
new  and  also  radially  arranged  plates  may  be  added  to  these,  which 
may  thus  also  themselves  possibly  be  accessory  structures. 


III.  Ophiupoidea. 

In  this  class,  the  plates  of  the  apical  system  do  not  appear  in  the 
embryo  in  exactly  the  same  order  as  in  the  Asteroidea.  First  the  five 
radials  and  the  central  plate  form,  and,  somewhat  later,  between  the  circle 
of  radials  and  the  central  plate,  the  five  basals  and  the  five  infrabasals 
appear.  In  many  Ophiuroidea,  an  embryonic  condition  of  the  apical 
system  is  retained  in  the  adult,  the  central  plate  being  surrounded  by 
the  circle  of  five  radials,  while  the  basals  and  infrabasals  are  wanting 


Fi<;.  287.—  Plates  of  the  apical  system  of  the  disc  of 
Ophiomusium   validum  (after  P.   H.   Carpenter).     For 

lettering  see  p.  317. 


FIG.  288.— Apical  system  of  a 
young  Amphiura  squamata  (after 
P.  H.  Carpenter).  For  lettering 
see  p.  317. 


(species  of  the  genera  Ophioglypha,  Ophiomastix,  Ophiopyrgus,  Ophiura, 
H'-iiiiphol is,  Ophioceramis,  Ophiopholis,  Ophiotrochus).  In  many  others, 
however,  there  are,  besides  the  radials,  the  five  basals,  which  may 
vary  greatly  in  size  (species  of  the  genera  Ophioglypha,  Ophiomastix, 
Ophiomusium,  Ophiura,  Ophiopholis,  Ophiozona,  Ophiactis,  Ophiolepis). 
In  Ophiomifra  wigua  there  is  only  the  central  plate  with  five  basals 
around  it.  In  some  Ophiuroidea  a  complete  apical  system  is  developed, 
infrabasals  being  added  to  the  basals,  the  radials  and  the  central 
plate  (isolated  species  of  Ophioceramis,  OphioglypJw,  Ophiozona,  Ophio- 
musiu.m  (Fig.  287),  Ophiolepis).  In  very  many  Ophiuroidea  the 
calcareous  plates  developed  at  the  apical  surface  of  the  disc  are  so 
numerous  that  it  is  then  impossible  to  recognise  among  them  the 
typical  plates  of  the  apical  system.  The  adult  Ophiuroidea  have  no 


OF    THF. 

ITN^  :TY 


328  COMPARATIVE  ANATOMY  CHAP. 

anus.     The  apertures  of  the  stone  canal  are  not  found  on  any  of  the 
apical  plates,  but  ventrally,  on  one  of  the  oral  shields. 

IV.  Pelmatozoa. 

In  no  other  class  of  the  Echinodermata  do  the  plates  of  the  apical 
system  form  so  large  a  part  of  the  skeleton  of  the  body  wall  (apart 
from  the  arms)  as  in  the  Pelmatozoa.  The  body  of  these  Echinoderms 
consists  of  a  central  calyx,  which  contains  the  viscera,  and  usually 
carries  jointed  appendages,  radially  arranged  at  its  edge ;  these  are 
the  arms  and  pinnula?.  Typically  the  Pelmatozoa  are  attached  to 
the  sea-floor  by  their  apical  poles,  with  or  without  the  intervention 
of  a  stem ;  in  some  the  stem  becomes  separated  from  its  attachment 
(Pentacrinus),  and  may  dwindle  in  size  (Millericrinus),  or  may  be  present 
only  in  the  embryonic  stages  (Antedon),  or  there  may  be  no  trace  of 
either  stem  or  attachment  (Marsupites).  The  oral  side  of  the  calyx 
(and  also  of  the  arms)  is  thus  turned  upwards,  while  the  apical  side 
of  the  calyx  (the  dorsal  cup)  is  turned  downwards  and  either 
surrounds  the  viscera  like  a  bowl  or  carries  them  like  a  dish.  The 
plated  test  of  this  bowl  or  dish  consists  exclusively,  or  for  the  greater 
part,  of  the  plates  of  the  apical  system :  the  basals  and  the  radials, 
to  which  infrabasals  may  be  added.  The  anal  aperture  always  lies 
interradially,  usually  on  the  oral  side  of  the  body  and  not  con- 
nected with  the  apical  system. 

Sub-Class  1.  Crinoidea. 

There  are  a  good  many  Crinoids  in  which  the  apical  system  is 
completely  developed.  The  five  radials  and  the  basals  are  constant, 
although  the  latter  may  be  hidden.  The  infrabasals  are  inconstant. 
The  Crinoids  in  which  the  latter  are  present  are  said  to  have  a 
dicyelie  base,  those  in  which  they  are  absent  have  a  monocyclic 
base. 

A  central  plate  has  been  observed  in  the  larva  of  Antedon.  It 
occurs  at  the  distal  or  root  end  of  the  larval  stem,  and  ultimately 
becomes  severed  from  the  animal. 

The  part  taken  by  the  plates  of  the  apical  system  in  the  construc- 
tion of  the  apical  capsule  varies  greatly.  In  the  stalked  larva  of 
Antedon  they  alone  form  the  skeleton  of  the  apical  side  of  the  calyx ; 
although  an  anal  interradial  has  a  transitory  existence.  The  same  is 
the  case  also  in  many  other  adult  Crinoids,  which  in  this  respect  show 
a  primitive  or  an  embryonic  character  (many  Inadunata  larviformia 
and  many  Inadunata  fistulata,  Encrinus,  Marsupites,  Holopus,  Hyocnuu.^ 
JBathycrinus,  and  a  few  Canaliculata :  PJiizocrinus,  Pentacrinns). 

In  most  Crinoids,  on  the  other  hand,  the  plates  of  the  typical  apical 
system,  i.e.  the  infrabasals  (where  these  occur),  basals  and  radials  do 
not  form  the  whole  skeleton  of  the  apical  capsule,  but  only  a  certain 


ECHINODSBMATA— MORPHOLOGY  OF  SKELETON        329 


(often  even  very  small)  part  of  it;  other  plates  take  part  in  its 
structure,  as  we  shall  see  more  in  detail  when  describing  the  peri- 
somatic  skeleton.  The  border  of  radials  round  the  apical  capsule 
becomes  more  or  less  markedly  disturbed  by  the  appearance  of 


-  \OA1S 

FIG.  -2SO.—  Apical  system  of  Cyatho- 
crinus.  For  lettering  see  p.  317.  <""/<.  Anal 
interred  iaL 


FIG.  29u.— Marsupites  ornatus.    P! 
the  dorsal  cup.     For  lettering  see  p.  317. 


i  .-A 


special    '•  anal   plates "   in   the   posterior  unpaired   interradius ;    these 
specialised  anals  occur  very  frequently  in  palaeozoic  Crinoids  (Fig.  291). 

The  Crinoids  with  dieyelie  base  (with  infrabasals,  Figs.  289 
and  290)  are :  (a)  most  Iimdumtta ;  (b)  among  the  Canierata,  the 
families  of  the  ReteocrinidiR  p.  p., 
Glyptasterid^,  and 
(c)  the  Articulatn. 
(Ichthyocrinida) ;  (d)  the  Camilieu- 
lafa,  in  which,  it  is  true,  the  infra- 
basals are  often  either  fused  with 
the  uppermost  joint  of  the  stem 
or  atrophied,  at  least  in  the  adult : 
such  are  conveniently  termed 
Pseudomonoeyelie. 

The  Crinoids  with  monoeyelie 

base  (without  infrabasals,  Fig.  291) 

are,  apart  from  a  few  Inadnnata, 

the     Camerate     families     of     the 

I'.iinocrinidce,    Phity- 

B:irra ndeocrin <d<r,  Eu<vluijh><-r'(n idee.} 

Instead  of  the  typical  five  infrabasals  and  five  basals  there  are 
very  often  found  four,  three,  or  even  only  two  plates  in  these  rings  ; 
this  is  especially  the  case  in  extinct  Crinoids  belonging  to  the  orders 
I/<"du/«>fii.  •  .  and  Arfwhtf-.i.  The  plates  are  then  almost 


FIG.  291.— Actinocriuus  proboscidalis.    Plates 
f  the  dorsal  «J/\^  lettering  see  p.  317. 


330  COMPARATIVE  ANATOMY  CHAP. 

always  of  unequal  size,  and  it  appears  not  unlikely  that  the  reduction 
of  their  number  was  caused  by  the  fusing  of  neighbouring  plates. 
These  characteristics  necessarily  destroy  the  strictty  radial  symmetry 
of  the  dorsal  cup. 

Still  further  fusions  may  occur  (among  the  Canaliculata). 

The  relative  sizes  of  the  plates  of  the  infrabasal,  basal  and  radial 
circles  vary  greatly,  but  this  is  of  no  great  interest  to  the  comparative 
anatomist. 


Sub-Class  2.  Blastoidea. 

The  Blastoidea  are  paleozoic  Pelmatozoa,  whose  stalked  armless 
body  very  often  has  the  appearance  of  a  bud  (Fig.  263,  p.  314). 
Seen  from  the  side,  the  body  is  an  oval,  truncated  sometimes  at 
the  apical,  sometimes  at  the  oral  end.  Seen  from  the  oral  or 
aboral  pole,  its  outline  is  in  by  far  the  greater  number  of  (regular) 
forms  regularly  pentagonal  with  rounded  projecting  angles,  some- 
times not  unlike  a  short  - 
armed  Star-fish  (Figs.  265 
and  266,  p.  314).  In  the 
irregular  Blastoids,  on  the 
contrary  (Eleutherocrinus,  As- 
trocrinus,  Fig.  267,  p.  315), 
the  radiate  structure  is  dis- 
turbed by  the  modified  form 
of  one  of  the  ambulacra. 
The  outline  of  the  ovoid 
body  of  Eleutkerocrinus,  seen 
from  the  apical  or  oral  pole, 
is  irregularly  pentagonal, 
with  three  shorter  and  two 
longer  sides,  the  latter  be- 
longing to  the  left  posterior 
and  the  unpaired  posterior 
interradii.  In  Astrocrinus, 
the  body  is  flattened  in  the 
direction  of  its  principal  axis, 


z,  the  two  larger  basals  ;  ir,  interradials  ;  r,  radials.  Or    aboral    pole,   almost    Sym- 

metrically    four-lobed,     the 

lobes  being  of  unequal  size.     The  largest  of  the  lobes  lies  diametrically 

site  the  abnormally  shaped  ambulacrum,  which  is  on  the  smallest 

formC(FiJ   o?7   p   315  T°  °ther  middle-sized  lobes  are  almost  dike  in 

The  whole'  body  of  the  Blastoids  is  plated.      The  test   consists, 

apart  from  the  ambulacra,  of  three  circles  of  plates  (Fie.  292)  two  of 

which  belong  to  the  typical  apical  system  of  the  Ech  noderma  a   while 


viii         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        331 

the    third   consists   of   perisomatic    plates,   which,  in   all   probability, 
correspond  with  the  primary  interradii  of  the  Crinoids. 

The  first  circle  at  the  apex  is  that  of  the  (interradial)  basal  plates. 
There  are  always  three  of  these,  one  smaller  and  two  larger  of  equal 
size,  as  also  occurs  in  the  Crinoids.  The  monocyclic  base  of  the 
Blastoidea  is  thus  symmetrical.  But  the  line  of  symmetry  (the  so- 
called  dorsal  axis),  which  passes  between  the  two  larger  plates  and 
through  the  small  unpaired  plate,  does  not  coincide  with  the 
symmetrical  (ventral)  axis  of  the  body,  which  passes  through  the 
mouth  and  the  anus,  the  latter  lying  in  the  posterior  interradius  on 
the  oral  surface.  The  smaller  unpaired  basal  plate  lies  in  the  left 
anterior  interradius.  If  we  imagine  the  two  larger  basal  plates  cut 
into  two  similar  parts  by  radial  lines  of  division,  we  obtain  the  five 
equal-sized,  strictly  radially  arranged,  and  interradially  placed  basals 
of  most  other  Echinoderms.  The  uppermost  ossicle  of  the  stem 
is  inserted  at  the  point  where  the  three  basals  of  the  Blastoids  meet. 

The  circle  of  the  basals  is  immediately  surrounded  by  that  of  the 
radials.  The  typical  number  of  five  is  always  retained  in  these, 
which,  in  regular  Blastoids,  are  strictly  radiate  in 
their  arrangement.  These  are  called  fork-pieces, 
because  each  of  them  is  produced  upwards,  i.e. 
orally,  in  the  shape  of  a  tuning-fork,  the  two 
limbs  holding  between  them  the  distal  end  of  an  *i 
ambulacrum.  The  radials  form  a  closed  circle, 
their  lateral  edges  being  contiguous. 

The  third  circle  of  plates  is  in  immediate 
contact  with  the  radials,  and  surrounds  the  peri- 
stome.  It  consists  of  five  interradial  plates, 
which,  in  regular  Blastoids,  are  strictly  radial ; 
these  are  the  interradials  or  deltoid  plates.  • CMX 

These  plates  do  not  form  a  closed  circle,  as  they  &  & 

are  separated  from  one  another  by  the  five  FlG  2o3.  _  Eieuthero- 
ambulacra.  The  apical  edges  of  each  deltoid  crinus  Cassedayi,  from 
plate  rest  on  the  oral  edges  of  the  contiguous  ^d  e^^m^^^entert 
forks  of  two  consecutive  radials  or  fork  pieces.  aa.bbj  A*°  pa.SSing  through 
The  relative  sizes  of  the  basals,  radials,  and  inter-  the  mouth  and  the  anus ; 
radials  of  the  Blastoids  vary  greatly  (cf.  figures).  ^^0^ 
One  of  the  five  interradials,  which  is  distinguished  an>  anal  side, 
as  the  posterior,  is  perforated  by  the  anus. 

In  the  irregular  Blastoids  (Fig.  293),  which  are  without  stems, 
all  the  plates  of  the  regular  forms  are  found,  but  are,  naturally, 
irregularly  developed.  The  radial  which  supports  the  modified  ambu- 
lacrum is  smaller  than  the  other  radials  and  differently  shaped.  It 
appears  shifted  quite  on  to  the  oral  surface.  At  the  same  time,  the 
pair  of  basals  (y  and  z)  which  flank  this  radial  are  much  prolonged 
orally  as  narrow  plates. 

It  cannot  at  present  be  decided  whether  there  are  skeletal  pieces 


332 


<<DMPARATUrE  ANATOMY 


CHAP. 


in  other  Echinodermata  homologous  with  the  interradials  of  the 
Crinoidea  and  the  Blastoidea.  In  all  endeavours  to  answer  tins 
question  the  following  plates  should  be  kept  in  mind:  in  the 
Ophiuroiilea  the  interradially  placed  plates  between  the  circle  of  radials 
and  the  oral  side  (Fig.  287,  p.  327),  and  among  the  Echinoidea,  in 
Tiarechinm  (Fig.  271,  p.  319),  the  central  of  the  three  interradial  plates 
of  an  interambulacral  area. 

Sub-Class  3.  Cystidea. 

The  spherical,  pear-shaped,  egg-shaped,  or  cup-shaped  body  of  the 
Cystidea  is  also  enclosed  in  calcareous  plates.  In  one  of  the  principal 
groups,  that  of  the  Eucystidea,  the  plating  consists  of  numerous  con- 
tiguous plates  arranged  without  any  recognisable  order.  In  this  case 


0 


FIG.  294.— System  of  plates  of  the  apical  capsule 
of  Caryocrinus  ornatus,  spread  out  (after  Hall).  For 
lettering  see  p.  317. 


FIG.  295.— Cystoblastus  Leuchten- 
bergi,  from  the  apical  side.  11,  Point 
of  insertion  of  the  stem ;  8,  anus ; 
10,  infrabasals  ;  12,  pectinated  rhombs. 


a  typical  apical  system  of  plates  cannot  be  distinguished.  In  the 
other  principal  group,  the  Cystocrinoidea,  certain  forms  of  which  show 
near  relationship  to  the  Crinoidea  and  Blastoidea,  the  test  consists 
of  a  relatively  small  number  of  plates,  and  a  true  apical  system  can 
be  found  round  the  apical  pole. 

The  forms  assumed  by  this  apical  system  may  be  grouped  around 
two  central  types  :  Caryoerinus  and  Eehinoencrinus.  The  group 
Caryocrinus  (Corylocrinus,  Hemicosmites,  Juglandocrinus)  has  its  plates 
arranged  in  six  rays ;  while  the  group  Eehinoencrinus  ( Callocystis, 
Lepadocrinus,  Apiocystls,  Cystoblastus,  Glyptocystis,  Pleurocystis,  Pruno- 
cystis,  Pseu<l;> •/•///.//.<,  etc.)  shows  the  typical  five-rayed  arrangement  of 
the  plates.  In  both  groups  the  base  is  dicyclic,  i.e.  there  is  a  circle 
of  infrabasals  inside  the  circle  of  basals. 

Caryocrinus,  six-rayed  (Fig.  294).— The  circle  of  infrabasals 
consists  of  four  plates,  two  larger  (which  are  contiguous)  and  two 
smaller.  Each  of  the  two  larger  plates  is  double.  Outside  the  circle 
of  the  infrabasals  lies  a  closed  circle  of  six  interradial  basals,  and 


VIH         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        333 

this  is  surrounded  by  a  closed  circle  of  six  radials.     These  plates, 
together   with   two   accessory   plates   (interradials  ?),  form   the  whole 
test    of   the    cup    of    the    attached    Caryocrinus,   from    the    point    of 
insertion  of  the  stem  to  the  base  of 
the    arms.       The    anus    lies    excen- 
trically  on  the  oral  surface,  in  the 
(interradial)     prolongation     of    the 
suture  between  the  two  larger  infra- 
basals  (cf.  Figs.  294,  295). 

Eehinoenerinus,  five-rayed  (Fig. 
296).— The  circle  of  infrabasals 
consists  of  four  plates,  one  large 
posterior  plate  and  three  smaller 
ones.  The  larger  plate  is  double 
(i.e.  consists  of  two  fused  plates). 
Outside  the  circle  of  infrabasals 
comes  the  closed  circle  of  the  five 


O 


FIG.  296.— System  of  plates  of  the  dorsal 
basalS,  and  outside  this  that   of   the    cup  of  EcWnoencrinus  armatus,  spread  out 

(after  Forbes).    For  lettering  see  p.  317. 

five  radials,  between  which  acces- 
sory pieces  are  intercalated,  the  homologies  of  which  cannot  be  made 
out.  The  anus  lies  posteriorly  to  the  right.  In  CystoUastus  the 
radials,  like  the  radials  or  fork-pieces  of  the  Blastoidea,  have  deep 
incisions  on  the  oral  side  for  the  reception  of  the  ambulacra  (cf.  Fig. 
259,  A  and  B,  p.  312,  and  Fig.  295). 


B.  The  Oral  System  of  Plates. 

In  certain  Echinodermata  (Pelmatozoa  and  Ophiuroidea)  there  is  a 
system  of  plates  surrounding  the  oral  (ventral,  actinal)  pole,  and  thus 
diametrically  opposite  to  the  apical  system.  This  system  develops 
round  the  left  ccelomic  vesicle  of  the  larva  in  a  way  similar  to  that  in 
which  the  apical  system  develops  round  the  right  vesicle.  The  oral 
system  is,  however,  much  simpler  than  the  apical,  and  consists  of  one 
single  circle  of  five  plates  (less  frequently  six,  in  the  six-rayed  arrange- 
ment of  the  whole  system) ;  these  plates,  placed  interradially,  corre- 
spond in  the  oral  system  with  the  basal  plates  of  the  apical  system, 
and  are  called  oral  plates. 

In  our  considerations  of  this  oral  system  we  again  find  the  best 
starting-point  to  be  the  stalked  larva  of  Antedon  (Pentacrinus  stage). 
In  a  young  stage  of  this  larva  the  oral  surface  of  the  calyx  appears 
vaulted  over  by  a  roof  closed  on  all  sides.  The  surface  of  the  calyx 
thus  forms  the  floor,  and  the  vault  the  roof,  of  a  closed  cavity,  which 
is  called  the  oral  or  tentacular  vestibule.  At  the  centre  of  the  floor 
the  oral  aperture  breaks  through,  connecting  the  intestine  with  the 
vestibule.  The  mouth  is  thus  at  this  stage  not  connected  with  the 
exterior.  The  fifteen  primary  tentacles,  which  rise  on  the  disc  of 
the  calyx,  also  cannot  project  externally,  but  are  covered  over  by  the 


COMPARATIVE  ANATOMY 


CHAP. 


334 

roof  of  the  vestibule.  This  roof  is  formed  of  five  interradial  lobes, 
supported  by  five  interradial  skeletal  plates,  the  oral  plates.  An  aper- 
ture only  arises  secondarily  at  the  apex  of  the  roof,  and  the  five  oral 
lobes  separate  in  such  a  manner  that  the  tentacles  can  project  through 
the  clefts  between  them.  The  mouth  is  now  in  open  communication 
with  the  exterior. 

At  first  the  five  oral  plates  rest  directly  on  the  oral  edges  of  the 
basal  plates  of  the  apical  system.  But  in  proportion  as  the  calyx 
increases  in  size,  and  the  arms  grow  out,  the  distance  between  the 
basals  and  the  newly-formed  radials,  which  support  the  arms,  on  the 
one  hand,  and  the  oral  plates  on  the  other,  becomes  greater  and  greater, 
since  the  latter  remain  at  the  centre  of  the  tegmen  calycis,  surround- 
ing the  mouth.  There  thus  arises,  between  the  bases  of  the  arms  and 
the  circle  of  the  oral  plates,  which  in  comparison  with  the  continually 

growing  calyx  be- 
comes more  and  more 
insignificant,  a  cir- 
cular zone,  the  peri- 
pheral zone  of  the 
tegmen  calycis.  The 
food  grooves  running 
out  from  the  mouth, 
passing  between  the 
five  oral  lobes,  tra- 
verse this  peripheral 
zone  of  the  tegmen 
to  the  bases  of  the 

FIG.  -297.— Haplocrinus  mespiliformis  (after  Wachsmuth  and  arms.           This       peri- 
Springer).   A,  From  the  anal  side ;  B,  from  the  oral  side.   1,  Orals ;  pheraj    zone    COntittU- 
2,  oral  pole  ;  3,  anus  ;  4,  radials  ;   5,  inferradial ;  6,  basals  ;  7,  first  ,.      .                       . 
brachial ;  8,  point  of  attachment  of  the  arm.  ally  increases  in  S1ZC, 

while  the  central  part, 

surrounded  by  the  five  oral  lobes,  does  not  grow  further,  and  forms 
an  ever-diminishing  central  region  of  the  tegmen  calycis.  Finally,  the 
oral  plates,  with  the  lobes,  are  entirely  resorbed,  and  the  minute 
central  zone  can  no  more  be  distinguished ;  the  whole  oral  surface  of 
the  Antedon  calyx  is  a  free  disc,  by  far  the  greater  part  of  which  has 
been  formed  outside  the  base  of  the  oral  pyramid.  In  the  centre  of 
this  oral  disc  the  mouth  lies  uncovered,  and  on  the  surface  of  the 
disc  the  food  grooves  are  visible  running  out  radially  to  the  bases 
of  the  arms. 

Among  the  immense  array  of  forms  comprised  under  the  crinoids 
we  find  a  few  groups  with  five  oral  plates  forming,  as  in  the  larva  of 
Antedon,  the  whole  skeleton  of  the  tegmen  calycis.  In  the  Inadunata 
larviformia,  type  Haplocrinus  (Fig.  297),  there  is  actually  a  closed 
pyramid  of  five  oral  plates,  which,  at  the  edge  of  the  calyx,  rest  on 
the  radials  of  the  dorsal  cup.  Only  at  the  bases  of  the  arms  do 
the  five  oral  plates  separate  to  form  five  radial  apertures,  through 


via         ECHINODERMATA—  MORPHOLOGY  OF  SKELETON        335 


which  the  food-grooves  pass  out  on  to  the  arms.  The  posterior  oral 
plate  is  somewhat  larger  than  the  others,  and  has  a  perforation 
which  may  be  the  anus  (?). 

The  same  condition  is  found  in  the  extant  genera  Holopus  and 
Hyocrinus  (Fig.  298),  the  extant  unstalked  genus  Thaumatocriims, 
and  the  extant  canaliculate 
genus  Bhizocrinus.  All  these 
genera  possess  five  oral  plates, 
which,  however,  are  separate, 
and  do  not  form  a  closed 
pyramid  ;  the  mouth,  there- 
fore, is  in  open  communica- 
tion with  the  exterior  between 
them.  Compared  with  the 
larva  of  Antedon  and  with 
Haplocrinus,  Holopus  shows 
the  most  primitive  (or  em- 
bryonic) condition,  since  in 
it  the  oral  pyramid  is  large, 
covering  nearly  the  whole  of 
the  tegmen,  so  that  between 

its  base  and    the  edge  of  the          FIG.  298.—  Hyocrinus   Bethellianus  (after  P.  H. 

Calyx  only  a  Very  Small   peri-  Carpenter)     Tegmen  calycis      1,  Axial  canal  of  the 

.J  J  J       .  *  braclnals  ;  2,   extension  of  body  cavity  in  the  arm  ; 

plieral      zone      remains.  In  3,  food  groove  of  the  arm  ;  4,  smaller  plates  of  the 

HyOCrimiS  (Fig.  298)  also,  and  tegmen;  5,  orals;  6,  anal  cone;  7,  oral  edges  of  the 

Thaumatocnnus  the  orals  are    ' 

still  of  considerable  size,  but  the  peripheral  zone,  which  is  beset  with 

small   closely  -crowded  plates,  is  somewhat  broader  than  in  Holopus 

(about  one-fifth  the  diameter 
of  the  whole  tegmen).  In 
Rhizocrinus  lofotensis  the  orals 
are  smaller,  and  in  Rhizocrinus 
Rawsoni  they  are  almost  rudi- 
mentary, so  that  the  zone 
which  surrounds  them  forms 
the  greater  part  of  the 
tegmen. 

In  the  Cyathocrinidce  (In- 
adunata  fistulata),  five  large 
plates  can  sometimes  be  dis- 
tinctly made  out  in  the  centre 
of  the  plated  tegmen  ;  some- 
FIG.  -299.  -system  of  plates  of  the  tegmen  of  times,  however,  irregular 

Platycrinus     tuberosus    (after    Wachsmuth    and 

Springer).     For  lettering  see  p.  317. 


peces     are     found      in      their 
placeg        When    they    an)    djg_ 

tinct,  the   posterior  plate   is   the    largest,  and   is   sometimes   shifted 
anteriorly  between  the  others.     In  all  cases  they  cover  the  mouth  in 


336  COMPARATIVE  ANATOMY  CHAP. 

such  a  way  as  to  hide  it.      These  plates  are   by  some  regarded   as 

In  the  Camerata  (Fig.  299)  five  supposed  oral  plates  (or)  can  almost 
always  be  distinguished  in  the  centre  of  the  richly  and  rigidly  plated, 
often  highly  arched,  tegmen.  They  close  together  firmly  over  the 
mouth.  The  posterior  oral  is  larger  than  the  rest,  and  presses  in 
between  them. 

As  far  as  is  known,  in  the  Articulate  (Ichthyocrinoidea)  also,  five 
orals  can  be  distinguished  at  the  centre  of  the  richly  but  loosely 
plated  tegmen.  But,  in  this  case,  they  are  separate,  and  surround  an 
open  mouth.  The  posterior  plate  is  larger  than  the  rest. 

In  the  Camliculata  (with  the  exception  of  the  above-named  genus 
Rhizocrinus)  the  orals  are  altogether  wanting  in  the  adult. 

In  the  Blastoidea  the  oral  region  is  covered  by  a  roof  consisting 
of  numerous  small  plates  usually  without  definite  arrangement,  which 
are  continued  as  covering  plates  over  the  ambulacra.  In  a  few  forms, 
however,  and  especially  in  Stephanocrinus,  five  orals  can  be  made  out. 
In  Stephanocrinus  these  five  interradial  orals,  resting  on  the  inter- 
radials  (i.e.  the  deltoid  pieces),  form  a  closed  pyramid  over  the  oral 
region. 

In  many  Cystidea,  also,  the  mouth  is  arched  over  by  an  oral 
pyramid.  In  Cyatliocystis,  the  five  oral  plates  forming  this  pyramid 
are  more  or  less  equal  in  size,  but  in  species  of  the  genera  Sph&ronis, 
Glyptosphcera,  and  Pirocystis  the  posterior  oral  is,  as  in  so  many 
Camerata,  larger  than  the  rest.  In  the  six-rayed  Cystid  Caryocrinu* 
this  latter  is  the  case,  one  of  the  six  orals  having  shifted  from  behind 
forward  between  the  other  five,  which  surround  it  symmetrically. 

In  the  Ophiuroidea,  on  the  oral  (lower)  side  of  the  disc,  there  is 
in  each  interradius  a  plate,  usually  distinguished  by  greater  size.  One 
of  these  plates,  which  are  called  bueeal  shields  (Fig.  245,  p.  300), 
is,  as  madreporite,  perforated  by  the  pores  of  the  water-vascular 
system.  In  the  pentagonal  larva  of  Amphiura  these  buccal  shields 
appear  at  the  edge  of  the  oral  side.  They  have  been  homologised, 
probably  correctly,  with  the  orals  of  the  Pelmatozoa. 

In  the  Asteroidea,  on  the  lower  surface  of  the  disc  at  the  edge  of 
the  mouth,  in  each  interbrachial  region,  there  occurs  a  skeletal  plate 
of  very  various  shape,  which  is  called  the  odontophore  (Fig.  310,  p. 
352).  These  plates,  which  might  be  described  as  the  proximal  or 
basal  plates  of  the  interbraehial  system,  may  correspond  with  the 
orals  of  the  Pelmatozoa  and  the  oral  shields  of  the  Ophiuroidea,  although 
they  may  be  pushed  below  the  surface  by  the  oral  plates  (the  first 
pairs  of  adambulacral  plates),  and  are  usually  completely  covered 
externally.  They  arise  early  in  the  larva  of  Asterias  (afteV  the  five 
terminal  plates,  the  five  basals,  the  apical  central  plate,  the  ten  oral 
ambulacral  plates,  and  twenty  other  ambulacral  plates  are  formed), 
interbrachially  between  the  oral  ambulacrals. 

Orals  have  not  been  discovered   in   the   Echinoidea.     Whether 


vin         ECHINODEEMATA — MORPHOLOGY  OF  SKELETON        337 

certain  pieces  of  the  calcareous  ring  of  the  Holothurioidea  corre- 
spond with  the  orals  of  other  Echinoderms  cannot  at  present  be 
determined. 


C.  The  Perisomatie  Skeleton.1 

All  those  skeletal  pieces  which  protect  the  body,  between  the  apical 
and  the  oral  systems,  taken  together,  form  the  perisomatie  skeleton 
of  the  Echinodermata.  It  is  obvious  that  the  extent  of  the  periso- 
matie skeleton  must  vary  inversely  with  that  of  the  polar  (apical  and 
oral)  systems.  Where  the  polar  systems  form  only  a  small  part  of 
the  body  wall  the  perisomatie  skeleton  is  the  more  strongly  developed, 
and  vice  versd.  In  the  Blastoidea,  for 
example,  nearly  the  whole  of  the  test 
is  formed  by  the  polar  systems 
(especially  the  apical),  while  in  most 
Echinoidea,  Asteroidea,  and  Ophiuroidea, 
the  perisomatie  system  covers  nearly 
the  whole  body.  Where  the  equatorial 
zone  of  the'  body  is  produced  into 
variously  shaped  branched  or  un- 
branched  arms,  as  in  most  Pelmatozoa, 
Asteroidea,  and  Ophiuroidea,  the  skeleton 
of  these  arms  is  exclusively  formed  by 
perisomatie  pieces.  It  is  at  present 
impossible  to  prove  any  definite 
homologies  between  the  parts  of  the 
perisomatie  systems  throughout  the 
Echinodermata. 


I.    Holothurioidea.  FlG  3oo._Microscopic  calcareous  bodies 

rji      •   •  7  of  Holothurioidea.    1,  Auchor  and  anchor 

In    the  CUtlS    Of  the  HolottlUnOUlea,  plate  of  Synapta  inhserens,  O.  F.  M. ;  2, 

as    well    in    the    body    Wall    as    in    the  "stool "of  Cucuinaria  longipeda,  Serap;  3, 

wall  of  the  tentacles,  ambulacra,  tube-  %%%£*££££&£& 

feet,  and  ambulacral  papillae,  there  are  sticopus  japonicus ;  5,  supporting  plate 
found  enormous  numbers  of  micro-  from  one  of  the  tube-feet  of  stychopus 

.      ,,  .  ,  ,      i'          f  japonicus;      6,     "stool"    of    Holothuria 

scopically  minute  calcareous  bodies  ot   Murray. .  7j  rod  from  the  ventral  ambula. 

definite  Shapes  (Fig.  300).      These  give  cral  appendages  of  Oneirophanta  mutabilis, 

the     integument     a     firm     and      rOUgh  Theel ;  S,  latticed  hemisphere ,of  Colochirus 

_.     .            ••!••£  cucumis,  Semp;  9,  "wheel     of  Acantho- 

COnSlStency.       Their    principal     Slgmtl-  trochus  mirabilis,  Dan.  and  Kor. 

cance  may  well  be  that  of  protection. 

These  small  calcareous  bodies  may  be  called,  according  to  their  shapes, 
"anchors,"  "wheels,"  "rods,"  "anchor  plates,"  "crosses,"  "lattices," 
"stools,"  "buckles,"  "biscuits,"  "cups,"  "rosettes,"  etc. 

1  On  the  author's  use  of  the  term  "  perisomatie,"  see  footnote,  p.  362. 
VOL.  II 


338  COMPARATIVE  ANATOMY  CHAP. 

The  shape  and  method  of  association  of  these  bodies  is  of  importance  for 
classification,  especially  for  distinguishing  one  species  from  another.  Nearly  all 
their  various  forms  can  be  traced  back,  in  a  way  which  cannot  here  be  further 
described,  Ito  a  common  form,  viz.  to  a  very  short  rod,  which  tends  to  branch 
dicho£oSsly  at  each  end.  In  some  Dendrochirotce  (Psolus,  Theelia,  etc.)  the 
calcareous  bodies  upon  the  (physiologically)  dorsal  side  of  the  body  attain  a 
specially  large  size  (1  to  5  mm.),  so  that  the  back  appears  to  the  naked  eye  to  be 
covered  with  scales  or  plates  (Fig.  228,  p.  287). 

In  the  Dendrochirotcc  an  anterior  part  of  the  body,  the  proboscis,  is  invaginable. 
At  the  posterior  boundary  of  this  proboscis  (when  evaginated)  five  calcareous  oral 
valves  are  found  in  a  few  genera.  When  the  proboscis  is  invaginated  these  come  to 
lie  close  together  in  the  form  of  a  rosette,  which  protects  the  aperture.  In  Psolus 
these  five  oral  valves  are  placed  interradially,  and  each  is  a  large  triangular  calcareous 
plate  (Fig.  228,  p.  287)  ;  in  Colochirus,  Actinocucumis,  etc.,  they  are  arranged  radially 
and  consist  of  compact  masses  of  calcareous  granules  and  ambulacral  papillae.  In 
many  Aspidochirota,  and  Dendrochirota  radially  or  interradially  arranged  anal 
valves  (anal  plates  or  anal  teeth)  also  occur  round  the  anus. 

II.  Eehinoidea. 

The  skeleton  of  the  Eehinoidea  forms  a  plated  covering  called  the 
test,  which  encloses  the  viscera.  The  greater  part  of  this  test  is 
composed  of  the  plates  of  the  perisomatic  system,  since,  as  a  rule,  the 
plates  of  the  apical  system  (the  central  plate,  the  periproctal  plates, 
the  basals  and  radials)  occupy  but  a  small,  and  even  sometimes  a 
minute,  area  at  the  apical  pole.  There  are,  however,  exceptions  to 
this  rule,  e.g.  the  Triassic  genus  Tiarechinus,  in  which  a  considerable 
portion  of  the  test  is  formed  by  the  plates  of  the  apical  system  (cf. 
Fig.  231,  p.  289). 

The  form  of  the  shell  is  thus,  as  a  rule,  in  the  Eehinoidea,  deter- 
mined by  the  perisomatic  skeleton.  The  horizontal  outline  of  the 
shell,  i.e.  the  outline  seen  when  an  Echinoid  shell  is  viewed  from  the 
oral  or  the  aboral  pole,  is  called  the  ambitus.  This  ambitus  in 
regular  Echinoids  is,  as  a  rule,  strictly  circular,  or  else  pentagonal  with 
rounded  corners ;  less  frequently  it  is  oval,  in  which  case  the  greatest 
diameter  of  the  ambitus  need  not  coincide  with  the  symmetrical  axis. 
In  irregular  Eehinoidea  the  ambitus  is  symmetrical,  being  generally 
elliptical  (lengthened  from  before  backward),  or  else  egg-  or  heart- 
shaped. 

In  all  EcMnoidea,  except  the  Spatangoida,  the  mouth  lies  at  the 
centre  of  the  oral  surface  of  the  test ;  in  the  Spatangoida  it  has  shifted 
anteriorly  on  this  surface.  The  mouth,  however,  always  remains  the 
centre  round  which  the  plates  of  the  perisomatic  skeleton  are 
grouped. 

We  have  already  seen  that  in  regular  endocyclic  forms,  the  anus 
lies  in  the  centre  of  the  apical  system,  but  in  exocyclic  forms  it  leaves 
the  apical  system  and  enters  the  posterior  interradius,  where  it  may 
approach  the  ambitus,  or  even  cross  it  on  to  the  oral  surface,  always, 
however,  remaining  in  the  posterior  interradius. 


vin         EGHINODERMATA — MORPHOLOGY  OF  SKELETON        339 

The  whole  perisome,  from  the  mouth  to  the  apical  system,  falls 
into  two  sections :  (1)  a  small  portion  surrounding  the  mouth,  the 
peristome  or  oral  area ;  and  (2)  the  larger  remaining  portion  be- 
tween the  peristome  and  the  apical  system,  the  corona.  In  the  peri- 
stome the  skeletal  pieces  are  usually  loosely  embedded  near  one 
another,  or  imbricate  one  with  the  other,  remaining  movable  one 
against  the  other.  Sometimes  the  peristome  is  membranous,  without 
skeletal  pieces.  In  the  corona  the  skeletal  pieces  are  usually  firmly 
connected  with  one  another  by  means  of  sutures,  like  the  plates  of 
the  apical  system,  together  with  which  they  form  a  rigid  test.  In 
dead  Echinoidea,  and  in  nearly  all  fossil  forms,  this  test  remains 
intact,  while  the  skeleton  of  the  peristome  falls  to  pieces,  and  is 
therefore  rarely  preserved. 

The  perisomatic  skeleton  in  all  Echinoidea  consists  of  two  systems 
of  plates,  which  run  from  the  apical  system  over  the  ambitus  to  the 
mouth  as  ten  meridional  zones  :  five  of  these  zones  or  systems  of 
plates  are  placed  radially,  and  these  are  called  the  ambulacra.  These 
five  zones,  on  which  the  tube-feet  rise,  are  always  in  contact  with  the 
five  radial  (ocular)  plates  of  the  apical  system,  so  that  each  ambulacrum 
touches  an  ocular  plate  with  its  apical  end.  The  ambulacral  plates 
are  perforated  for  the  passage  of  the  ambulacral  vessels,  which  serve 
for  swelling  the  tube-feet.  The  five  other  zones  or  systems  of  plates 
are  interradially  placed,  and  are  called  interambulaera  or  interambu- 
lacral  plate  systems.  They  alternate  regularly  with  the  ambulacra. 

Considering  the  perisomatic  skeleton  of  the  Echinoidea  more  closely,  the  follow- 
ing special  points  are  worth  attention. 


(a)  The  Number  of  the  Vertical  or  Meridional  Rows  of  Plates  in  the  Ambulacra 
(radii)  and  Interambulacra  (interradii). 

In  all  Euechinoidca  (from  Devonian  times  up  to  the  present),  the  corona  consists 
of  twenty  meridional  rows  of  plates,  ten  of  which  united  in  pairs  belong  to  the 
ambulacral  system,  and  ten  also  in  pairs  to  the  interambulacral  system.  Five 
double  rows  of  ambulacral  plates  thus  regularly  alternate  with  five  double  rows  of 
interambulacral  plates. 

In  the  exclusively  Palaeozoic  Palccechinbidea,  the  number  of  meridional  rows  of 
plates  in  both  ambulacra  and  interambulaera  varies.  The  number  of  rows  in  all 
the  five  ambulacra  and  in  all  the  five  interambulaera  of  individuals  of  one  and  the 
same  species  is,  however,  always  the  same. 

In  the  ambulacra,  however,  the  number  of  rows  of  plates  in  the  Palaeechinoidea 
is  usually  two.  The  Mclonitidw  (Fig.  301)  form  the  only  exception,  having  four  to 
ten  rows  in  each  ambulacrum. 

In  the  interradii,  on  the  other  hand,  the  number  of  rows  of  plates  varies. 
Bothriocidaris  has  only  one  single  row  of  plates  in  each  interradius.  In  all  other 
Palccechinoidea  there  are  more  than  two  (3-11)  rows  of  plates  in  each  interradius 
(Fig.  230,  p.  289).  The  interesting  genus  Tiarechimis  (Fig.  231,  p.  289)  is  dis- 
tinguished by  the  great  simplicity  of  its  interradial  system  of  plates  ;  in  each  inter- 
radius there  are  only  four  plates,  a  single  one  at  the  edge  of  the  peristome — the 
large  marginal  plate  of  the  peristome— and  three  intercalated  between  this  and  the 


340 


COMPARATIVE  ANATOMY 


CHAP. 


adjoining  apical  system,  these  plates  being  separated  by  meridional  (perpendicular) 

sutures. 

The  plates  of  the  Eckinoidea  are  most  frequently  pentagonal.  In  the  two  per- 
pendicular rows  of  an  ambulacrum  or 
an  interambulacrum  the  consecutive 
plates  usually  alternate  in  such  a  way 
that  the  suture  between  the  two  rows 
forms  a  zigzag  line.  The  sutures 
between  the  plates,  which  lie  one 
below  the  other  in  a  row,  usually  run 
horizontally  (Fig.  232,  p.  291). 

(6)  The  Pores  perforating  the  Plates 
of  the  Ambulacral  System. 

As  a  rule,  in  the  Echinoidea,  the 
pores  occur  in  pairs.  These  double 
pores  occur  only  on  the  ambulacral 
plates.  One  double  pore  belongs  to 
each  ambulacral  foot.1  From  the 
ampulla,  under  the  test  (at  its  inner 
side),  two  canals  run  out,  which, 
running  separately  through  the  plate, 
unite  at  the  base  of  the  tube-foot  to 

form  a  single  canal,  which  runs  through  the  foot  and  ends  blindly  at  its  tip. 
Originally,  there  was  only  one  pair  of  pores  on  each  ambulacral  plate.  Where  two 
or  more  pairs  occur  on  one  plate,  the  plate  can  be  proved  to  be  composed  of  just  as 
many  fused  plates  as  there  are  pairs  of  pores.  Primary  plates  are  such  as  reach 
from  the  lateral  edge  of  a  two-rowed  ambulacrum  as  far  as  the  median  suture 
between  the  two  rows  of  ambulacral  plates.  Half  plates  are  such  as  do  not  reach 
the  suture,  and  included  plates  such  as  do  not  reach  the  edge  of  the  ambulacrum. 
Isolated  plates  reach  neither  the  edge  nor  the  median  suture  of  the  ambulacrum. 

Besides  the  double  pores  there  are,  in  the  Clypeastroida  and  Spatangoida,  single 
pores  as  well,  to  which  small  tentacles  belong.  The  arrangement  of  these  pores 
varies,  and  they  are  often  not  confined  to  the  ambulacra,  but  are  also  found  on  the 
interradii,  especially  on  the  oral  surface.  Occasionally  they  are  scattered,  often  in 
grooves,  the  so-called  ambulacral  grooves,  which  radiate  out  from  the  peristome, 
and  may  stretch  more  or  less  far  towards  the  ambitus  or  even  beyond  it,  and  may 
be  more  or  less  branched. 


FIG.  301.— Apical  system  and  adjoining  peri- 
some  of  Melonites  multipora,  Norw.  (after  Meek 
and  Worthen).  For  lettering  see  p.  31V. 


(c)  The  Symmetry  of  the  Echinoid  Shell. 

The  test  of  the  regular  Echinoids  (Cidaroida,  Diadematoida,  and  most 
Palceechinoidea),  viewed  superficially,  appears  to  be  strictly  radiate.  The  anal  area 
lies  at  the  apical,  and  the  oral  area  at  the  diametrically  opposite  oral  pole.  All  the 
ambulacra  and  interambulacra  appear  similar  one  to  the  other,  and  the  ambitus, 
with  few  exceptions,  is  circular  or  regularly  pentagonal  with  rounded  corners.  In 
the  Holedypoida  also  the  test,  as  a  rule,  appears  radial,  with  regard  both  to  the 
circular  (or  regularly  pentagonal)  form  of  the  ambitus  and  to  the  similar  develop- 
ment of  the  ambulacra  and  interambulacra.  The  peristome  occupies  its  place  at  the 
centre  of  the  oral  surface.  Notwithstanding  this,  the  longitudinal  axis  and  the 

1  For  the  different  forms  and  arrangements  of  these  feet  or  tentacles,  cf.  section  on  the 
ambulacral  system,  p.  416  et  seq. 


vin         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        341 

plane  of  symmetry  can  be  recognised  in  the  Holedypoida  at  the  first  glance,  because 
the  anal  area  has  shifted  out  of  the  apical  system,  and  into  that  interradius  which  is 
called  the  posterior  interradius.  The  same  is  the  case  in  the  Clypeastroida,  and,  in 
a  still  higher  degree,  in  the  Spatangoida.  In  the  Clypeastroida  the  peristome  with 
the  mouth  still  remains  in  the  centre  of  the  oral  surface,  or  only  very  slightly  shifts 
away  from  this  position.  But  the  ambitus  is  no  longer  circular  or  regularly 
pentagonal ;  its  outline  appears  symmetrically  lengthened  or  shortened  in  the 
direction  of  the  longitudinal  axis,  in  such  a  way  that,  even  in  a  superficial  view,  the 
plane  of  symmetry  is  discoverable.  Apart  from  the  fact  that  the  posterior  inter- 
radius is  at  once  recognisable  by  the  anus  lying  in  it,  it  is  often  further  distin- 
guished in  the  Scutellidcc  by  a  perforation  through  the  test  (lunula),  which  never 
occurs  in  the  other  interradii.  Further,  in  the  Scutellidcc,  the  bilateral  symmetry 
is  often  distinctly  indicated  by  the  number  and  arrangement  of  the  radial  lunulse, 
or  of  the  marginal  incisions  (Figs.  233-235,  pp.  292,  293). 

The  bilateral  symmetry,  which  is  most  pronounced  in  the  Spatangoida,  culmi- 
nates in  the  remarkable  family  of  the  Pourtalesiidce.  The  ambitus,  which  varies 
greatly  in  details,  is  frequently  egg-shaped,  or  heart-shaped,  and  in  Pourtalesia 
flask-shaped.  Not  only  does  the  anus  always  lie  somewhere  in  the  posterior  inter- 
radius, but  the  oral  area  also  shifts  from  the  centre  of  the  oral  surface,  moving  more 
or  less  far  along  this  surface  anteriorly.  In  the  Cassidulidce  all  the  transition  stages 
between  a  central  and  a  frontal  position  of  the  oral  area  occur.  Since  the  mouth, 
with  the  oral  area,  always  forms  morphologically  the  centre  of  all  the  systems  of  radii, 
in  shifting  anteriorly  it  necessarily  draws  along  with  it  the  systems  radiating  out 
from  it.  We  shall  return  later  on  to  the  dissimilarity  in  the  ambulacra,  and  especially 
to  the  abnormal  development  of  the  anterior  ambulacra,  and  consequent  formation  of 
the  bivium  and  trivium,  to  the  special  form  of  the  peristome  of  the  Spatangoida,  etc. 

The  apical  system  also  does  not  always  remain  at  the  dorsal  centre  of  the  test, 
but  shifts  more  or  less  far  forward  (less  frequently  backward),  and  the  highest  point 
of  the  test  may  then  come  to  lie  in  front  of  (less  frequently  behind)  its  central 
point  (Figs.  236-238,  pp.  294,  295). 

We  have  seen  that  in  exocyclic  Echinoidea  (in  which  the  anal  area  lies  some- 
where in  the  posterior  interradius)  the  longitudinal  axis  and  the  plane  of  symmetry 
can  easily  be  made  out  even  in  a  superficial  examination,  they  can  also  be  dis- 
covered by  careful  observation,  even  in  regular  endocyclic  Echinoidea,  which  are 
apparently  strictly  radiate.  When  describing  the  apical  system,  the  constant 
relation  of  the  outer  apertures  of  the  pores  of  the  stone  canal  to  the  right  anterior 
basal  plate,  was  pointed  out.  These  relations  never  quite  disappear,  and  where 
the  apical  system  is  retained,  they  define  with  certainty  the  longitudinal  axis 
and  the  plane  of  symmetry. 

Further,  even  where  the  apical  system  has  not  been  retained,  it  is  always 
possible,  as  has  been  proved  by  a  very  careful  investigation  of  the  Echinoid  test, 
to  determine  the  longitudinal  axis  and  the  plane  of  symmetry  by  the  definite  and 
constant  arrangement  of  the  plates  of  the  test,  both  in  regular  and  irregular  endo- 
cyclic and  exocyclic  Echinoids.  This  constant  relation  of  the  plates  to  one  another 
is  expressed  in  Loven's  law. 

Let  the  test  of  any  Spatangoid  be  laid  with  the  dorsal  (apical)  side  on  a  perpen- 
dicular surface,  in  such  a  way  that  the  mouth  is  directed  upward,  and  the  posterior 
unpaired  interradius  (between  the  bivium)  downward.  Let  the  five  ambulacra  be 
then  marked  with  the  figures  I,  II,  III,  IV,  V  (Fig.  302),  starting  from  the  left 
lower  ambulacrum  (the  right  posterior  of  the  animal)  and  proceeding  in  the  direc- 
tion in  which  the  hands  of  a  watch  travel.  Two  plates  of  each  ambulacrum,  the 
so-called  marginal  peristome  plates,  take  part  in  forming  the  boundary  of  the 
peristome.  The  first  marginal  plate  which  is  met  with  in  each  ambulacrum,  when 


342 


COMPARATIVE  ANATOMY 


CHAP. 


these  ten  plates  carefully,  we  see  that  those  indicated  by  la,  lla,  I 


FIG.  302.— Kleinia  luzonica  (Gray).    Apical  system,  spread  out  (after  Loven).     fa,  Fascicles. 
Further  explanations  in  the  adjoining  text. 

are  larger  and  possess  two  pores  each,  while  the  smaller  plates  Ib,  lib,  Ilia,  I\  b 
and  Va  have  only  one  pore  each.  Only  the  ambulacra  I  and  V,  i.e.  the  two 
posterior  ambulacra,  are  thus  bilaterally  symmetrical,  while  the  two  (paired) 
anterior  ambulacra  17  and  IV,  and  the  two  rows  of  plates  of  the  anterior  unpaired 
ambulacrum  III,  are  asymmetrical.  This  law  holds  for  all  Echinoidea  (not  only 
for  adults  but  for  their  young  stages  also)  ;  the  plates  la,  Ha,  lllb,  IVa,  V6  are 


vm         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        343 


IV 


FIG.  303. — Toxopneustes  drcebachiensis  juv.,  4  mm.  in  diam.     The  whole  system  of  plates 
spread  out  in  one  plane  (after  Loven).    B,  Peristome  plates.     D,  teeth. 

marked  by  common  characters,  and  are  distinguished  from  the  plates  Ib,  lib,  Ilia, 
IVb,  Va,  which  also  resemble  one  another.  These  different  characters  are,  it  is  true, 
often  not  very  evident. 


344 


COMPARATIVE  ANATOMY 


CHAP. 


As  a  further  example,  let  us  take  the  test  of  a  young  Toxopneustes  drcebachiensis, 
4  mm.  in  diameter  (Fig.  303).  If  we  examine  it  we  shall  find  that,  of  the  ten 
ambulacral  plates  bordering  the  peristome,  five,  belonging  to  different  ambulacra, 
are  of  greater  size  (consisting  each  of  three  primary  plates),  and  show  three  double 
pores,  while  the  five  others  are  smaller  (consisting  of  but  two  primary  plates)  and  are 
perforated  by  only  two  double  pores.  "We  can  place  the  test  in  only  one  position,  viz. 
that  given  in  the  figure,  in  which  the  formula  la,  Ila,  lllb,  IVa,  ~Vb,  and  Ib,  lib,  Ilia, 
I VJ,  Va  holds  good.  In  this  we  see  that  a  median  plane,  corresponding  with  that  of 
the  irregular  Echinoidea,  can  be  established  also  for  the  regular  Echinoidea.  The 
accuracy  of  this  law  can  be  proved  by  investigating  the  position  of  the  madreporite. 
In  the  above'case  this  actually  lies  in  the  right  anterior  basal  plate  between  the  radii 
II  and  III. 

Loven's  law  also  applies  to  other  plates  besides  the  ambulacral  "marginal  plates  of 
the  peristome. 

It  may  be  remarked  in  passing  here  that  the  system  of  marking  above  described 
can  be  used  for  naming  all  the  plates  of  the  Echinoid  test ;  in  this  way  we  have 
the  ambulacra  I-V,  the  ambulacral  rows  of  plates  la,  Ib,  Ila,  lib,  Ilia,  lllb,  IVa, 
IV6,  Va,  and  V6,  and  in  the  apical  system  the  radials  I-V.  If  we  mark  the  inter- 
radii  (interambulacra)  1  -5,  starting  from  the  one  lying  to  the  left  of  ambulacrum  I, 
and  proceeding  in  the  direction  of  the  hands  of  a  watch  (viewing  the  test  orally), 
we  get  the  interambulacral  rows  of  plates  la,  Ib,  2a,  26,  3a,  3b,  4a,  46,  5a,  5b,  and 
the  basals  1-5.  The  madreporite  lies  in  basal  2.  The  consecutive  plates,  countino- 

along  each  row  of  ambu- 

«$ 

6    i  £^(©_o;  ©x^r^    3 


lacral  and  interambulacral 
plates,  start  from  the  edge 
of  the  oral  disc. 

The  arrangement  of 
plates  revealed  by  Loven's 
law,  taken  together  with 
the  special  position  of  th.e 
madreporite,  and  with  the 
excentric  position  of  the 
anus  in  the  anal  area  of 
the  regular  Echinoids,  show 
us  that,  strictly  speaking, 
no  Echinoid  is  either  radi- 
ally, or  bilaterally,  sym- 
metrical. 


FIG.  304.  -Peristome  and  neighbouring  parts  of  the  test 
of  Cidaris  hystrix,  Lamk.  (after  Loven). 


(d)  The  Relation  of  the 
Ambulacral  and  In- 
terambulacral Plates 
to  the  Peristome. 

be 


Three    cases    must 
distinguished. 

1.  The  plates,  both  of 

J  ambulacraand  of  the  interambulacra,  arecontinued  in  a  modified  form  over  the  edge 

tome,  and  on  the  peristome  itself,  towards  the  mouth  (Cidaroida,  Fig.  304). 

2.  Only  the  ambulacral  plates  are  continued  on  to  the  oral  integument  (Diade- 

tiaida),  forming  either  several  concentric  rings  of  plates  (Streptosomata,  Echino- 

vte)   or  as  five  pairs  of  plates  lying  isolated  in  the  integument,  the  so-called 

buccal  plates  (Stercosomata). 


vin         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        345 

3.  Xeither  the  anibulacral  nor  the  interambulacral  plates  are  continued  on  to  the 
peristome  (Ifolectypoida,  Clypeastroida,  Spatangoida). 

Among  the  Palceechinoidea  also  there  are  forms  in  which  the  perisomatic  plates 
reach  as  far  as  the  mouth ;  in  Lepidocentrus,  indeed,  they  do  this  in  such  a  way  as 
to  make  it  impossible  to  distinguish  the  coronal  from  the  peristomal  plates. 

Apart  from  the  peristome  plates  just  mentioned,  the  oral  area  is  beset  all  over 
with  small  irregularly  arranged  calcareous  bodies. 

With  regard  to  the  number  of  coronal  plates  which  border  the  peristome  (mar- 
ginal plates  of  the  peristome),  it  is  to  be  noted  that  in  regular  Echinoidea  (Cidaroida, 
Jti'fderiiatoida),  and  even  in  most  Ifolectypoida,  ten  pairs  occur,  five  ambulacral  and 
five  interambulacral.  There  are,  however,  certain  Holectypoida  in  which,  in  one  or 
several  interradii,  only  a  single  marginal  plate  occurs.  In  the  Clypeastroida  (Fig. 
306)  and  Spatangoida  (Fig.  302)  the  peristome  is,  as  a  rule,  bordered  by  five  pairs  of 
anibulacral  and  five  single  interambulacral  marginal  plates.  Exceptions  to  this  rule 
are  found  in  the  Spatangoid  division,  the  Cassiduloidea,  where,  for  example,  among 
the  Echijioticidcc,  Echinoncus  and  Amblypygiis  have  two  marginal  plates  in  their 
second  and  fourth  interradii  and  only  one  in  the  others. 

(e)  Manner  in  which  the  Skeletal  Plates  are  Connected. 

In  most  Euechinoidca  the  plates  of  the  skeleton,  at  least  those  of  the  corona,  are 
firmly  and  immovably  connected  together  by  means  of  sutures,  and  thus  form  a 
rigid  test.  This  is  not  the  case  in  very  many  Palceechinoidea,  and  among  the 


FIG.  305.— Oral  area  of  Cidaris  papiUata,  Leske,  from  within  (after  Loven). 
apo,  Perignathous  apophyses. 

Encchinoidea  in  the  Diadcmatoid  Echinothuridce  ;  also,  as  far  as  the  skeleton  of  the 
peristome  is  concerned,  in  the  Cidaroida  (Fig.  305).  The  edges  of  the  plates  here 
overlap,  i.e.  they  are  imbricated.  In  the  Echinothuridce  the  plates  are  divided  from 
one  another  by  strips  of  uncalcified  connective  tissue,  which,  to  some  extent, 
allow  the  test  to  change  its  shape.  The  imbrication  of  the  anibulacral  plates  is  in 
a  direction  opposite  to  that  of  the  interambulacral.  Viewing  the  test  from  without, 


346 


COMPARATIVE  ANATOMY 


CHAP. 


the  imbrication  of  the  ambulacra  is  adoral,  i.e.  the  oral  edge  of  each  plate  overlaps 
the  apical  edge  of  the  next  in  order  below  it,  whereas,  in  the  interambulacra,  the 
imbrication  is  apical.     Lateral  imbrication  also  occasionally  occurs. 
Slight  imbrication  is  also  found  in  certain  Spatangoida. 

(/)  Special  Modifications  of  the  Ambulacra. 

In  all  Echinoidea,  in  which  the  mouth  remains  at  the  centre  of  the  oral  surface, 
the  five  ambulacra  are  alike  in  length,  breadth,  and  in  the  arrangement  of  their 

in 


F,o.  306.-Sy*tem  of  plates  of  a  Clypeastroid  (Encope  Valenciennesi,  Agass.),  spread  out 

(after  Loven). 


P™mi"el'CefS'  etc:,  They  0»'y  ™y  »  length  when  the  apieal  system,  towards 

from  thf     J"g   f7        .PeriSt°me  aCTOSS  the  ambitus'  they  c°n™'«e'  *•  shifted 
-he  centre  of  the  ap.cal  hemisphere  to  a  somewhat  anterior  (less  frequently 
1-ostenor)  pos.bon.     If  the  test  of  such  an   Echinoid,  in   which   the  ambulacra 


vni         ECHINODERMATA—  MORPHOLOGY  OF  SKELETON        347 

are  of  unequal  length  owing  to  the  shifting  of  the  apical  system,  be  viewed  from 
the  oral  side,  the  ambulacra  still  form  a  regular,  or  almost  regular,  five -rayed 
star  round  the  central  oral  aperture  or  peristome.  Where,  however,  as  in  the 
Spatangoida,  the  peristome  with  the  mouth  has  moved  from  the  centre  of  the  oral 
surface  (on  which  the  Echinoids  creep),  and  is  shifted  more  or  less  anteriorly,  and 
finally,  where  in  the  Pourtalesia  it  comes  to  lie  quite  on  the  anterior  ambitus,  the  parts 
taken  by  the  five  ambulacra  in  the  formation  of  the  oral  surface  are  necessarily  very 
different.  The  unpaired  anterior  ambulacrum  (III)  and  the  two  anterior  and 
lateral  ambulacra  (II  and  IV)  shorten  and  form  an  ever  smaller  portion  of  the 
whole  ambulacra!  area  of  the  oral  (ventral)  surface,  in  proportion  as  the  peristome 
with  the  mouth  shifts  forward.  They  form  together  the  trivium.  Conversely,  the 
two  posterior  radii  at  the  same  time  lengthen  and  form  an  increasingly  large 
portion  of  the  ambulacral  area  of  the  ventral  surface.  They  form  the  bivium.  The 
length  of  the  ambulacra  of  the  trivium  and  the  bivium  in  the  apical  direction  is  of 
course  determined  by  the  position  of  the  apical  system.  If  this  system  shifts  for- 
ward, the  trivium  is  shortened  apically  ;  if  backward,  the  ambulacra  of  the  trivium 
(especially  the  anterior  unpaired  ambulacrum)  are  lengthened,  while  those  of  the 
bivium  are  shortened.  This  grouping  of  the  ambulacra  into  an  anterior  trivium 
and  a  posterior  bivium  is  especially  clear  on  the  apical  surface  of  those  Spatangoida 
which  have  a  diffused  apical  system,  e.g.  the  Collyritidce  and  Pourtalesiidce  (cf.  pp. 
324,  325).  Since  the  apical  ends  of  the  ambulacra  are  always  in  contact  with  the 
radial  plates  of  the  apical  system,  and  since,  further,  in  the  diffused  apical  system 
the  two  posterior  radials  I  and  Y,  which  are  separated  from  the  anterior,  are 
shifted  posteriorly,  the  apical  ends  of  the  two  posterior  ambulacra  (the  bivium)  are 
also  necessarily  separated  from  the  three  anterior  ambulacra  (the  trivium)  by  a  con- 
siderable space  (Fig.  284,  p.  325). 

In  the  Palccechinoidea,  and  among  the  Euechinoidea  in  the  Cidaroida,  the  Diade- 
matoida,  nearly  all  Holeetypoida,  and  many  Spatangoida,  the  ambulacra  throughout 
their  whole  courses  have  a  similar  structure,  and  are  similar!}7  provided  with  pores. 
In  the  Clypeastridce  and  many  Spatangoida,  however,  the  ambulacra  are  modified  on 
the  apical  side  in  a  characteristic  manner ;  they  are  petaloid,  each  ambulacrum  forming 
a  petalodium  (Figs.  233,  234,  p.  292  ;  236,  p.  294,  and  306).  Such  a  petalodium 
arises  by  the  divergence  of  the  two  rows  of  large  double  pores  of  each  ambulacrum 
from  one  another  immediately  on  leaving  the  apex,  and  their  reapproximation  and 
junction  before  they  reach  the  ambitus.  The  two  rows  of  pores  of  each  petalodium 
make  a  figure  like  a  lancet-shaped  leaf,  and  the  five  petaloids  together  form  round 
the  apex  a  graceful  rosette  of  leaves,  which  recalls  the  petals  of  a  flower.  On  the 
remaining  plates  of  the  ambulacra,  i.e.  those  not  forming  the  petalodium,  the  pores 
are  single  and  small ;  they  are,  further,  few  in  number  and  scattered.  Between 
the  regular  ambulacra  and  those  which  have  apical  petaloids  there  are  many  transi- 
tion forms,  occurring  often  within  one  and  the  same  family.  One  of  these  transi- 
tions is  specially  frequent ;  the  two  rows  of  pores  of  a  petalodium  do  not  unite 
at  their  oral  ends  but  remain  open.  The  ambulacra  are  then  called  sub-petaloid. 
Such  petaloids  are  often  very  long. 

The  petaloids  often  sink  in  (Fig.  236,  p.  294),  and  then,  not  infrequently,  serve 
as  brood  cavities,  or  marsupia,  for  containing  the  young. 

Just  as  the  ambulacra  occasionally  form  petaloid  rosettes  round  the  apical  system, 
so,  in  the  family  of  the  Cassidulidw  (sub-order  Cassiduloida  of  the  order  Spate  n- 
yoida),  can  they  form  rosettes  of  so-called  phyllodes  round  the  peristome  (Fig.  307). 
The  five  phyllodes,  in  which  the  well-developed  double  pores  lie  thickly  crowded 
together,  sink  in,  while  the  five  interradial  marginal  plates  of  the  peristome  between 
them  are  contrariwise  bulged  out.  The  five  interradial  cushions  form,  together  with 
the  five  radial  phyllodes,  what  is  called  a  floscelle. 


348  COMPARATIVE  ANATOMY  CHAP. 

The  anterior  unpaired  ambulacrum  in  many  exocyclic  Echmoidea  differs  greatly, 
th  in  s^Tlnn  the  number,  arrangement,  and  form  of  its  pores,  from  the  other 


both  in  s 


a 
V 

FIG.  307.-0ral  perisome  of  Cassidulus  pacificus,  Ag.,  with  the  five  phyllodes  (after  Loven). 

four.  This  variation  in  the  anterior  ambulacrum  is  found  almost  exclusively  in  the 
order  Spatangoida,  especially  in  the  Cassiduloid  family  Plesiospatangidce  and  in  the 
sub-order  Spatangoidea  (here  especially,  and,  to  a  very  marked  degree,  in  the  family 
of  the  Spatangidce). 

((/)  Special  Modifications  of  the  Interradii. 

We  can  here  only  point  out  certain  conditions  occurring  in  the  order  Spatan- 
goida. 

In  the  sub-order  Spatangoidea  an  extraordinary  asymmetry  of  the  two 
posterior  interradii  1  and  4  prevails  (cf.  Fig.  302,  p.  342).  The  right  posterior  in- 
terradius  1  is  always  so  modified  near  the  peristome  that  two  plates  fuse,  thus  con- 
trasting with  the  left  posterior  interradius,  which  remains  only  slightly  if  at  all 
modified.  This  fusion  takes  place  either  between  the  second  and  third  plates  of  the 
row  la,  or  the  two  second  plates  of  rows  la  and  1&,  or  the  second  and  third  plates 
of  row  b  and  the  second  plate  of  row  a.  In  the  last  case,  the  second  plates  of  the 
two  rows  of  interradius  4  are  also  fused. 

Since,  in  the  Spatangoida,  the  peristome,  with  the  mouth,  is  shifted  forward  on 
the  oral  surface,  the  posterior  unpaired  interradius  occupies  a  considerable  portion  of 
the  ventral  surface  (and  this  is  also  the  case  in  the  Cassiduloidca  with  mouth  shifted 
forward).  It  is  often  somewhat  bulged  out,  and  the  region  occupied  by  it  on  the 
oral  side  is  known  as  the  plastron.  It  takes  part  in  the  limitation  of  the  peristome 
by  means  of  a  single  crescent-shaped  plate,  which  is  known  as  the  labrum  in  those 
forms  which  have  a  projecting  under-lip  to  the  transverse  peristome  (cf.  Fig.  302, 
p.  342).  In  many  Spatangoida  the  labrum  is  followed  posteriorly  by  two  large  sym- 
metrically arranged  plates  (sternum),  which  again  are  followed  by  two  smaller  but 


vin         ECHINODERMATA — MORPHOLOGY  OF  SKELETON        349 

still  not  insignificant  plates  (episternum).  The  test  is  then  amphisternal.  In 
other  forms,  however,  the  arrangement  of  the  plates  on  the  plastron  (apart  from  the 
labruni)  approaches  the  usual  arrangement,  i.e.  the  plates  of  the  two  rows  alternate 
more  or  less  distinctly,  so  that  the  median  suture  which  divides  them  forms  a  zig- 
zag line.  This  arrangement,  as  compared  with  that  first  described,  is  older  and  more 
primitive.  The  test  is  then  called  meridosternal. 

In  most  Clypmstrid.ai  the  interambulacra  are  interrupted,  i.e.  they  do  not  run 
continuously  from  the  apical  system  to  the  peristome,  but,  near  the  latter,  are 
crowded  out  by  the  broad  plates  of  the  ambulacra  which  touch  one  another  inter- 
radially,  so  that  the  five  interradial  marginal  plates  of  the  peristome  are  completely 
isolated  from  the  remaining  portions  of  the  interambulacra  (Fig.  306).  Not  infre- 
quently, the  paired  interambulacra  are  interrupted  and  the  unpaired  posterior  inter- 
ambulacrum  is  uninterrupted. 

(h)  Form  of  the  Peristome. 

In  most  Echinoidea,  i.e.  in  those  in  which  the  peristome  retains  its  central  posi- 
tion, its  shape  is  pentagonal,  or  decagonal,  or  round,  less  frequently  oval  or  oblique, 
or  quite  irregular,  often  with  branchial  incisions.  But  where  the  peristome  is 
shifted  anteriorly,  as  in  the  sub-order  Spatangoidea,  the  peristome  is  transverse  and 
crescent-like,  with  depressed  anterior  upper-lip  and  raised  posterior  under-lip.  The 
peristome,  however,  is  always  central  in  the  embryo,  and  is  originally  pentagonal. 

(i)  Ornamentation. 

The  outer  surface  of  the  plates  of  the  Echinoid  test  are  beset — in  many  different 
ways,  which  are  of  importance  in  classification — with  numerous  larger  or  smaller  pro- 
minences, granules,  etc.,  on  which  spines  and  pedicellariae  are  planted. 

In  the  sub-order  Spatangoidea,  narrow,  finely  granulated  streaks  or  bands  run,  in 
definite  arrangement,  along  the  surface  of  the  test,  and  carry  small  rudimentary 
spines  or  pedicellarise.  These  are  called  fascioles  or  Semites  (Fig.  302,  p.  342).  The 
following  systematically  important  forms  of  fascioles  are  to  be  distinguished  : — 

1.  The  peripetaloid  fascicle  encircles  the  apical  rosette  of  petaloids. 

2.  The  lateral  or  marginal  fascicle  runs  round  the  shell  near  the  ambitus. 

3.  The  lateral  subanal  fasciole  branches  off  from  the  peripetaloid  fascicle  and 
runs  below  the  anus. 

4.  The  subanal  fasciole  forms  a  ring  below  the  anus  (between  the  latter  and 
the  peristome).     They  may  give  off  anal  branches  which  run  up  on  each  side  of  the 
anus,  and  occasionally  unite  above  it  to  form  an  anal  fasciole. 

5.  The  internal  fascioles  run  around  the  apex  and  the  anterior  ambulacrum. 
The  tentacles  and  plates  in  those  regions  which  are  encircled  by  the  internal 

and  subanal  fascioles  are  modified. 

One  very  varied  form  of  ornamentation  of  the  Echinoid  test,  which  arises  early 
during  postlarval  development,  is  due  to  the  deposit  of  calcareous  substance  on  the 
plates,  and  is  known  as  epistroma. 

(£)  Marginal  Incisions  or  Perforations. 

These  are  often  to  be  found  in  the  flat  disc-shaped  test  of  the  Sadellidcc,  in 
some  or  all  of  the  ambulacra,  and  not  infrequently  also  in  the  posterior  inter- 
ambulacrum.  The  edge  of  the  shell  is  at  first  entire,  but  during  growth  marginal 
indentations  and  incisions  make  their  appearance,  and  these  may  close  to  form  per- 
forations (lunula).  (Figs.  234,  235,  pp.  292,  293,  and  306,  p.  346.) 


350 


COMPARATIVE  ANATOMY 


CHAP. 


(0  The  Perignathic  Apophysial  Girdle  (Figs.  308,  and  348,  p.  402). 

In  all  Echinoidea  in  which  the  mouth  is  armed  with  five  teeth,  moved  by  a  com- 
plicated masticatory  apparatus,  i.e.  in  all  Eclmwidea  except  the  Spatangoida  and  a 
few  Holectypoida,  processes,  directed  apically  inwards,  are  found  at  the  peristomal 
edge  of  the  test ;  these  serve  for  the  attachment  of  the  muscles  and  bands  of 
masticatory  apparatus.       They  either  consist  solely  of  the   ambulacral   or   inter- 
ambulacral marginal  plates  of  the  peristome  bent  round  inwards,  or  else  a  few  oi 
plates  next  in  order  also  take  part  in  their  formation. 

These  processes  may  be  divided  into  those  Avhich  rise  on  the  ambulacral  marginal 
plates   and  those  which  rise  on  the  interambulacral  marginal  plates.     The  former 

may  be  called  the   ambulacral   apophyses, 
the  latter  the  interambulacral  apophyses. 

The  apophysial  circle  is  closed  or  inter- 
rupted. In  the  former  case,  Avhich  is  best 
illustrated  by  the  Diadematoida  (Fig.  308,  A), 
an  apophysis  rises  on  the  peristomal  margin 
of  each  ambulacral  area  on  each  side  of  the 
ambulacral  suture.  The  tAvo  apophyses  of 
one  and  the  same  ambulacrum  usually  unite 
at  their  free  ends,  Avhich  project  into  the 
body,  in  such  a  way  as  together  to  form  a 
kind  of  arch  ;  this  is  called  an  auricle,  and 
affords  passage  for  some  of  the  important 
organs  (for  the  trunks  of  the  radial  ambulacral 
vessels,  of  the  nerves,  etc. ).  There  are  thus, 
in  all,  ten  ambulacral  apophyses,  which  may 
unite  in  pairs  to  form  five  auricles.  The 
interambulacral  apophyses  project  less  far  into 
The  tAvo  apophyses 

of  one  and  the  same  interambulacrum  together 
form  a  ridge  AA'hich  runs  along  the  edge  of 
the  peristome,  and  connects  two  neighbour- 
ing auricles  ;  these  ridges  are  generally  fused 
Avith  one  another  and  with  the  auricles. 

Such  a  closed  apophysial  ring,  which  rises  on  the  edge  of  the  peristome  and  pro- 
jects into  the  body,  may  be  compared  to  a  circular  Avail  with  high  arched  gateAvays 
at  five  radially  arranged  points.  The  five  arched  gateAvays  Avould  represent  the 
auricles,  i.e.  the  five  pairs  of  ambulacral  apophyses,  and  the  circular  Avail  Avould 
be  formed  of  the  five  pairs  of  interambulacral  apophyses. 

In  the  Cidaroida  (Fig.  308,  B  and  C)  the  apophysial  ring  is  interrupted.  The 
ambulacra!  apophyses  are  A\-anting,  but  the  interambulacral  apophyses  are  all  the 
more  strongly  developed,  and  form  ear-shaped  processes.  The  tAvo  apophyses  of  an 
interambulacrum  are  connected  by  a  suture  at  their  bases,  but  diverge  at  their  tips. 
When  the  two  interambulacral  apophyses  standing  at  the  sides  of  an  ambulacrum 
approximate  above  it  (the  ambulacrum),  but  without  fusing,  a  false  auricle  may 
be  formed. 

The  ambulacral  apophyses  are  also  Avanting  in  a  feAv  Holectypoida ;  Avhere  they 
are  present,  they  do  not  unite  in  pairs  to  form  auricles. 

In  all  Clypeastroida,  the  apophysial  ring  is  interrupted,  and  consists  either  of 
ambulacral  or  of  interambulacral  apophyses. 


FIG.  308.— The  perignathic  apophyses 
of  a  radius  and  of  the  two  neighbouring 
interradii  of  various  Echinoidea.  A, 
Diadematoid.  The  apophyses  of  the 
ambulacral  plates  (am)  form  true  auriculas  the  interior  of  the  body, 
(cutr).  B,  Cidaroid.  Apophyses  are  formed, 
not  by  the  ambulacral  but  by  the  inter- 


interambulacral  plates  have  fused. 


vin        ECHINODERMATA—  MORPHOLOGY  OF  SKELETON        351 


III.  Asteroidea. 

Here  also  the  perisomatic  portion  forms  by  far  the  greater  part 
of  the  whole  skeleton.  Only  in  a  few  forms  does  the  apical  system 
constitute  a  distinctly  appreciable  element  in  the  skeleton.  Further, 
the  oral  system  also,  even  if  we  include,  besides  the  orals  (odonto- 
phores,  proximal  plates  of  the  interbrachial  system),  the  terminals,  as 
radials  belonging  to  the  oral  system,  forms  but  a  very  small  fraction 
of  the  whole  skeleton. 

The  skeleton  of  the  Asteroidea  is  distinguished  from  that  of  most 
Ecliinoidea  by  its  mobility.  It  is  not  a  rigid  capsule,  but  its  principal 
plates  are  articulated  one  with  another,  and  are  movable  one  upon 
another  by  means  of  muscles.  The  arms  can  bend  upwards  and 
downwards,  and  also  occasionally,  to  a  certain  degree,  laterally  (in  the 
horizontal  plane).  The  ambulacral  furrows  may  be  deep,  or  shallow. 
The  disc  is  sometimes  shortened  in  the  direction  of  the  principal  axis, 
i.e.  flattened. 

In  the  perisomatic  skeleton  of  the  Asteroidea  three  principal  parts 
may  be  distinguished  :  (1)  the  ambulacral,  (2)  the  interambulacral, 
and  (3)  the  accessory. 

(a)  The  Ambulacral  Skeleton. 

From  the  free  end,  or  tip,  of  each  arm  or  ray  a  large  median  groove 
runs  on  the  oral  side  to  the  centre  of  the  disc,  and  here  runs  into  the 


Fir;.  30;<.  -Transverse  section  through  the  brachial  skeleton  of  Astropecten  aurantia- 
cus  (Gray) ;  original.  For  lettering  see  p.  317.  sa,  Supports  of  the  ambulacral  plates  or  supra- 
ambulacral  plates ;  ad,  adambulacral  plates ;  p.^paxillse ;  1,  position  of  the  radial  canal,  etc. ; 
•2,  ampullse  ;  3,  ambulacral  feet. 

mouth.  In  the  base  of  this  ambulaeral  furrow  rise  the  ambulacral, 
or  tube-feet  in  two  or  four  longitudinal  rows  (Figs.  239,  243,  pp.  296, 
298,  and  343,  p.  396).  The  plates  of  the  ambulacral  skeleton,  which 


352 


COMPARATIVE  ANATOMY 


CHAP. 


may  be  compared  with  vertebrae,  and  are  the  principal  pieces  of  the 
skeleton,  form  a  long  roof  over  the  ambulacral  furrow,  which  opens 
downwards.  In  a  transverse  section  through  the  arm  of  an  Asteroid 
(Fig.  309)  we  see  that  the  roof  of  the  furrow  invariably  consists  of 
four  skeletal  pieces.  Two  of  these  pieces— the  ambulaeral  ossicles 
(am)— form  the  greater  part  of  the  roof.  They  lie  symmetrically  to 
the  median  plane  of  the  arm,  and  articulate  with  one  another  along 
the  ridge  of  -  the  roof.  The  two  other  skeletal  pieces — the  adambu- 
laeral  °ossieles  (ad) — meet  the  diverging  edges  of  the  ambulacral 
ossicles,  and  so  lie  at  the  edge  of  the  furrow,  or,  in  other  words,  at 
the  lower  lateral  edges  of  its  skeletal  roof. 

The  general  form  of  the  ambulacral  ossicles  is  that  of  transversely  elongated 
clasps.  They  are  arranged  in  two  longitudinal  rows  in  close  proximity  to  one 
another,  and  in  this  way  form  the  roof,  which  arches  over  the  groove  along  the  whole 
of  its  course",  from  the  tip  of  the  arm  to  the  mouth. 

In  the  Euasteroidea  (to  which  sub-class  all  recent  forms  belong)  the  ambulacral 
ossicles  of  the  two  rows  are  arranged  in  pairs,  each  ossicle  on  one  side  of  the  roof 


FIG.  310.— Scheme  of  the  oral  skeleton  of  the  Asteroidea,  from  the  inner  side  (after  Ludwig). 
or,  Oral  plate  (odontophore)  ;  MI,  first  lower  transverse  muscle  of  the  jambulacral  furrow  ;  Mi,  the 
iuterradial  muscle;  I-VI,  first  to  sixth  ambulacral  ossicles;  1-6,  first  to  sixth  adambulacral 
ossicles ;  a,  b,  c,  d,  e,  f,  apertures  for  the  ampullae  of  the  tube-feet. 

corresponding  with  one  on  the  other  side.  In  the  Palwasteroidea,  on  the  contrary, 
the  ossicles  alternate,  at  least  in  the  middle  part  of  the  arm. 

The  (smaller)  adambulacral  plates  usually  alternate  regularly  with  the  ambulacral 
plates. 

We  must  here  emphasise  the  important  fact  that  the  ambulacral  ossicles  of  the 
Asteroidea  lie  much  deeper  than  the  skeletal  pieces  of  the  same  name  in  the 
Echinoidea.  In  the  latter  class  they  are  quite  superficial,  the  radial  trunks  of  the 
water  vascular  system,  as  well  as  the  radial  nerves  and  the  spaces  of  the  schizocoel, 
are  to  be  found  on  their  inner  side  ;  whereas,  in  the  Asteroidea,  these  organs  lie  on 
the  outer  side  under  the  ambulacral  roof.  Of  the  whole  ambulacral  vascular  system 
only  the  ampullae  lie  on  the  inner  sides  of  the  ambulacral  ossicles,  i.e.  that  turned 
towards  the  general  body  cavity. 

Between  every  two  consecutive  ambulacral  ossicles  there  is  one  (and  never  more 
than  one)  aperture  for  the  passage  of  a  tube -foot.  The  number  of  ambulacral 
ossicles  in  a  row  thrs  always  corresponds  quite  accurately  with  the  number  of  the 
tube-feet  on  the  same  side  of  the  ambulacral  furrow. 

Each  aperture  for  the  passage  of  a  tube-foot  normally  lies  in  the  corner  between 


, 


viii         ECHINODERMATA—  MORPHOLOGY  OF  SKELETON^     353 

two  ambtilacral  ossicles  and  an  adambulacral  ossicle  (cf.  Fig.  310).  In  those 
Asteroids  which  have  four  longitudinal  rows  of  tube-feet,  however,  these  apertures, 
at  some  distance  from  the  mouth,  alternate  regularly  in  such  a  way  that  the 
laterally  placed  aperture  of  one  interstitium  is  followed  by  a  more  median  aperture 
in  the  next  interstitium,  the  next  again  being  lateral,  and  so  on.  The  connecting 
line  between  the  apertures  of  one  and  the  same  side  of  an  ambulacrum  in  this  case 
forms  a  zigzag,  the  angle  of  which  is  the  more  pointed  the  narrower  the  ambulacral 
ossicle.  The  consequence  of  this  is,  that  the  tube-feet  which  stand  in  the  corners  of 
the  zigzag  line  appear  arranged  in  two  rows,  that  is,  in  the  whole  ambulacrum,  in 
.four  rows. 

The  oral  aperture,  which  always  lies  in  the  centre  of  the  ventral 
surface  of  the  disc,  and  into  which  the  ambulacral  furrows  of  the  arms 
converge,  is  surrounded  by  a  circle  of  firmly  connected  calcareous 
pieces,  the  external  edges  of  which  are  in  immediate  contact  with  the 
ambulacral  and  adambulacral  ossicles.  This  circle  forms  the  oral 
skeleton  of  the  Asteroidea.  It  is  extremely  probable  that  its  separate 
pieces  (which  in  the  five-rayed  forms  number  thirty,  and  in  forms  with 
a  greater  number  of  rays  are  six  times  as  numerous  as  the  rays)  are 
merely  the  transformed  and  more  firmly  connected  proximal  ossicles 
of  the  ambulacral  and  adambulacral  rows.  In  this  case,  in  each  ray 
or  arm,  the  first  two  pairs  of  ambulacral  and  the  first  pair  of  adambu- 
lacral plates  of  these  rows  (in  Ctenodiscus,  the  first  three  ambulacral 
and  the  first  two  adambulacral  pairs  of  plates)  would  take  part  in  the 
formation  of  the  oral  skeleton.  The  oral  skeleton  is  ambulaeral  (in 
many  Cryptozonia)  or  adambulaeral  (in  the  Phanerozoma  and  some 
Cryptozonia\  according  as  the  ambulacral  or  the  adambulacral  portions 
of  the  circle  project  the  further  into  the  oral  cavity. 


(b)  The  Interambulaeral  Skeleton. 

This  comprises  the  ambitus,  i.e.  the  whole  surface  of  the  body 
between  the  oral  (or  ventral)  and  the  apical  (or  dorsal)  regions,  on  both 
of  which,  however,  interambulacral  plates  may  be  found.  The  inter- 
ambulacral  skeleton  thus  forms  the  lateral  walls  of  the  arms.  The 
pieces  constituting  it  are  called  marginal  plates,  and  are  arranged  in 
each  lateral  wall  in  two  rows,  one  above  the  other.  The  upper  row 
consists  of  the  supramarginal  (Fig.  309  sm)  and  the  lower  of  the  mfra- 
marginal  (im)  plates.  It  only  rarely  happens  (e.g.  in  Luidia)  that 
the  marginal  plates  agree  in  number  and  length  with  the  ambulacral 
ossicles.  The  marginal  plates,  which  in  the  order  PJmnerozonia  are 
large  and  well  developed,  become  reduced  in  that  of  the  Cryptozonia, 
being  difficult  to  distinguish  externally.  They  may  be  altogether 
wanting,  or  else  represented  merely  by  microscopically  small  rudi- 
ments. The  row  of  inframarginal  plates  may  be  separated  from  that 
of  the  adambulacral  ossicles  by  a  row  of  small  intermediate  plates.  In 
the  same  way  a  row  of  small  intermediate  plates  may  be  intercalated 
between  the  two  rows  of  marginal  plates. 

VOL.  II  2  A 


354  COMPARATIVE  ANATOMY  CHAP. 

(c)  The  Accessory  Skeletal  System. 

In  this  system  may  be  included  all  those  plates  or  ossicles  which  occur  in  those  parts 
of  the  body  not  covered  by  the  ambulacral  and  marginal  systems.  This  accessory  system 
is  very  variously  developed,  and  a  comparative  study  of  it  cannot  here  be  under- 
taken. The  plates  differ  greatly  in  size,  form  and  ornamentation,  and  arrangement, 
sometimes  being  scattered  or  lying  loosely  near  one  another,  or  else  closely  approxi- 
mated, sometimes  imbricating  or  reticulating  by  means  of  anastomoses  of  skeletal 
pieces. 

Not  infrequently  either  the  whole,  or  parts,  of  the  accessory  skeleton  are  reduced. 
It  is  often  covered  by  a  considerable  layer  of  integument,  and  is  difficult  to  dis- 
tinguish externally.  Its  plates  may  diminish  greatly  in  size,  even  becoming  micro- 
scopically small,  but  they  are  rarely  altogether  wanting. 

Three  sub-divisions  of  the  accessory  skeleton  may  be  distinguished  : — 

1.  The  dorsal,  abactinal,  or  apical  accessory  system,  when  present,  consists 
of  skeletal  plates  developed  in  the  dorsal  integument  of  the  disc  and  in  the  arms. 
We  have  seen  above  that  in  the  Asteroidea  the  apical  system  only  rarely  takes  any 
recognisable  part  in  the  formation  of  the  dorsal  skeleton.     There  are,  nevertheless, 
forms  (e.g.  Cnemidaster]  in  which  the  large  and  distinct  plates  of  the  apical  system 
form  almost  the  whole  of  the  dorsal  protection  of  the  disc. 

2.  The  ambital  accessory  system  consists  of  the  intermarginal  plates  already 
mentioned  as  occasionally  being   intercalated   between  the   supra-  and  the  infra- 
marginal  rows  of  plates. 

3.  The  ventral,  actinal,  or  oral  system  in  the  same  way  consists  of  the  already 
mentioned  intermediate  plates  which  may  occur  between  the  inframarginal  and  the 
adambulacral  plates.      It  is  most  developed  in  those   forms   in  which  the  disc 
increases  in  size  at  the  expense  of  the  arms.  i.e.  in  forms  whose  outline  is  more  or  less 
pentagonal.    The  ventral  accessory  plates  then  fill  up  the  larger  or  smaller  triangular 
regions  between  the  ambulacral  farrows  on  the  lower  side  of  the  disc. 

Finally,  two  other  skeletal  systems  which  occasionally  occur  in  the  Asteroid 
body  must  be  mentioned. 

In  a  certain  number  of  Star-fish  each  ambulacral  ossicle  is  connected  by  a  skeletal 
plate,  or  more  rarely  by  a  row  of  two  to  three  firmly  united  plates,  through  the 
body  cavity,  with  a  marginal  plate  of  its  own  side,  or  else  with  a  laterally  placed 
accessory  plate.  These  simple  or  compound  skeletal  pieces,  which  are  limited  to 
the  arms,  and  which  here  correspond  in  number  with  the  ambulacral  ossicles,  are 
called  supports  to  the  ambulacral  ossicles  or  supraambulacral  plates  (Fig.  309  ««.). 

The  other  skeletal  system,  which  occurs  especially  in  Asteroidea  with  large  discs, 
but  is  altogether  wanting  in  many  forms,  is  called  the  interbrachial  system.  It 
continues  the  divisions  between  the  arms,  either  completely  or  incompletely,  into 
the  interior  of  the  disc,  and  consists  either  of  interbrachial  walls,  running  from  the 
oral  to  the  actinal  skeleton,  or  of  interbrachial  chains  of  skeletal  plates  descending 
vertically  to  the  oral  skeleton.  In  each  interradius  a  proximal  plate  of  this  inter- 
brachial skeleton,  however,  always  enters  into  closer  relations  with  the  oral  skeleton. 
These  plates  are  the  orals,  already  mentioned  in  the  section  on  the  oral  system. 

At  the  free  end  of  each  arm  in  every  Asteroid  there  is  to  be 
found  a  single  median  skeletal  plate,  which  is  sometimes  of  consider- 
able size  and  distinctly  visible,  sometimes  small  and  inconspicuous  ; 
it  carries  on  its  lower  side  a  visual  organ.  These  plates  are  called 
ocular  plates  o;-  terminals.  According  to  recent  investigations  they 
develop  very  early  (apparently  first  of  all  the  plates)  over  the  left 


vin        ECHINODERMATA— MORPHOLOGY  OF  SKELETON        355 

coelomic  vesicle.  They  must  thus  belong  to  the  oral  system,  and 
perhaps,  in  this  system,  correspond  with  the  radials  in  the  apical 
system. 

In  the  development  of  the  Asteroidea  the  formative  centre  of  each 
newly  appearing  plate  in  a  radius  of  the  perisomatic  system  is  always 
immediately  proximal  to  the  ocular  plate  of  the  arm.  At  these 
points  new  plates  continually  appear  between  those  last  formed  and  the 
ocular  plates,  which  thus  always  remain  at  the  free  tips  of  the  arms. 

(d)  Comparison  of  the  Perisomatic  Skeleton  of  the  Asteroidea  with  that  of 
the  Echinoidea. 

The  ocular  plates  (terminals)  of  the  Asteroidea  bear  to  the  newly  appearing 
plates  of  the  perisomatic  skeleton  relations  altogether  similar  to  those  which  the 
radials  (also  "oculars  ")  of  the  apical  system  in  the  Echinoidea  bear  specially  to  the 
ambulacral  plates.  Since  it  has  not  been  proved  that  the  radial  plates  of  the 
Echinoidea  arise  over  the  right  ccelomic  vesicle,  it  is  possible  that  they,  although 
lying  high  up  at  the  apex,  belong  genetically  to  the  oral  system,  and  correspond 
with  the  terminals  of  the  Asteroidea.  The  radials  should  then  not  be  represented  in 
the  apical  system  of  the  Echinoidea. 

In  a  comparison  of  the  skeletons  of  the  Echinoidea  and  the  Asteroidea  we  should 
then  have  to  suppose  that  in  the  former  the  ambulacra  have  been  lengthened  round 
over  the  ambitus  to  the  apex  ;  and  that,  further,  the  body  took,  on  the  form 
of  a  pentagonal  pyramid,  by  the  abbreviation  of  the  arms  and  the  elongation  of 
the  principal  axis  of  the  body  ;  and  that,  therefore,  the  whole  region  occupied  by 
the  accessory  skeleton  of  the  Asteroid  has  disappeared.  The  marginal  plates  of  the 
Asteroid  would  then  correspond  with  the  interambulacral  plates  of  the  Echinoid, 
and  the  adambulacral  ossicles  of  the  former  with  the  ambulacral  plates  of  the  latter. 
A  comparison  of  the  ambulacral  plates  of  the  Echinoid  with  the  plates  of  the  same 
name  in  the  Asteroid  is  rendered  difficult  by  the  difference  of  position  of  the  two, 
the  former  being  superficial,  epiambulacral,  and  epineural,  and  the  latter  deeper, 
subambulacral,  and  subneural.  The  ambulacral  ossicles  of  the  Asteroid  would  thus 
not  be  represented  in  the  skeleton  of  the  Echinoid. 


IV.  Ophiuroidea. 
(a)  Skeleton  of  the  Arms. 

The  brachial  skeleton  of  the  Ophiuroidea  consists  typically  of  six 
longitudinal  rows  of  plates,  a  dorsal  row  (dorsal  shields),  a  ventral 
row  (ventral  shields),  two  lateral  rows  (lateral  shields),  and  a  double 
row  of  internal  ossicles  lying  in  the  axis  of  the  arm.  This  system  is 
jointed,  or  segmented,  in  quite  a  regular  manner — one  dorsal,  one 
ventral,  one  axial  piece  and  two  lateral  pieces  together  composing 
a  skeletal  segment  (Fig.  311). 

The  external  pieces  together  form,  in  each  arm,  a  jointed  tube, 
which  determines  the  shape  of  the  arm.  Most  of  the  lateral  shields 
carry  spines;  on  each  shield  there  are  usually  four  of  these,  one 
above  the  other,  so  that  each  longitudinal  row  of  shields  is  armed 
with  four  longitudinal  rows  of  spines.  The  tube -feet  emerge  at 


356 


COMPARATIVE  ANATOMY 


regular  segmental  intervals  through  apertures  which  lie  on  each  side 
between  the  ventral  shields  and  the  lateral  shields  belonging  to  them 


."•-* 

^  s  771  /*# 

FIG.  311.— Transverse  section  through  the  arm  of  an  Ophiurid  (after  Ludwig).  Diagram. 
ss  Lateral  shields  ;  ds,  dorsal  shields ;  cl,  cavity  of  the  arm  (ccelora) ;  oc,  spines ;  am,  the  ambu- 
lacral  plates  (vertebrae) ;  x,  loop  of  tentacle  canal  in  the  groove  on  the  distal  face  of  the  ossicle  (c/. 
next  tig.,  A  4) ;  rte,  tentacular  canal  of  the  radial  vessel  (ra)  of  the  water  vascular  system  ;  te, 
feeler  (tentacle) ;  re,  radial  pseudoluemal  vessel ;  rn,  radial  nerve  strand  ;  bs,  ventral  shield. 


PIG.  312.— Vertebral  ossicles  (ambulacral  plates)  of  Ophiarachna  incrassata  (after  Ludwig), 
to  show  the  articulating  prominences  and  depressions,  etc.  A,  Three  vertebral  ossicles  from  the 
side.  B,  Vertebral  ossicles  from  the  proximal  (adoral),  and  C,  from  the  distal  (aboral)  side.  D,  Three 
vertebral  ossicles  from  the  ventral  side,  pr,  Proximal ;  di,  distal ;  ra,  radial  trunks  of  the  water 
vascular  system;  rn,  radial  nerve  trunk;  rv,  radial  pseudohsemal  canal.  1,  Point  at  which  the 
branch  of  the  radial  water  vascular  trunk  running  to  the  tube-foot  passes  out  of  the  substance 
of  the  vertebral  ossicle  at  its  distal  side ;  2,  point  where  this  branch  re-enters  the  ossicle ; 
4,  channel  between  these  two  points,  which  receives  the  loop  of  the  branch  belonging  to  the 
tube-foot;  3,  depression  for  the  lower  intervertebral  muscle;  5,  channel  for  the  radial  water 
vascular  trunk  ;  6,  depression  for  the  tube-foot ;  7,  channel  for  the  branch  of  the  nerve  running  to 
the  tube-foot ;  8,  pseudohsemal  vessel  to  the  same  ;  9,  nerve  branch  to  the  same  ;  10,  branch  of  the 
water  vascular  system  to  the  same,  which  at  12  passes  into  the  substance  of  the  ossicle,  and  at  13 
out  of  the  latter  and  into  the  tube-foot ;  11,  point  at  which  the  nerve  branch  (14)  running  to  the 
upper  intervertebral  muscle,  enters  the  vertebral  ossicle. 

(c/.  Fig.  245,  p.  300).     At  the  edge  of  these  apertures  there  are  smaller 
spines  or  scales. 

The  axial  double  plates  are  called  vertebral  ossicles,  a  very  suit- 


vin         ECHINODERMATA—  MORPHOLOGY  OF  SKELETON        357 

able  name,  since  they  play  a  part  altogether  similar  to  that  of  the 
vertebrae  of  the  axial  skeleton  in  Vertebrate  animals.  In  a  large 
majority  of  cases  the  two  lateral  portions  of  a  vertebral  ossicle  are 
fused  in  the  median  plane  in  such  a  way  that  no  sutures  are  now  to 
be  seen.  These  ossicles,  however,  arise  ontogenetically  as  two,  at  first 
entirely  distinct,  lateral  pieces,  which  only  fuse  later.  There  are, 
further,  certain  deep-sea  Ophiuroidea  (OphioMus,  Fig.  313)  in  which 
each  vertebral  ossicle  consists,  even  in  the  adult,  of  two  distinct  slender 
pieces,  articulated  one  with  the  other. 

The  vertebral  ossicles  fill  up  the  greater  part  of  the  skeletal  tube 
formed  by  the  dorsal,  ventral,  and  lateral  shields.  Between  them  and 
the  tube,  in  dried  skeletons, 
only  small  spaces  are  to  be 
found,  which  dorsally  contain 
continuations  of  the  body 
cavity  of  the  disc,  while  ven- 
trally  they  contain  the  radial 
water  vascular  trunk,  the 
radial  nerve  cord,  the  epineural 
canal,  and  the  pseudohaemal  am 
vessel.  The  lateral  branches 
of  the  radial  vessels  of  the 

water    Vascular    system,  before        FiG.313.-OpMohelusimbella,Lym.   A  macerated 

entering  each  tube -foot,   pass  J°int  from  near  the  tjp  of  an  arm> frora  the  dorsal  side 

l          'A         4.U       (after  Lyman).     ds,  Dorsal ;  ss,  lateral  shield ;  am, 
through,     On      each      Side,     the    Lnbulacral  ossicles  ;  «p«,  ho^k  spines. 

substance     of     the     vertebral 

ossicle  of  the  corresponding  segment,  nearer  the  distal  than  the 
proximal  end  of  the  ossicle.  The  consecutive  vertebral  ossicles  of  the 
arms  articulate  one  with  another,  and  are  connected  by  means  of  four 
intervertebral  muscles.  The  contraction  of  the  two  upper  inter- 
vertebral  muscles  brings  about  the  upward  curving,  and  the  contrac- 
tion of  the  two  lower,  the  downward  curving,  of  the  arms.  The 
horizontal  (lateral)  movement  is  brought  about  by  the  contraction  of 
the  upper  and  lower  muscles  of  the  same  side.  The  vertical  movement 
of  the  arms  is  very  slight  in  true  OphiuridaB,  whereas  in  the  Euryalidce 
the  arms  can  be  completely  rolled  up  orally  (cf.  Fig.  246,  p.  301). 

Small  accessory  plates  may  occur  in  addition  to  the  dorsal  shields.  The  super- 
ficial brachial  skeleton  is  much  reduced  in  the  Astrophytidce  (Euryalidce)  and  the 
Ophiomyxidce,  and  the  arms  are,  in  these  animals,  covered  by  a  soft  integument,  in 
which  only  small  skeletal  pieces  occur.  In  other  forms  the  brachial  skeleton  is  so 
covered  by  an  integument,  often  containing  small  embedded  skeletal  pieces,  that  it 
is  either  partly  or  altogether  invisible  externally. 

At  the  distal  end  of  each  arm  in  the  Ophiuroidea  there  is,  as  in 
the  A*f<''roi'1"ii,  an  unpaired  median  terminal,  which  surrounds  the 
tip  of  the  radial  water  vascular  trunk  (the  terminal  tentacle)  in  the 
form  of  a  short  skeletal  ring.  Since,  in  the  A*t>-rm<lfii,  the  terminal 


358  COMPARATIVE  ANATOMY  CHAP. 

plate  receives  the  terminal  tentacle  in  a  channel  on  its  ventral  side,  it 
is  important  to  note  that  "  the  terminal  plate  in  the  Ophiuroidea  also 
originally  forms  a  channel  opening  downwards,  and  only  later  closes 
to  form  a  ring." 

The  relation  between  the  terminals  and  the  developing  brachial 
skeleton  is  the  same  in  the  Ophiuroidea  as  in  the  Asteroidea.  The 
oldest  skeletal  segment  is  the  one  lying  most  proximally  (orally),  and 
of  the  following  segments  the  more  distal  are  always  the  younger. 
The  plates  which  compose  each  newly  appearing  skeletal  segment 
always  arise  at  the  end  of  the  arm,  on  the  proximal  side  of  the  ter- 
minal, which  thus  remains  at  the  extreme  tip  of  the  arm. 

When  we  consider  the  paired  elements  of  the  vertebral  ossicles  and  the  relative 
positions  of  the  skeletal  plates  and  the  water  vascular  system,  we  are  able  to  estab- 
lish the  following  homologies  between  the  components  of  the  brachial  skeletons  of 
the  Ophiuroidea  and  Asteroidea. 

OPHIUROIDEA.  ASTEROIDEA. 

The  two  lateral  halves  of  the  vertebral  Ambulacral  ossicles. 

ossicles. 

Lateral  shields.  Adambulacral  ossicles. 

Ventral  shields.  Not  represented. 

(b)  The  Oral  Skeleton. 

The  most  important  and  constant  plates  of  the  oral  skeleton,  in 
the  Ophiuroidea,  as  in  the  Asteroidea,  are  the  specially  modified 
proximal  plates  of  the  brachial  skeleton.  The  most  satisfactory  view 
which  has  been  propounded  as  to  the  morphological  worth  of  the 
oral  skeleton  is  that  it  consists  essentially  of  the  ambulacral  ossicles 
(the  halves  of  the  vertebral  ossicles),  adambulacral  ossicles  (lateral 
shields),  and  ventral  shields  of  the  first  and  second  proximal  skeletal 
segments  of  the  arms. 

If  we  look  at  the  oral  region  of  any  Ophiuroid  from  without,  i.e. 
from  the  free  oral  surface  of  the  disc,  or  from  within,  i.e.  after  removal 
of  the  apical  cover  of  the  disc  and  the  viscera,  we  see  the  mouth  in  the 
centre  of  the  disc  as  a  rosette-like,  or  star-shaped,  aperture.  The  slits 
arranged  radially  round  the  centre  are  called  the  bueeal  fissures.  Be- 
tween them  lie  the  triangular  oral-angles  (Figs.  245,  p.  300,  and  314). 
Five  pairs  of  large  plates  form  the  frame  surrounding  the  mouth  ; 
these  are  the  oral -angle  plates  (Fig.  314).  At  the  interradial 
angle  of  each  of  these,  i.e.  the  angle  which  projects  towards  the 
centre  of  the  oral  aperture,  two  neighbouring  angle  plates  meet. 
Each  angle  plate  has,  on  the  side  facing  a  buccal  fissure  of  the  oral 
aperture,  two  depressions  for  receiving  the  first  tube-feet  which  have 
shifted  into  the  oral  aperture,  and  are  known  as  oral  tube-feet,  or  oral 
tentacles.  There  are  often  in  addition,  in  the  dorsal  side  (that  facing 
the  body  cavity)  of  the  circle  of  oral-angle  plates,  two  circular  furrows 


vin         Ei'HIX<)I>ERU ATA— MORPHOLOGY  OF  SKELETON        359 

or  channels,  one  of  which  receives  the  nerve  ring  and  the  other  the 
water  vascular  ring. 

In  Astrophyton  part  of  the  water  vascular  ring  is  entirely  enclosed 
within  the  oral-angle  plates. 

Closer  examination  reveals  the  fact  that  each  oral-angle  plate 
consists  of  two  fused  plates,  a  proximal  and  a  distal.  The  former, 


ta 


D 


FH;.  314.— Oral  skeleton  of  the  Ophiopya  longispinus.  Lym.,  from  within;  above,  an  inter- 
radial  region  of  the  cover  of  the  ilisc.  rs,  Radial  shields  ;  am,  vertebral  ossicle  ;  omj,  peristomal 
plates  ;  ptcb.  depressions  for  the  oral  tentacles  ;  o.m-t+cdi,  oral-angle  plates  ;  fb,  bursal  apertures ; 
tit.  torus  angularis ;  D,  teeth ;  ibr,  interbrachial  region  ;  9sge,  bursal  scale ;  gp,  genital  plate 
(after  Lyman). 

directed  towards  the  centre  of  the  mouth,  fuses  with  the  corre- 
sponding piece  of  its  associated  oral-angle  plate,  the  two  forming  the 
oral  angle.  The  distal  plate  at  its  distal  end  is  in  contact  with  a 
corresponding  plate  on  the  opposite  side  of  the  buccal  fissure.  The 
former  of  these  constituents  of  each  oral-angle  plate  is  regarded  as  an 
adambulaeral  plate  of  the  first  brachial  segment,  taking  part  in  the 
formation  of  the  oral  skeleton,  while  the  distal  plate  is  regarded  as  an 


360  COMPARATIVE  ANATOMY  CHAP. 

ambulacral  ossicle  of  the  second  skeletal  segment.  It  is  the  latter 
which  is  provided  with  furrows  for  the  nerve  and  the  water  vascular 
rings,  and  with  depressions  for  the  oral  feet  (two  on  each  piece). 
The  distal  portions  of  each  pair  of  oral-angle  plates,  which  together 
border  a  buccal  fissure,  would  thus  correspond  with  the  lateral  halves 
of  a  brachial  vertebral  ossicle,  not  fused  together. 

In  viewing  the  under  (oral)  side  of  the  disc  of  an  Ophiuroid  (Fig. 
245,  p.  300)  we  can  easily  recognise  the  interradially  placed  bueeal 
shields  (scuta  buccalia),  which  are  usually  large,  and  have  already 
been  mentioned  as  belonging  to  the  oral  system.  At  the  sides  of 
each  buccal  shield,  between  it  and  the  neighbouring  oral-angle  plates, 
lie  two  skeletal  plates,  which  are  known  as  lateral  bueeal  shields 
(scutella  adoratia).  That  these  last-mentioned  plates  belong  to  the 
same  row  as  the  adambulacral  plates  (lateral  shields)  of  the  arms  can 
generally  easily  be  seen.  They  are  the  adambulaeral  plates  of  the 
second  segment  taking  part  in  the  formation  of  the  oral  skeleton. 
The  third  pair  of  adambulacral  plates  is  thus  the  first  pair  of  lateral 
shields  in  the  arm. 

Again  viewing  the  oral  skeleton  from  the  dorsal  or  apical  side  (Fig. 
314),  we  see  that  above  the  ten  oral-angle  plates  lie  ten  other  plates, 
which  usually  to  a  greater  or  lesser  extent  roof  over  the  water  vascular, 
and  the  nerve  furrows.  These,  the  peristomal  plates,  thus  lie  upon 
the  inner  sides  of  the  oral-angle  plates,  i.e.  the  sides  facing  the  body 
cavity.  The  peristomal  plates  belonging  to  two  neighbouring  radii 
meet  interradially,  and  may  fuse  together  to  form  single  plates.  The 
two  peristomal  plates  belonging  to  one  and  the  same  radius  may,  in 
the  same  way,  touch  one  another  (in  which  case  the  ten  plates 
together  form  a  closed  circle),  or  their  radial  ends  may  remain  more 
or  less  apart.  Accessory  peristomal  plates  sometimes  occur ;  in  other 
cases  these  are  altogether  wanting.  The  peristomal  plates  are  con- 
sidered to  represent  the  ambulaeral  ossicles  (halves  of  the  verte- 
bral ossicles)  of  the  first  segment  of  the  oral  skeleton,  a  view 
which  does  not  appear  to  be  certainly  established,  chiefly  because  they 
are  in  no  way  connected  with  the  tube-feet.  The  two  pairs  of  tube- 
feet  of  each  radius  of  the  oral  skeleton,  as  has  been  pointed  out,  belong 
to  its  two  oral-angle  plates. 

At  the  distal  end  of  each  of  the  oral  slits  radially,  viewed  from 
without,  there  is,  in  many,  indeed,  in  most  Ophiuroidea,  a  plate  which 
also  takes  part  in  the  limitation  of  the  oral  cavity  (Fig.  245,  p.  300). 
This  plate  can  at  once  be  recognised  as  the  most  proximal  plate  in 
the  row  of  ventral  shields.  It  is  the  ventral  shield  of  the  second 
segment  of  the  oral  skeleton.  The  lateral  shields  belonging  to  them 
are  the  lateral  bueeal  shields. 

In  a  row  with,  but  dorsally  to,  this  ventral  shield,  within  the  buccal 
issure,  there  is  a  second  plate  (which,  however,  may  occasionally  be 
wanting) ;  this  varies  greatly  in  size  and  form,  and  is  to  be  regarded 
as  the  ventral  shield  of  the  first  segment  of  the  oral  skeleton. 


viii         ECHIXODERIIA  TA  —MORPHOLOG  Y  OF  SKELETON        36  1 


The  following  table  embodies  this  view  of  the  oral  skeleton,  viz. 
that  it  consists  of  modified  pieces  of  the  first  two  skeletal  segments  of 
the  radii  (arms). 


Skeletal  Segment  of  the  arm. 


-n<l  <  Uistal)s|S^t  °f  the  oral 


1st  (Proximal)  Segment  of  the 
oral  skeleton. 


The  two  halves  of  the  The   distal  portions  of       The      two     peristomal 

vertebral    ossicle    (ambu-  the  two  oral-angle  plates    plates  of  a  radius     (Fig. 

lacral    plates)    (Figs.  311  belor    ' 

and  314  am).  (Fig. 


to     a     radius    314 


lacral    plates)    (Figs.  311    belonging 
Tig.  314 

The  two  lateral  shields  The  two  lateral  buccal        The  proximal   portions 

(adambulacral  plates)  shields  of  a  radius  (Fig.    of  the  oral -angle   plates 

i  Fius.   311  ss,  and  245,  4,  245,  5,  p.  300). 
p.  300). 


belonging  to  a  radius  (Fig. 
314 


The      ventral      shields        Externally  visible  ven-        Inner  ventral  shield  of 
!  (Figs.  245,  1,  p.  300,  and    tral  shield  of  each  radius    the  oral  skeleton. 
311  bs).  of  the  oral  skeleton  (Fig. 

245,  8,  p.  300). 


Accessory  Parts  of  the  Oral  Skeleton. 

V 

At  each  oral  angle  (at  the  point  where  two  neighbouring  oral-angle  plates  meet 
interradially),  on  the  side  facing  the  buccal  cavity,  there  lies  a  vertical  row  of  small 
skeletal  pieces,  which  may  fuse  together  to  form  the  torus  angularis  (Fig.  386,  ta, 
p.  486).  This  carries  the  teeth  (D)  which  project  into  the  buccal  cavity.  The  oral- 
angle  plates  themselves  carry,  at  the  edges  which  are  visible  externally,  i.e.  from 
the  ventral  side,  small  spine-like  skeletal  pieces.  Of  these  those  which  project  into 
the  buccal  fissures  are  called  oral  papillae  ;  while  those  which  rise  at  the  tips  of  the 
oral  angles,  and  are  turned  to  the  axis  of  the  buccal  cavity,  are  called  dental 
papillae.  Consequently,  in  each  oral  angle,  the  teeth  above  mentioned  lie  dorsally 
to  the  dental  papilla?. 

Accessory  Skeletal  Plates  of  the  Disc. 

Lower  side. — The  pieces  already  described  as  appearing  at  the  surface  on  the  lower 
side  of  the  disc,  and  which  belong  to  the  oral  system  (oral  shields)  or  to  the  oral 
.skeleton  (oral-angle  plates,  lateral  buccal  shields,  ventral  shields),  hardly  ever  form 
the  whole  ventral  carapace  of  the  disc.  On  the  contrary,  between  the  roots  of  the 
arms  (iuterbrachially  or  interradially)  these  plates  leave  free  spaces  (Figs.  245,  p.  300, 
and  314  ibr] ;  these  are  often  triangular,  and  are  sometimes  covered  with  plates  which 
vary  in  size  and  number,  and  frequently  imbricate,  or  else  they  consist  of  soft  integu- 
ment with  small  skeletal  granules  scattered  through  it.  These  interbrachial  regions 
of  the  disc  may  be  armed  with  spines  of  varying  length. 

On  either  side  of  the  root  of  each  arm,  on  the  ventral  surface  of  the  disc,  there 
are  one  or  two  fissures  or  slits  ;  if  two.  one  proximal  and  the  other  distal.  These 
bursal  apertures  (Figs.  245,  246,  pp.  300,  301.  and  Fig.  314)  lead  into  the  lnirs«v. 
which  will  be  described  later.  The  adradial  edge  of  each  of  these  slits  is  usually 
supported  by  a  single  skeletal  piece,  the  genital  plate,  while  the  interbrachial  edge 
is  plated  with  a  row  of  scales,  which  is  directly  continued  into  the  plating  of  the 
neighbouring  interbrachial  region. 

Upper  (apical)  side  of  the  disc. — It  follows  from  what  was  said  above  (p.  327) 


362  COMPARATIVE  ANATOMY  CHAP. 

that  the  apical  system,  whether  complete  or  incomplete,  forms,  in  many  Ophiuroidea, 
even  in  adults,  the  greatest,  or  at  any  rate  a  considerable,  part  of  the  dorsal  carapace 
of  the  disc.  Those  regions  which  are  not  covered  by  the  apical  system  are  plated 
by  the  perisomatic  skeleton.  The  plates  of  this  skeleton  vary  much  in  size,  form, 
number,  and  arrangement,  and  not  infrequently,  especially  in  cases  where  the  apical 
system  does  not  consist  of  large  distinct  plates,  the  dorsal  integument  of  the  disc  is 
soft,  and  only  provided  with  scattered  skeletal  pieces,  which  are  sometimes  micro- 
scopically small. 

Ten  large  perisomatic  plates  appear  most  constantly  (even  more  constantly  than 
any  of  the  circle  of  plates  of  the  apical  system)  ;  one  pair  of  these  lies  near  the  base 
of  each  arm.  These  are  called  the  radial  shields  (Figs.  244,  p.  299,  and  314  rs), 
and  are  often  present  even  when  there  are  no  large  plates  in  the  rest  of  the  dorsal 
carapace  of  the  disc.  Sometimes  the  radial  shields,  covered  with  a  soft  integument, 
reach  from  the  base  of  the  arm  to  near  the  centre  of  the  disc,  their  presence  being 
then  outwardly  marked  by  a  graceful  rosette  formed  of  five  pairs  of  radial  ridges. 

V.  Crinoidea. 
(Of.  the  apical  and  oral  systems  of  this  class,  pp.  328-333). 

The  perisomatic  skeleton  of  the  Crinoidea  consists  of:  (1)  The 
perisomatic  skeleton  of  the  calyx  ;  (2)  the  skeleton  of  the  arms  and 
pinnulae  ;  (3)  the  skeleton  of  the  stem. 

(a)  The  Perisomatie  l  Skeleton  of  the  Calyx. 

In  this  are  included  all  the  skeletal  pieces  of  the  calyx,  which  do 
not  belong  either  to  the  apical  system  (central,  infrabasals,  basals,  and 
radials)  or  to  the  oral  system  (orals). 

In  the  young  stalked  larva  of  Antedon  the  skeleton  of  the  calyx 
has  no  perisomatic  pieces  ;  it  consists  exclusively  of  the  typical  plates 
of  the  oral  and  apical  systems  (Fig.  270,  p.  318). 

The  only  forms  in  which  this  stage  persists  throughout  life  are 
those  of  the  type  Inadunata  larviformia,  e.g.  ^enus  Haplocrinn* 
(Fig.  297,  p.  334). 

In  all  other  living  and  extinct  Crinoidea  a  perisomatic  skeleton  is 
developed,  although  it  varies  in  extent  to  an  extraordinary  degree. 

This  skeleton  may  consist  of  very  various  systems,  and  may  be 
developed  both  in  the  dorsal  cup  and  the  tegmen. 

".  One,  or  several,  or  even  many,  pieces  may  appear  only  in  the 
posterior  or  anal  interradius,  especially  in  the  dorsal  cup  supporting 
or  bordering  the  anus.  These  anals,  which  characterise  the  posterior 
interradius,  more  or  less  markedly  disturb  the  regularly  radial 
structure  of  the  calyx. 

k   In  ah  the  five  interradii  one  or  many  pieces  may  occur,  both 


Mr   F.  A.  Bather  (Xutxml  .Science,  vol.  vi.  pp.  418,  419  :  1895),  in  reviewing  the 
al  Gtouiaa  edition  of  this  work,  adduces  strong  reasons  against  the  use  of  the  term 
<  here  employed  by  the  author.     The  term  is,  nevertheless,  retained  in 
tSl,i    S    t     Ve™°nhecause  lts  exc1™™  «een,ed  to  necessitate  the  entire  rearrangement 


viii         ECHINODERMATA — MORPHOLOGY  OF  SKELETON        363 

in  the  dorsal  cup  and  in  the  tegmeu.  They  are  called  interradials. 
In  the  tegmen  they  develop  in  the  zone  between  the  orals  and  the  edge 
of  the  calyx,  and  belong  to  the  interambulacral  system  of  plates. 
Usually,  only  the  interradials  of  the  dorsal  cup  are  so  called,  although 
they  are  not  infrequently  continued,  between  the  bases  of  the  arms, 
into  the  interradial  system  of  plates  of  the  tegmen  without  any  sharp 
boundary. 

i\  The  proximal  portions  of  the  arms  may,  to  a  greater  or  lesser 
extent  (to  their  first,  second,  etc.  divisions),  be  taken  into  the  calyx, 
in  which  case  the  skeletal  segments  of  the  arms  (brachials)  become 
perisomatic  plates  of  the  dorsal  cup,  and  are  known  as  fixed  braehials 
(primary,  secondary,  etc.,  formerly  called  radials  of  1st,  2nd,  etc. 
orders).  (For  the  meaning  of  these  names,  see  below,  the  section  on 
the  brachial  skeleton,  p.  370.) 

'/.  Just  as  interradials  may  appear  in  the  dorsal  cup  between  the 
five  radials  and  the  fixed  branchials  of  the  five  radii,  so  the  branches 
of  each  arm  incorporated  into  the  calyx  may  themselves  be  connected 
by  intercalated  plates.  Those  which  lie  between  brachials  of  the 
second  order  are  then  called  interdistiehals  or  interseeundibraehs, 
those  between  brachials  of  the  third  order  (after  the  second  forking) 
interpalmars  or  intertertibraehs,  etc. 

When  more  than  five  free  arms  rise  from  the  edge  of  the  calyx,  i.e. 
when  some  length  of  the  arms  and  their  branches  is  incorporated  into 
the  calyx,  the  food-grooves  running  over  the  tegmen  from  the  mouth 
to  the  periphery  divide  dichotomously  in  such  a  way  that  the  number 
of  grooves  ultimately  corresponds  with  that  of  the  free  arms.  The 
regions  between  the  branches  of  the  five  primary  radial  food-grooves 
are  as  a  rule  also  plated  with  small  interambulaeral  pieces. 

- .  The  food-grooves  running  over  the  tegmen  from  the  mouth 
to  the  bases  of  the  arms  very  often  have  a  skeleton  of  their  own, 
which  may  be  continued  into  the  ambulacral  furrows  of  the  arms  and 
their  branches.  This  ambulaeral  skeleton  may  consist  of  lateral 
plates  (which  border  the  furrow  laterally)  or  of  covering  plates  (which 
cover  the  furrows,  changing  them  into  passages  or  tunnels),  or  of  both 
these  sorts  of  plates.  Subambulacral  plates  may  also  occur. 

Special  Remarks  on  the  Perisomatic  Skeleton  of  the  Crinoid  Calyx. 

In  the  Inadunata  larviformia  (Type:  Haplocrinus]  there  is  no  perisomatic 
skeleton  of  the  calyx.  This  latter  consists  exclusively  of  the  plates  of  the  apical 
and  oral  systems  (five  basals,  five  radials,  three  of  which  are  transversely  divided, 
and  five  orals). 

The  first  perisomatic  plate  of  the  calyx  occurs  in  related  forms,  interradially,  in 
the  radial  circle,  and  rests  upon  the  posterior  basal  ;  it  is  the  anal. 

As  a  type  of  the  Inadunata  fistulata,  we  shall  first  select  Cyathocrinus.  In  the 
dorsal  cnp  only  one  perisomatic  plate  appears,  which  is  found  resting  on  the 
posterior  basal,  between  the  two  posterior  radials  (Fig.  289,  p.  329).  The  apical 
capsule  thus  altogether  resembles  that  of  the  Larviformia.  The  tegmen  calycis, 
on  the  contrary,  shows  an  entirely  different  condition,  which,  however,  may  vary 


364 


COMPARATIVE  ANATOMY 


CHAP. 


considerably  in  the  different  species,  and  indeed  in  different  individuals  of  one 
and  the  same  species.  The  orals  now  no  longer  occupy  the  whole  of  the  tegmen, 
but  are  supposed  by  some  writers  to  be  represented  by  certain  plates  which  occur 
at  its  centre,  and  vary  in  number  and  regularity,  often  being  replaced  by  small 
irregularly  arranged  perisomatic  plates.  The  mouth  is  always  hidden  beneath 
them.  From  these  supposed  orals  the  five  ambulacral  furrows  run  over  the  tegmen 
to  the  bases  of  the  five  much  branched  arms.  Each  furrow  is  covered  or  bordered 
by  two  or  four  rows  of  alternating  covering  plates.  Inter-radially  there  are  5  plates 
(deltoids),  the  edges  of  which  meet  beneath  the  ambulacrals  and  form  the  floors  of 
the  furrows.  They  sometimes  appear  at  the  surface  for  a  certain  distance  between 
the  ambulacrals  ;  in  other  cases,  they  are  even  here  covered  by  more  or  less  numerous 
interambulacrals. 

In  some  species  of  Cyathocrinus,  and  in  many  related  Inaduiiata  the  posterior  or 
anal  interradial  area  bulges  out  to  form  a  ventral  or  anal  sac,  which  is  sometimes 
cylindrical,  sometimes  club-shaped  or  bladder-like  (Fig.  315).  This  anal  sac,  besides 

the  hind-gut,  probably  contained  a  large  part  of 
the  body  cavity.  It  is  covered  with  numerous 
plates  arranged  in  vertical  rows.  The  plates 
of  the  neighbouring  rows  alternate  in  some 
species.  The  anus  lies  sometimes  near  the  tip 
of  the  sac,  sometimes  on  its  anterior  side,  and  is 
often  encircled  by  special  plates.  The  anal  sac 
may  attain  such  dimensions  that  it  is  as  long 
as,  or  even  longer  than  the  arms.  The  first 
tendency  to  the  formation  of  such  an  anal  sac  is 
met  with  in  Hybocrinus,  in  which  the  posterior 
interradial  region  of  the  tegmen  is  somewhat 
though  still  only  slightly  bulged. 

The  Inadunata  so  far  mentioned  are  palaeo- 
zoic  forms.      From  them  certain  more  recent 
types  may  be  derived.     In  Encrinus  (Trias)  the 
anal   sac   has  again  become  a  short  cone.     In 
forms   closely   related    to    this   genus,    and   in 
Marsupites   (Chalk),    the    anal    pieces   as   well 
have  disappeared,   so  that,  while  the  base  is 
FIG  315 .-Cyathocrinus  longimanus,    dicyclic,  the  regularly  radial  dorsal  cup  consists 
after  Angelm,  from  the  anal  side,  after         i       c  \ •>        ->    .         -  . ,  .     , 

removal  of  the  greater  part  of  the  arms.  ^  °f  the  Pktea  °f  the  aP1Cal  8ystem>  P6"80' 
pr,  Ventral  sac ;  x,  anal  plate ;  r,  radials ;  niatic  pieces  being,  in  this  system,  altogether 
'•",  hasals.  '  wanting. 

The  same  is  the  case  in  the  dorsal  cup  of  the 

family  Holopulce  (Lias  to  present  time),  Hyocrinidce  (Lias  to  present  time),  Bathy- 
crinidce  (present  time).  In  the  tegmen  calycis  of  these  forms  we  first  notice  that  the 
large  anal  sac  of  the  Cyalhocrinidce  is  reduced  to  a  small  anal  tube.  In  Holopus, 
between  the  base  of  the  open  oral  pyramid  and  the  edge  of  the  calyx,  there  is  only  a 
very  narrow  zone  beset  with  irregular  perisomatic  plates.  This  zone  is  still  wider  in 
Hyocrinus  (cf.  Fig.  298,  p.  335),  and  is  thickly  covered  with  numerous  small  plates. 
Between  the  ambulacral  furrows  lie  the  interambulacral  plates  ;  the  furrows,  im- 
mediately on  emerging  from  between  the  oral  plates,  are  bordered  and  covered  by 
lateral  and  covering  plates.  In  the  posterior  interambulacral  area,  near  the  edge  of 
the  tegmen,  sometimes  excentrically,  there  rises  the  short  conical  plated  anal  tube, 
•ith  the  anus.  In  Pathycriniis,  where  the  orals  are  either  wanting  or  reduced,  the 
interradial  region  is  either  naked  or  plated  with  small  pieces.  The  ambulacral 
furrows  havp  lateral  plates  only.  The  anus  lies  on  a  very  short  papilla-like  anal  cone. 


la: 


vin         ECHINODERMATA— MORPHOLOGY  OF  SKELETOX        365 


The  Canal  iculata,  like  the  more  recent  Inadunuta  (Lias  to  present  time),  are 
distinguished  by  the  regular  radial  structure  of  the  dorsal  cup,  in  which  interradials 
only  exceptionally  occur,  and  special  plates  in  the  posterior  mterradius  (anals)  never 
occur.  Very  often  (Apiocrinus,  PJiizocrinus,  Antedonidce)  two  or  more  brachial 
plates  following  the  radials  of  the  calyx  are  incorporated  as  "fixed  brachials"  into 
the  dorsal  cup. 

In  connection  with  the  tegmen  calycis,  it  must  be  noted  that  among  the  Canali- 
culata,  orals  appear  in  the  adult  only  in  PJiizocrinus.  As  a  rule,  the  tegmen  calycis 
is  plated  in  the  interambulacral  regions  with  numerous  loosely  connected  skeletal 
pieces,  which  vary  in  size  according  to  the  species  and  genus.  These  small  plates 
are  perforated  by  pores.  This  skeletal  covering  is  not  infrequently  continued  on  to 


afa 


FIG.  31'5.—  Tegmen  calycis  of  Metacrinus  angu- 
latus,  P.  H.  Carp,  (after  P.  H.  Carpenter),  o,  Mouth; 
hr,  arms  ;  p,  pinnulae  (both  broken  off)  ;  ta,  anal  tube, 
near  which  there  is  a  second  abnormal  tube  ta]  ;  rpa, 
covering  plates  of  the  ambiilacral  furrows. 


FIG.  317.— Actinometra  strota,  P.  H. 
Carp,  (after  P.  H.  Carpenter).  Tegmen 
calycis.  o,  Mouth;  an,  anus;  afa,  food 
grooves  of  the  arms  •  afd,  the  same  of  the 
tegmen  ;  p1}  two  pinnula?,  which  take  the 
place  of  one  of  the  two  posterior  arms. 


the  bases  of  the  arms,  and  occasionally  runs  out  between  these  apically  in  such  a 
manner  as  to  be  visible  in  the  interradii  of  the  dorsal  cup. 

The  ambulacral  furrows  of  the  tegmen  calycis  are  rarely  open,  but  usually  covered 
with  covering  plates  and  often  bordered  by  lateral  plates  (Fig.  316).  Occasionally 
the  mouth  also  may  be  covered  with  perisomatic  plates,  but  it  is  usually  open. 

The  anal  tube  in  the  posterior  interradius  varies  in  size  and  in  its  position  within 
this  interradial  area.  Its  plating  agrees  with  that  of  the  interambulacral  area  on 
which  it  is  found. 

The  interambulacral  areas  may  also  be  naked,  i.e.  covered  with  integument 
containing  only  very  small  calcareous  corpuscles. 

A'1in.i.tin':ii'i.t.  is  the  only  recent  Crinoid  in  which  the  mouth  is  found 
placed  quite  excentrically  (anteriorly)  on  the  tegmen,  and  the  anus, 
which  lies  in  the  enlarged  posterior  interradial  area,  comes  to  lie 


366 


COMPARATIVE  ANATOMY 


CHAP. 


almost   centrally   (Fig.    317).     In    consequence   of    this   shifting    the 
ambulacra  are,  of  course,  very  unequal  in  length. 

The  (paleozoic)  Camerata  are  distinguished  by  the  tendency  to  strong  development 
of  the  perisomatic  skeleton  in  the  calyx,  and  by  the  plates  being  so  firmly  inter- 
connected as  to  form  a  rigid  test.  In  the  formation  of  the  dorsal  cup,  the 
bases  of  the  arms  are  incorporated  to  a  certain  extent  in  such  a  manner  that  their 
lower  brachials  become  fixed  plates  of  the  cup.  In  the  five  interradii  of  the  dorsal 
cup,  interradials  appear,  to  which,  in  the  posterior  interradius,  special  anal 
plates  are  often  added.  In  those  cases  in  which  the  arms  take  part  in  the  formation 
of  the  capsule  beyond  their  first  branchings,  interdistichals,  etc.,  may  connect  the 
branches  firmly  together. 

The  tegmen  calycis  also  consists  of  plates  which  are  usually  very  numerous  and 
firmly  connected  together.  Just  as  the  mouth  is  always  covered  by  characteristically 
arranged,  closely  fitting,  orals,  so  also  the  ambulacral  furrows  are  never  open,  but  are 
always  arched  over  by  large  covering  plates,  some  of  which  may  be  distinguished  by 


FIG.  318.— Actinometra  (after  P.  H.  Carpenter).  Diagrams  to  illustrate  the  courses  of  the  food 
grooves  over  the  tegmen  calycis.  ArE2,  the  directions  of  the  five  pairs  of  arms.  In  the  centre  the 
anal  tube. 

their  greater  size.  In  the  older  forms,  the  tegmen  is,  as  a  rule,  rather  flat,  and  the 
covering  plates  of  the  ambulacral  skeleton  appear  at  the  surface.  In  the  course  of 
the  geological  development  of  the  palaeozoic  epoch,  however,  the  tegmen  bulges  out 
more  and  more,  and  finally  forms  a  high,  firmly  plated  "vault"  or  dome  (Figs. 
253,  254,  pp.  307,  308),  which,  immediately  behind  its  centre,  may  be  prolonged 
to  form  a  tube,  often  of  greater  length  than  the  arms,  with  the  anus  at  its  tip. 
Where  such  a  highly  arched  dome  is  developed,  the  interambulacral  plates,  which 
border  the  ambulacral  furrows,  send  out  processes  over  the  latter.  The  processes 
(which  are  closely  joined  to  one  another)  from  one  side  of  the  ambulacra  meet  and 
ecome  firmly  connected  with  those  from  the  other,  so  that  the  ambulacral  furrows 
with  their  skeletons  are  completely  arched  over,  and  are  not  externally  visible. 

This  condition  was  until   quite  recently  wrongly  explained   as   follows      The 

vnerata  possessed  an  inner,  naked,  or  merely  loosely  plated  tegmen,  in  which  the 

ambulacra  ran  from  the  mouth  in  the  centre  to  the  periphery,  and  this  tegmen 

was  arched  over  by  a  firmly  plated  vault  in  such  a  way,  that  between  the  tegmen 

and  the  vault  there  was  a  free  space. ) 

The  interradial  plates  of  the  tegmen  are  often  continued  directly,  i.e.  without  a 
boundary  lm<-,  into  the  interradial  plates  of  the  dorsal  cup. 

The  anus,  surrounded  by  special  plates,  lies  in  the  posterior  interradius. 


viii         ECHINODERMATA—  MORPHOLOGY  OF  SKELETON        367 

a.  The  apical  capsule  or  dorsal  cup.— In  Platycrinus,  the  dorsal  cup  (cf.  Fig. 
254,  p.  308)  still  consists  exclusively  of  the  plates  of  the  apical  system  (three  basals 
and  five  large  radials).  The  arms  are  free  from  their  bases.  A  plate  which  is  found 
in  each  interradius,  between  the  bases  of  the  free  arms  and  between  the  radials,  may 
be  considered  to  belong  almost  as  much  to  the  tegmen  as  to  the  dorsal  cup.  In 
Hexacriiius  the  radial  structure  of  the  apical  capsule  is  essentially  disturbed  by  the 
appearance  of  an  anal  plate,  which  presses  in  between  the  two  posterior  interradials 
in  the  posterior  interradius,  and  to  which,  in  the  direction  of  the  tegmen,  two  or 
three  other  anals  may  be  added.  Further,  in  each  radius,  the  one  small  primary 
brachial  plate  present  has  become  a  fixed  plate  of  the  apical  system.  As  a  further 


FIG.  31t'.—  Gilbertsocrinus  tuberculosus,  Hall  (after  Wachsmuth  and  Springer).  The  system 
of  plates  of  the  dorsal  cup  and  of  the  interradial  appendages  IB.  Ba,  point  of  attachment  of  the 
anus  ;  B/,  commencement  of  the  free  portions  of  the  arms.  For  other  lettering  see  p.  317. 

example  we  may  take  Dimerocrinus  (Glyptasterida-),  in  which  the  dorsal  cup  is  still 
more  complicated.  In  each  radius  the  radial  is  followed  by  two  primary  brachials, 
which  are  incorporated  into  the  dorsal  cup.  In  each  case  the  second  of  these 
brachials  is  followed  by  two  or  three  secondary  brachials,  which  are  also  fixed  in 
the  dorsal  cup,  the  last  of  them  carrying  a  free  arm.  In  each  interradius  there  are 
several  interradials  ;  first  a  large  plate  which  lies  between  the  primary  brachials,  and 
then  two  more  lying  at  the  level  of  the  secondary  brachials.  The  posterior  interradius 
is  broader  than  the  others.  The  first  plate  here  lies  between  the  radials,  and 
agrees  with  them  in  size,  then  follows  a  second  row  of  three  plates,  and,  orally 
from  these,  various  small  plates  which  lead  over  on  to  the  tegmen  calycis.  Inter- 
distichals  may  also  occur.  Mdvuriiiux  (Fig.  252,  p.  307)  and  Dorycrimis,  etc.  agree 
with  Dimcrwrinus  in  these  points. 

'  In  Gilbertsocrinus  (Rliodocrinidce]  also,  the  two  primary  brachials  and  the  two  or 
three  secondary  brachials  are  incorporated  into  the  dorsal  cup  (Fig.  319).  In  each  of 


COMPARATIVE  ANATOMY 


CHAP. 


368 

the  five  interradii  there  are  several  (twelve)  interradials,  the  arrangement  of  which 
is  shown  in  the  figure.  The  anal  interradius  is  hardly  distinguishable  from  the 
other  interradii.  The  distichals  or  secondary  brachials  are  connected  by  smaller 

The  perisomatic  skeleton  of  the  dorsal  cup  of  Actinocrinus  (Fig.  291,  p.  329) 
•is  very  like  that  of  Gilbertsocrinus ;  but  the  anal  interradius  is  much  larger  than 
the  others,  and  its  plates  are  divided  into  two  lateral  groups  by  the  intercalation  of 
a  vertical  'row  of  anal  plates.  This  is  also  the  case  in  Batocrinus  (Actinocrinida) . 
Here,  however,  not  only  the  5x2  primary  brachials  and  the  10x2  secondary 
brachials,  but  also  the  20  x  2  tertiary  branchials  are  incorporated  into  the  dorsal 
cup.  In  Strotocrinus  (regalis)  an  extreme  form  is  found  (Fig.  320).  The  calyx 

& 


Fro.  320.— Strotocrinus  regalis  (after  Wachsmutli  and  Springer).  The  apical  border.  The 
conical  portion  of  the  dorsal  cup  is  broken  away  (as  far  as  the  distichals  di)  and  shows  the 
t.'-iiH-n  with  the  anus,  the  mouth  and  the  food' grooves.  The  dotted  lines  denote  the  manner  of 
branching  of  the  fixed  arms,  an,  anus  ;  If,  fixed  joints  of  the  arms,  which  form  the  border ;  B/,  the 
free  arms  which  run  out  from  the  edge  of  the  border ;  ire,  interambulacral  region  of  the  tegmen 
calycis  ;  am,  ambulacra  ;  pf,  fixed  pinnule. 

is  very  large.  The  dorsal  cup  consists  of  a  small  conical  portion  above  the  stalk, 
followed  by  a  border  spread  out  horizontally.  In  each  radius  each  radial  is  followed 
by  two  primary  brachials.  The  second  costal  is  in  each  case  followed  by  the  two 
secondary  brachials  (making  ten  in  all).  Up  to  this  point  the  above  mentioned 
plates,  together  with  the  apical  system,  form  the  conical  part  of  the  dorsal  cup. 
The  plates  which  follow  form  the  horizontally  expanded  border.  Each  distichal  is 
followed  by  a  principal  row  of  (six)  plates,  which  runs  radially  to  the  edge  of  the 
border,  where  the  last  plate  carries  a  free  arm.  Accessory  rows  branch  alternately 
from  these  principal  rows,  three  on  each  side.  These  also  run  to  the  edge  of  the 
border,  and  the  last  plate  of  each  row  carries  an  arm-branch.  Seventy  free  arm- 
branches  in  all  thus  rise  from  the  edge  of  the  border.  In  the  interradii,  in  the 
intcidistichal  regions,  and  between  all  the  further  branches  of  the  fixed  arms,  inter- 


a/n, 


vin         ECHINODERMATA — MORPHOLOGY  OF  SKELETON        369 

radials,  interdistichals.  etc.  are  found  binding  the  brachials  into  the  rigid  horizontal 
border.  Their  number  and  arrangement  are  best  elucidated  by  the  figure.  The  anal 
interradius  is  not  distinguished  from  the  others  in  any  marked  manner. 

b.  The  Tegmen  calycis.— The  tegmen  of  Marsupiocrinus  (ccelatiis)  is  only  slightly 
vaulted.  It  is  plated  with  numerous  small,  firmly  connected  pieces  (Fig.  321). 
Among  these,  we  can  easily  distinguish  the  covering  plates  of  the  ambulacra,  which 
thus  here  come  to  the  surface,  and 
can  easily  be  distinguished  from 
the  somewhat  larger  iuterradial 
and  interambulacral  plates.  In 
the  centre  of  the  tegmeii  lie  the 
five  orals,  arranged  in  the  manner 
which  is  characteristic  of  the 
Camcrata,  and  behind  these, 
subcentrally,  the  anal  aperture, 
surrounded  by  special  plates. 

If  the  ambulacral  covering 
plates  are  larger  and  more  massive, 
as  in  many  species  of  the  genus 
Platycrinus,  it  is  then  more 
difficult  to  distinguish  the  inter- 
radial plates  of  the  tegmen  from 
them. 

The  genus  Agaricocrinus 
affords  examples  of  the  specially 

strong  development  of  single  covering  plates  of  the  ambulacral  skeleton,  which  are 
called  radial  dome  plates.  The  tegmen  is  highly  vaulted. 

An    extraordinarily   highly   vaulted    tegmen    is   found    in   the    Adinocrinidce 
(Actinocrinus,  Batocrinus,  Figs.  253,  254,  pp.  307,  308).      It  is  regularly  and  firmly 

&z 


an. 


FIG.  321.— Tegmen  calycis  of  Marsupiocrinus  coelatus 
(after  Wachsmuth  and  Springer),  or,  Orals;  am,  am- 
bulacra ;  cp,  covering  plates  of  the  ambulacral  furrows ; 
ia,  interambulacral  region. 


FIG.  322.— Part  of  the  dorsal  cup  of  Forbesiocrinus,  spread  out.  For  lettering  see  p.  317. 
In  addition,  IO,  one  of  the  four  similar  interradial  regions ;  IA,  the  deviating  anal  interradial 
region ;  pal,  palmars. 

plated  with  large  strong  plates  more  or  less  equal  in  size.     Nothing  can  be  seen 
of  the  ambulacral  skeleton  externally,  it  having  been  pressed  down,  or  rather,  over- 
grown, as  already  described,  by  the  interambulacral  plates.     In  the  posterior  inter- 
VOL.  II  2  B 


OF   THh 

UNIVI 


370  COMPARATIVE  ANATOMY  CHAP. 

radius,  immediately  behind  the  centre  of  the  tegmen,  this  dome  is  produced  still 
further  into  a  long  tube  similarly  plated  ;  this  is  the  anal  tube,  on  the  tip  of  which 
lies  the  anus. 

The  Articulata,  so  far  as  the  perisomatic  plates  of  the  calyx  are  concerned, 
agree  with  the  Camerata  in  that  the  ossicles  of  the  arms  are  sometimes  incorporated 
into  the  dorsal  cup  as  far  as  to  their  second  or  third  divisions  (Fig.  322),  the 
primary,  secondary,  and  often  also  the  tertiary  brachials  becoming  fixed  plates 
of  the  dorsal  cup.  The  number  of  the  brachials  in  each  arm  and  its  branchings 
varies.  Three  primary  brachials  are  often  found  in  each  radius.  But  these  fixed 
brachials  are  not,  as  in  the  Camerata,  rigidly  connected  inter  se  and  with  the  radials, 
but  are  articulated.  The  spaces  on  the  dorsal  cup  between  the  radii  and  between 
their  branchings  are  filled  either  with  quite  small,  loose,  and  irregular  calcareous 
corpuscles  or  scales,  or  with  small,  definitely  arranged  plates  (interradials,  inter- 
distichals,  etc.).  In  the  posterior  interradius  there  are  often,  in  addition,  special  anal 
plates  frequently  asymmetrically  arranged. 

The  tegmen  calycis  of  one -species  of  the  genus  Taxocrinus  is  well  known.  The 
radii  and  their  branchings  are  bulged  out  while  the  interradii  are  depressed.  From 
the  central  mouth,  which  is  open  and  surrounded  by  five  orals,  the  five  ambulacral 
furrows  run  out,  dividing  dichotomously  in  correspondence  with  the  branching  of 
the  arms.  Each  ambulacral  furrow  has  a  floor  of  two  longitudinal  rows  of  sub- 
ambulacral  plates,  is  bordered  by  lateral  plates,  and  closed  in  by  two  longitudinal 
rows  of  covering  plates.  The  covering  plates  in  the  two  rows  are  alternately 
arranged,  their  interlocking  forming  a  zigzag  line,  and  it  is  very  probable  that  they 
were  movable,  i.e.  that  they  could  be  raised  and  depressed.  The  interambulacral 
regions  contain  a  large  number  of  small,  loose,  irregular  plates.  In  the  posterior 
interradius,  at  the  edge  of  the  tegmen,  there  is  a  plated  process  (anal  tube  ?). 

For  Thaumatocrimis,  see  "The  Systematic  Review"  (p.  309). 

(b)  The  Brachial  Skeleton. 

The  calyx  of  the  Crinoidea  carries  at  its  edge  (on  the  boundary 
between  the  tegmen  calycis  and  the  dorsal  cup)  five  arms,  which 
are  rarely  simple,  but  usually  branched,  and  in  the  living  animal  are 
beautifully  extended.  The  arms  can  be  made  by  stimulation  to 
fold  together  over  the  tegmen.  They  are  found  in  this  position 
also  in  dead  animals,  and  therefore  almost  always  in  fossilised 
individuals. 

The  arms,  which  contain  important  inner  organs,  are  supported 
by  a  special  brachial  skeleton.  This  consists  of  consecutive  calcareous 
pieces,  the  braehials,  which  are  either  firmly  connected  or  articulated 
with  one  another.  The  brachials  are  deepened  on  their  oral  side, 
that  which  is  directed  upwards  when  the  arms  are  spread  out,  to  form 
a  more  or  less  deep  longitudinal  groove  along  the  arms  and  all  their 
branches  this  is  the  ambulaeral  groove.  In  the  base  of  this  groove 
he  the  most  important  inner  organs  of  the  arms  (radial  canals,  water 
vessels,  outgrowths  of  the  body  cavity,  etc.).  The  soft  integument 
which  covers  these  organs,  and  stretches  over  the  ambulacral  grooves 

the  brachial  skeleton  is  in  its  turn  depressed  to  form  a  channel 

mtegumental  channels,  which  accurately  correspond  with  the 

ambulacral  grooves  of  the  skeleton,  are  called  food  grooves.     At  the 


vin         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        371 

bases  of  the  free  arms  they  pass  into  the  ambulacral  grooves  or  food 
grooves  of  the  tegmen  calycis,  which  run  to  the  mouth. 

The  arms,  when  divided,  as  they  normally  are,  usually  branch 
dichotomously ;  occasionally,  however,  they  give  off  alternating 
branches,  which  may  again  branch  alternately.  In  most  Crinoids  the 
arms  and  their  branches  carry,  at  the  sides  of  the  ambulacral  grooves, 
closely  crowded  and  alternating  processes  ;  these  are  rod-shaped,  and  end 
in  a  point,  and  are  known  as  pinnules.  The  skeleton  of  these  pinnules 
resembles  that  of  the  arm,  and,  like  the  latter,  is  jointed.  The  pinnules 
may  best  be  regarded  as  the  ultimate  branchings  of  the  arms,  and  it  is 
very  probable  that  in  the  numerous  palaeozoic  Inwlumta  that  have  no 
pinnules  the  last  branches  of  the  arms  fulfilled  their  functions.  ' 

The  brachial  skeletons  in  the  Crinoidea  are  always  direct  continuations  of  the 
radials  of  the  apical  capsule.  The  first  plate  which  follows  the  apical  radial  radially 
must  be  considered,  morphologically,  as  a  brachial  or  ossicle  of  the  arm,  although 
it  is  only  rarely  (e.g.  in  the  Inadniiata)  a  free  ossicle.  The  terms  introduced  to 
denote  the  various  orders  of  brachials  are  almost  as  numerous  as  the  writers  them- 
selves. It  is  the  clearest  plan  to  speak  of  them  as  brachials  of  the  first,  second, 
third,  etc.  orders,  or  as  primary,  secondary,  tertiary,  etc.  brachials.  Such  a  plan 
was,  however,  soon  found  too  cumbrous  for  practical  purposes,  and  was  supplanted 
by  the  terms  costals,  distichals,  palmars,  and  postpalmars.  To  these  terms,  how- 
ever, considerable  exception  may  be  taken,  and  it  seems  simplest  to  adopt  the  intel- 
ligible and  congruent  terms  primibrachs,  secundibrachs,  tertibrachs,  etc.,  which  are 
capable  of  indefinite  extension,  and  are  readily  symbolised  as  IBr,  IIBr,  etc. 

It  has  already  been  pointed  out,  in  the  section  which  treated  of  the  perisomatic 
plates  of  the  calyx,  that  brachials  are  incorporated  into  the  dorsal  cup  in  many, 
indeed,  iu  the  great  majority  of  Crinoids.  We  can  accordingly  distinguish  free 
brachials  from  fixed  brachials,  the  latter  being  those  which  have  become  peri- 
somatic  plates  of  the  dorsal  cup.  The  first  brachials  to  be  so  incorporated  are 
naturally  the  primibrachs,  the  next  the  secundibrachs,  the  tertibrachs  may  also  then 
become  fixed.  In  describing  the  skeleton  iu  detail,  therefore,  the  terms  fixed  primi- 
brachs, fixed  secundibrachs,  etc.  are  used,  and  the  number  of  these  plates  in  each 
arm  is  given.  The  arrangements  found  in  the  various  divisions  of  the  Crinoidea 
in  this  respect  have  been  already  briefly  described  in  the  preceding  section.  That 
of  the  Iiiadv.nata  is  the  simplest,  since  in  them  the  arms  are  free  from  their  very 
bases  (hence  the  name),  the  first  primibrach  being  a  free  ossicle  of  the  arm  ;  the  most 
complicated  condition  is  that  of  certain  Camerata  (Adinocrinoidca,  etc.).  in  which 
the  brachials  of  several  orders  are  incorporated  into  the  calyx,  and  being  connected 
by  interradials,  interdistichals,  etc.  lend  to  the  dorsal  cup  its  rich  plating. 

In  branched  arms  those  joints  above  which  the  divisions  or  branchings  take  place 
are  called  axillary,  c.cj.  we  have  an  axillary  costal,  axillary  distichal,  or,  as  they 
may  more  simply  be  called,  primaxil,  secundaxil,  etc.  (lax,  Ilax,  etc.). 

With  regard  to  the  distribution  of  the  pinnulse,  it  is  the  rule,  at  least  in  modern 
Crinoids,  that  the  axillary  joints  never  carry  pinuulse,  and  that  where  two  joints 
are  connected  by  syzygial  sutures  or  by  ligaments,  pinnulre  arc  also  wanting  on  the 
lower  or  proximal  joint. 

There  are  three  different  ways  in  which  the  free  brachials  may  be  arranged.  The 
arms  may  consist  of  a  single  row  of  joints,  the  brachials  being  superimposed  in  a 
single  series  with  parallel  surfaces  of  contact  (uniserial).  Again,  the  joints  may 
*'  alternate,"  if  they  are  wedge-shaped,  and  if,  in  the  row,  the  thick  and  the  thin  sides 


372 


COMPARATIVE  ANATOMY 


CHAP. 


of  the  wedges  regularly  alternate.  Or  again,  the  joints  may  be  arranged  in  two  series 
or  rows,  the  contact-surfaces  of  the  one  row  alternating  with  those  of  the  other,  and 
the  two  rows  themselves  interlocking  along  a  zigzag  line  (biserial). 

The  Articulata,  many  Canaliculata,  and  the  recent  Inadunata  have  the  joints 
of  their  arms  arranged  in  single  rows.  This  condition  has  been  proved  to  be 
ontogenetically  and  phylogenetically  primitive,  i.e.  for  the  palaeozoic  Inadunata 
and  the  Camerata.  The  majority  of  palaeozoic  Inadunata  have  uniserial  arms, 
but  towards  the  end  of  the  palaeozoic  period  forms  appeared  with  alternating  rows 
(e.g.  Potcriocrinus),  and  finally  some  genera  in  which  the  brachials  may  be  biserial 
at  the  tips  of  the  arms  (Eupachycrinus,  JErisocrinus,  Hydreionocrinus). 

Most  of  the  Camerata  (an  order  limited  to  the  palaeozoic  age)  have  biserial 
arms.  But  by  far  the  greater  number  of  the  Lower  Silurian  species  have  uniserial 


br 


Fn;.  323.— Part  of  the  arm 
of  a  Crinoid.  Diagram  showing 
the  transition  from  the  uniserial, 
through  the  alternating,  to  the 
biserial  arrangement  of  the 
brachials. 


FIG.  324.— Part  of  the  disc  formed  by  the  arms 
of  Crotalocrinus  rugosus  (after  Wachsmuth  and 
Springer).  2,  The  trabeeulte  connecting  the  arms ; 
/»-,  the  arms  with  the  covering  plates  (cpa)  over  their 
food  grooves  ;  in  3  these  covering  plates  are  removed. 


arms.  In  the  Upper  Silurian,  however,  but  few  forms  persisted  with  such  arms, 
and  they  are  found  side  by  side  with  species  and  genera  with  alternating,  or  with 
two  rows  of,  brachials. 

In  Crinoids  whose  arms  have  two  rows  of  joints,  the  uniserial  and  the  alternate 
stages  are  passed  through  ontogenetically.  It  must,  further,  be  specially  emphasised 
that  not  a  single  case  is  known  of  arms  being  formed  of  two  rows  of  brachials 
throughout  their  whole  length,  i.e.  from  the  radials  of  the  calyx  to  their  tips.  At 
their  bases  the  arms  always,  for  a  certain  distance,  have  a  single  series  of  brachials, 
then  they  have  alternating  brachials,  and  finally  two  rows.  The  transformation  of 
the  uniserial  arm  into  an  alternate,  and  finally  into  a  biserial  one  commences, 
ontogenetically  and  phylogenetically,  at  the  tip  of  the  arm,  and  proceeds  from  that 
point  towards  the  base. 

The  food  grooves  of  the  anus  resemble  those  of  the  calyx.  They  are  sometimes 
naked  and  open,  and  at  others  provided  with  a  variously  developed  ambulacral 
skeleton,  consisting  either  only  of  lateral  plates,  or  of  lateral  and  covering  plates. 
Subambulacral  plates  may  also  occur  in  the  floor  of  the  food  grooves,  dividing  them 
from  the  subjacent  organs  of  the  ambulacral  furrows  of  the  skeleton  (body  cavity  of 
the  arms,  genital  strands,  pseudohaemal  canals,  etc.).  Where  covering  plates  are 
present  there  are  two  rows  which  alternate  and  interlock  in  such  a  way  as  to  form  a 


YIII         EGHINODERMATA— MORPHOLOGY  OF  SKELETON        373 

median  zigzag  line.     These  plates  can  be  raised  and  depressed  in  the  living  animal ; 
when  they  are  raised  the  food  groove  is  open,  when  depressed,  it  is  shut. 

An  altogether  peculiar  arrangement  is  found  in  the  arms  of  the  genus  Crotalo- 
c i- in  us  (Upper  Silurian,  England,  Sweden),  which  is  thought  by  some  to  belong  to 
the  Caiiicrata.  The  free  arms  branch  extraordinarily  frequently,  the  separate  branches 
being  closely  crowded  together,  and  forming  together  a  wide  expanded  coherent 
disc  round  the  calyx,  resembling  the  fully  open  corolla  of  a  flower.  As  many  as  500 
to  600  branches  may  in  some  forms  reach  the  edge  of  this  disc  (C.  rugosus,  Fig.  324). 
Each  ossicle  of  the  arms  has  two  lateral  processes,  which  become  connected  with 
similar  processes  of  the  corresponding  ossicles  of  the  neighbouring  arms  or  branches, 
so  that  the  disc  formed  in  this  way  by  the  skeleton  of  all  the  free  arms  is  lattice-like. 
At  definite  distances  from  the  calyx  the  brachials  are  of  equal  length,  so  that  they, 
as  well  as  the  sutures  which  lie  between  the  consecutive  brachials,  seem  to  be  arranged 
in  regular  concentric  rings  round  the  calyx.  The  whole  brachial  disc  was  very  flexible, 
and  could  be  rolled  up  over  the  calyx  from  its  periphery.  In  C.  pulchcr,  the  brachial 
disc  falls  into  five  broad  radial  lobes,  which,  when  the  disc  closes  over  the  calyx,  over- 
lap like  the  petals  of  a  bud.  The  food  grooves  are  covered  by  double  longitudinal 
rows  of  alternating  covering  plates. 

(<•)  The  Stem  (Columna). 

The  great  majority  of  Crinoids  are  attached  to  the  bottom  of  the 
sea  by  means  of  a  jointed  stem.  Among  recent  Crinoids  only  the  Ante- 
doiiidn:'  and  Thaumatocrinus  are.  in  the  adult  condition,  non-pedunculate 
and  unattached.  The  stalked  condition  is  undoubtedly  the  more 
primitive,  for  (1)  the  Crinoids  show  very  markedly  the  habitus 
characteristic  of  many  attached  animals,  and  (2)  all  free  and  unstalked 
Antedonidce  pass  through  an  early  stalked  and  attached  stage.  The 
stem,  which  varies  greatly  in  length  and  thickness,  consists  of  a  series 
of  calcareous  ossicles  one  above  the  other,  the  uppermost  of  which  is 
connected  with  the  centre  of  the  apical  system,  and  carries  the  calyx 
with  its  arms. 

The  ossicles  of  the  stem  (columnals  >  vary  greatly  in  shape.  They  may  be  flat  and 
disc-like,  or  long  and  cylindrical  ;  sometimes  they  are  gradually  thickened  towards 
each  end  in  such  a  way  as  to  resemble  dice-boxes.  Further,  the  columnals  in 
different  parts  of  one  and  the  same  stem  may  be  very  different.  The  external  outline 
of  the  ossicles  in  transverse  section  is  sometimes  pentagonal,  sometimes  round, 
rarely  elliptical.  They  are  connected  with  one  another  more  or  less  firmly  by  sutures, 
or  else  are  movably  articulated.  The  stem  throughout  its  whole  length  is  pene- 
trated by  a  central  canal  (axial  canal),  which  thus  runs  through  all  the  consecutive 
columnals.  AVithin  this  canal  run  the  ccelomic  canals  (continuations  of  the  chambered 
organ)  and  nerves.  The  size  of  the  canal  in  transverse  section  differs  as  much  as 
its  shape.  The  outline  of  its  section  seems  most  frequently  to  be  pentagonal  or  quinque- 
lobate,  but  it  is  not  infrequently  round.  Occasionally  also  the  central  canal  is  sur- 
rounded by  five  narrower  peripheral  canals. 

Xew  ossicles  are  added,  as  the  animal  grows,  at  the  upper  end  ;  at  first  they  are 
small  and  flat,  and  often  concealed  within  the  stem.  The  most  constant  place  of 
their  appearance  is  between  the  uppermost  columnal  and  the  base  of  the  calyx. 
New  ossicles  may,  however,  also  be  intercalated  between  two  already  formed  ossicles, 
but  this  almost  always  takes  place  at  the  upper  end  of  the  stem.  In  a  growing  stem 
the  ossicles  in  the  upper  part  vary  greatly  in  length,  the  shortest  being  the  youngest. 


COMPARATIVE  ANATOMY 


CHAP. 


374 

At  definite  intervals  the  stem  may  carry  whorls  of  so-called 
cirri  These  are  jointed  processes  of  the  stem,  pointed  at  their  tips, 
and  perforated  by  a  longitudinal  canal  which  communicates  with  the 
central  canal  of  the  stem  (Figs.  257,  258,  pp.  311,  312). 

The  cirri  are,  as  observations  on  living  animals  have  shown,  very  mobile.  Five 
cirri  as  a  rule,  belong  to  one  whorl,  being  inserted  on  the  five  sides  of  the  nodal 
ossicle  Between  two  consecutive  nodes  there  are  a  varying  number  of  columnals 
which  do  not  carry  cirri.  These  together  form  an  internode.  Whereas  in  the 
Inadunata,  Articulata,  and  Camerata  cirri  are,  as  a  rule,  wanting,  or  only  present 
at  the  lower  part  of  the  stem,  in  the  Cwutliculata  (Pentacrinidce)  nodes  are  found 
along  the  whole  length  of  the  stem  between  the  consecutive  internodes.  In  the 


FIG.  325.— Diagram  to  elucidate  Wachsmuth  and  Springer's  rule.    A,  Crinoid  with  dicyclic 
base.    B,  Crinoid  with  inonocyclic  base.     For  lettering  see  p.  317. 

recent  species  of  Pentacrinus  and  Metacrinus,  each  nodal  ossicle  is  connected  with 
the  next  ossicle  of  the  internode  below  it  by  a  syzygial  suture. 

Peculiar  relations  exist  between  the  stem  and  the  base  of  the  apical  capsule, 
according  to  the  "  rule  of  Wachsmuth  and  Springer,"  given  in  the  diagram  Fig.  325. 
In  Crinoids  with  dicyclic  base  (i.e.  where  the  base  consists  of  basals  and  infrabasals, 
with  pentagonal  stem  and  five-rayed  central  canal,  the  five  edges  or  angles  are  interradi- 
ally  arranged,  while  the  five  rays  of  the  central  canal  and  the  five  cirri  of  each  whorl 
are  radially  arranged.  In  Crinoids  with  monocyclic  base  (i.e.  where  the  base  consists 
exclusively  of  the  basals,  Fig.  325  B)  the  reverse  is  the  case.  In  those  Crinoids  which 
possess  cirri,  and  in  which  the  stern  and  central  canal  are  not  round,  the  character 
(monocyclic  or  dicyclic)  of  the  base  of  the  calyx  can  be  determined — apparently  with 
great  certainty— from  an  examination  of  the  stem.  This  is  of  importance  in  forms 
in  which  the  infrabasals  are  very  small,  or,  being  covered  by  the  uppermost  joint 
of  the  stem,  are  hidden,  or  when  they  occur  only  in  a  young  stage.  Such  forms  are 
said  to  be  constructed  on  a  dicyclic  plan,  and  have  been  called  "  pseudo-monocyclic." 
It  is  possible  that  certain  genera  in  which  Wachsmuth  and  Springer's  rule  appears 
to  be  violated  may  eventually  be  proved  pseudo-monocyclic.  Meanwhile,  however, 
the  rule  is  not  absolutely  universal. 

The  lower  part  of  the  Crinoid  stem  is  called  the  root.     It  serves, 
in  various  ways,  to  attach  the  body  to  the  sea  floor.     If  the  latter 


vni         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        375 


be  muddy  or  sandy,  the  base  of  the  stem  puts  out  lateral  branches, 
the  so-called  root-cirri,  the  numerous  ramifications  of  which  penetrate 
the  sea  floor  in  all  directions.  The  end  of  the  stem  itself  may  at 
the  same  time  branch  like  the  root-cirri.  When  the  sea  floor  is 
rocky,  the  root-cirri  spread  out  more  horizontally,  accommodating 
themselves  to  the  surface  to  which  they  are  attached,  and  becoming 
cemented  to  it  at  their  ends  by  means  of  a  calcareous  secretion. 

Further,  it  is  almost  certain  that  individuals  of  some  species,  e.g.  Pentacrinidce 
and  some  Palaeozoic  crinoids,  are  capable  of  free  locomotion  when  the  stem  is  either 
voluntarily  or  accidentally 
broken.  Such  locomotion  is 
no  doubt  chiefly  promoted  by 
the  movements  of  the  arms, 
the  cirri  serving  rather  for 
attachment. 

In  Holopus  (Fig.  250,  p. 
305)  the  stem  is  wanting. 
The  calyx,  which  resembles 
a  reversed  cone  in  shape,  is 
cemented  to  the  substratum 
by  means  of  an  irregularly 
expanded  calcareous  mass. 

The  Antedonidce  are 
stalked  and  attached 
only  in  the  young  stage. 
The  larval  stem  re- 
sembles that  of  the 
Bourgueticrinidae  (Fig. 
326) ;  the  cirri  are 
developed  only  on  its 
uppermost  ossicle,  on 
which  five  radially  ar- 
ranged cirri  first  appear, 

then     five     interradially         FIG.  326. —Several  stalked  young  stages  of  Antedon 
arranged.      At  a  certain    Phalangium  (A) ;  Antedon  spec.  (B) ;  Antedon  tuberosa  (C) ; 
rTffiori          -        fV,      anfi  Antedon  multispina  (D),  after  P.  H.  Carpenter.    For 
oe»     '  "    lettering  see  p.  317.     cia,  Points  of  attachment  of  the  cirri. 

different      species,     the 

calyx  together  with  the  uppermost  columnal  (i.e.  the  one  carrying  cirri), 
which  is  fused  with  the  infrabasals  to  form  the  eentrodorsal,  separates 
from  the  stem,  the  latter  remaining  attached  to  the  place  where  it  was 
fixed.  Above  the  cirri  already  formed,  i.e.  between  them  and  the  base 
of  the  calyx,  new  whorls  of  cirri  continuously  appear  on  the  centro- 
dorsal,  which  constantly  increases  in  size,  so  that  we  are  tempted  to 
consider  this  piece  as  part  of  the  stem,  consisting  entirely  of  nodal 
ossicles  fused  together  without  intervening  internodes. 

The  Comatulidae  can  both  swim  by  the  rowing  movements  of  their  arms,  and 
creep  by  means  of  the  cirri  and  of  the  arms.  They  can,  further,  anchor  themselves 
by  their  cirri,  the  arms  being  then  directed  upwards. 


376  COMPARATIVE  ANATOMY  CHAP. 


(d)  The  Manner  of  Connection  between  the  Skeletal  Pieces.1 

Under  this  head  we  have  to  consider  the  method  of  connection  between  the 
ossicles  of  the  arms  and  of  the  pinnulse,  between  the  plates  of  the  apical  capsule,  and 
between  the  columnals. 

Perhaps  the  clearest  view  of  the  great  diversity  which  prevails  in  this  matter  is 
obtained  by  assuming  that  the  plates  composing  an  echinoderm  skeleton  develop 
in  a  stroma  of  connective  tissue  fibrils  ;  all  the  plates  might  thus  be  supposed  to  have 
been  originally  but  loosely  united  by  such  fibrils. 

This  condition  persists  in  what  is  known  as  the  loose  suture.  The  ossicles  of  the 
pinnules  in  many  living  Crinoids  are  united  in  this  way. 

From  this  loose  suture  we  can  obtain  all  the  many  variations  which  are  found 
either  in  the  direction  of  greater  rigidity  or  of  greater  flexibility. 

In  the  former  case  we  have  : — 

1.  The  close  suture,  also  known  as  synostosis,  in  which  the  connecting  fibres 
are  short,  and  "the  joints  closely  and  immovably  fitted  together,  though  they  can  be 
separated  by  the  action  of  alkalies,"  e.g.  the  radials  of  Antedon. 

2.  The  syzygy,  which  is  a  special  case  of  close  suture,  viz.   that  in  which,  if 
pinnules  or  cirri  are  carried  by  the  ossicles,  the  lower  one  loses  its  pinnule  or  cirrus. 

The  two  components  of  a  syzygy  are  termed  epizygal  and  hypozygal. 

3.  Anchylosis,   in  which  the   two   plates  or  ossicles  are   immovably  cemented 
together  by  an  unbroken  deposit  of  calcareous  substance,  which,   however,  is  less 
solid  than  that  of  the  plates  themselves  (e.g.  the  basals  of  Bathycrinus  and  the 
radials  of  Rhizocrinus). 

On  the  other  hand  we  have  the  modifications  in  the  direction  of  greater  flexibility 
which  lead  up  gradually  to  the  development  of  true  muscles,  the  original  undiffer- 
entiated  fibrils  becoming  muscularly  contractile.  Such  muscular  articulations  may 
indeed  be  very  highly  specialised,  with  interlocking  ridges  and  teeth  on  the  opposed 
facets  of  the  ossicles. 

As  all  these  different  modifications  pass  into  one  another  by  imperceptible  stages, 
it  is  not  always  easy  to  say  whether  in  any  special  case  we  have  to  do  with  muscular 
articulation  or  with  a  less  specialised  form  of  connection.  It  now  seems  probable 
that  many  of  the 'fibrous  connections  which  were  at  one  time  thought  to  be  only 
elastic  fibres  are  really  muscles. 

For  instance  in  the  arms  of  living  Crinoids,  a  pair  of  muscles  on  the  oral  side  are 
counteracted  by  a  pair  of  bundles  of  "elastic  "  fibres  on  the  dorsal  side.  The  action  of 
the  muscles  in  contracting  rolls  the  arms  up  orally,  and  on  the  muscles  relaxing,  the 
"elastic"  fibres  expand  the  arms  again.  Now  it  is  clear  that  if  these  fibres  were 
thus  simply  elastic,  Crinoids  would  die  with  their  crown  of  tentacles  expanded, 
whereas  the  reverse  is  the  case. 

Again,  the  cirri  are  actively  movable,  often  (e.g.  in  Pentacrinus)  more  so  than 
the  arms,  although  no  muscular  articulations  occur  in  them. 

From  these  facts  it  is  rightly  argued  that  the  fibrils  in  these  cases,  though  differ- 
ing histologically  from  the  true  muscles,  are  yet  to  some  extent  muscular. 

That  all  these  various  connections  are  in  reality  derivations  from  some  common 
primitive  form  of  connection,  we  gather  from  the  fact  that  in  the  stems  of  Crinoids 
we  may  have  anchylosis,  close  suture,  syzygy,  loose  suture,  and  true  muscular 
articulation. 

1  This  passage  was  rewritten  in  accordance  with  the  tenour  of  Mr.  Bather's  criticism 
in  Natural  Science,  vol.  vi.  pp.  420,  421. TR. 


viii         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        377 

(>-)  The  Nerve  Canals  of  the  Arms  and  of  the  Apical  Capsule. 

(Figs.  327-330). 

The  skeletal  joints  of  each  arm  (the  brachials)  are  perforated  by 
an  axial  canal,  which  is  continued  to  the  tip  of  the  arm,  and  into  the 
pinnulae,  Where  the  arms  fork  or  branch  in  various  ways,  the  axial 
canal  does  the  same.  It  contains  nerve  strands,  and  may  thus  be 
suitably  considered  as  a  nerve  canal.  This  canal  is  continued  right  into 
the  base  of  the  dorsal  cup,  perforating  the  radials,  basals,  and  also  the 
infrabasals,  when  present.  All  the  nerve  canals,  and  the  nerves  within 
them,  converge  towards  the  apex  of  the  calyx,  where,  either  in  the  base 
of  the  dorsal  cup  itself  (surrounded  by  the  basals  in  stalked  Crinoids),  or 
partly  enclosed  within  the  centrodorsal  (Antedonulce),  lies  the  central 
organ  of  this  nervous  system,  which  surrounds  the  so-called  five- 
chambered  sinus  in  the  shape  of  a  cup  or  capsule.  From  this  point 
the  already  mentioned  central,  or  axial,  canal  runs  through  all  the 
ossicles  of  the  stem,  giving  off  lateral  branches  to  the  cirri. 

The  nerve  strands  arise  from  this  apical  central  nervous  system  at  five  interradial 
points.  The  five  interradial  strands  fork  either  in  the  basals  or  in  the  radials. 
Within  the  radials  each  branch  of  a  strand  becomes  connected  with  a  branch  of  the 
neighbouring  strand,  and  from  these  radially  arranged  points  of  junction  the  radial 
nerve  strands  originate  which  pass  from  the  radials  into  the  costals,  a'nd  are 
continued  into  the  ossicles  of  the  arms  and  of  their  branchings.  Within  the  circle 
of  radials  there  is,  besides,  a  circular  commissure  between  the  radiating  nerve 
strands  ;  the  following  diagrams  illustrate  the  courses  taken  by  these. 

In  the  Pentacrinida.',  Encrinidce,  and  Antedonidce,  where  the  nerve  strands 
divide  in  the  first  axillaries,  there  is  a  peculiar  chiasma  nervorum  brachialium, 
which  is  shown  in  the  diagrams. 

In  Encrinus,  and  it  is  said  also  in  Pcntacrinus,  the  nerve  strands  which  run 
through  the  ossicles  of  the  arms  are  double.  But  whereas,  in  Encrinus,  they  run 
separately,  and  are  enclosed  in  separate  canals,  in  Pcntacrinus  they  lie  in  a  common 
canal. 

Many  palaeozoic  Crinoids,  and  above  all  the  Camcrata  (with  the  exception  of  the 
Crotalocrinoids),  appear  to  have  no  differentiated  nerve  canals  in  the  arms. 


(/)  The  Water  Pores. 

In  the  Canaliculata  (e.g.  Pentacrinus,  AuMm,  Adi  no  metro)  the 
tegmen  calycis,  whether  naked  or  plated,  is  perforated  by  so-called 
water  pores,  whose  significance  will  be  discussed  more  in  detail 
later  on. 

If  the  tegmen  is  plated,  many  or  all  of  the  plates  of  the  interambulacral  areas 
are  perforated  by  one  or  more  such  pores.  In  Pcntacrinus  dccorus,  as  many  as  twenty 
pores  are  found  on  one  plate.  The  total  number  of  pores  varies  greatly  in  different 
genera  and  species.  In  Antcdon  rosacea  it  has  been  estimated  at  1500,  and  in  other 
forms  may  be  even  greater.  The  pores  are  usually  limited  to  the  tegmen,  and  are 
least  plentiful  in  the  posterior  interradius.  They  may,  however,  also  occur  on  the 


FIGS.  327-330.— Diagrams  to  illustrate 
the  course  of  the  axial  canals  and  the 
nerve  strands  within  them  in  the  dorsal 
cup  and  the  first  brachial  joints  of  En- 
crinus  (Fig.  327,  after  Beyrich),  Rhizocrinus 
lofotensis  (Fig.  328,  after  P.  H.  Carpenter), 
Antedon  rosaceus  (Fig.  329)  and  Bathy- 
crinus  aldrichianus  (Fig.  330,  after  P.  H. 
Carpenter).  In  Fig.  327  only  the  distal  ends 
of  the  interradial  canals  are  represented. 
The  parts  which  are  transversely  streaked 
run  superficially  on  the  inner  side  of  the 
basal  plates. 


FIG.  330. 


CH.  viii     ECHIXODERMATA— MORPHOLOGY  OF  SKELETON      379 

edge  of  the  calyx  between  the  bases  of  the  arms,  and  in  the  genus  Actinometra, 
where  they  are  chiefly  developed  near  the  ambulacral  furrows,  they  have  occasionally 
been  observed  on  the  lowest  pinnuke  as  well,  and  even  on  pinnule  in  the  middle 
or  towards  the  ends,  of  the  arms. 

In  Rhizocrinus,  in  each  interradius  of  the  tegnien  calycis  there  is  only  one  water 
pore  perforating  the  oral  plate.  In  Hyocrinus  the  anal  oral  plate  is  perforated  by 
two  pores  ;  the  other  oral  plates  either  have  one  pore  each  or  else  none  at  all. 
Further,  in  this  genus,  2  to  7  pores  occur  in  the  plates  of  the  interambulacral  areas 
lying  between  the  oral  pyramid  and  the  edge  of  the  calyx,  except  in  the  posterior 
interambulacral  area,  where  they  are  wanting. 

It  is  impossible  to  decide  with  certainty  whether  the  pores  which,  in  certain 
L'<i-i,i(r<ita  (Actinocrinidce,  Melocrinida;llhodocrinida'},  occur  at  the  edge  of  the  calyx  at 
the  bases  of  the  arms  (and  correspond  with  the  arms  in  number)  are  the  homolcgues 
of  these  water  pores.  The  same  applies  to  the  slit-like  pores  which  are  supposed  by 
some  writers  to  perforate  the  edges  of  the  plates  of  the  ventral  sac  in  the  Inadunata 
fistulata  (along  the  sutures),  and  to  the  pores  which  are  found  along  the  brachial 
farrows  in  the  Inadunata  larviformia.  These  pores  may,  in  some  cases,  have 
been  connected  with  hydrospires  (see  the  next  two  sections).  In  many  Inadunata 
there  is  what  appears  to  be  a  true  madreporic  plate  in  the  posterior  interradius  of 
the  tegnien. 

VI  Blastoidea. 

One  part  of  the  perisomatic  skeleton  of  the  Blastoidea  has  already 
been  described  in  connection  with  their  apical  system.  There  are 
five  interradial  or  deltoid  plates,  which  surround  the  oral  region 
(peristome),  and  radiate  out  from  it  (Fig.  331).  These  deltoid  plates 
do  not  form  a  closed  circle,  i.e.  do  not  touch  one  another  laterally, 
but  are  separated  from  one  another  by  the  proximal  portions  of  the 
five  ambulacra. 

In  describing  the  rest  of  the  perisomatic  skeleton,  which,  apart 
from  the  stem,  consists  exclusively  of  the  skeleton  of  the  ambulacra, 
it  is  useful  to  select  a  few  typical  forms. 


(a)  The  Ambulaeral  Skeleton. 

1.  Pentremites. — Fig.  263,  p.  314,  shows  a  representative  of 
this  genus  in  profile,  Fig.  331  from  the  oral  side.  The  five  ambulacral 
regions,  or  ambulacra,  together  form  a  five-leaved  rosette  round  the 
peristome  (Fig.  331,  A,  B,  C,  D,  E).  They  are  separated  from  one 
another  by  the  five  (interradial)  deltoid  plates  (3).  The  larger,  distal 
part  of  each  ambulacrum  fits  in  between  the  two  limbs  (10,  106)  of 
the  forked  radials. 

On  the  egg-  or  pear-shaped  body  the  ambulacra  stretch  as  far  as 
to  the  equator,  or  even  further,  towards  the  apical  pole. 

The  skeleton  of  each  ambulacrum,  when  most  complete,  consists 
of  the  following  parts  : — 

(a)  One  lancet  plate  (6). 

(b)  One  lower  lancet  plate  (12). 


380 


COMPARATIVE  ANATOMY 

(c)  Two  rows  of  side  plates  (5). 

(d)  Two  rows  of  accessory  side  plates  (8). 

(e)  Two  rows  of  pinnulae  (2). 

(/)  Two  groups  of  folds  of  hydrospire  pouches  (13). 
(g)  A  double  row  of  covering  plates  (1). 


CHAP. 


D 


FHJ.  331.-  Diagram  of  the  organisation  of  a  Pentremites  (original).  A,  B,  C,  D,  E,  the  five 
ambulacra.  A,  Ambulacrum  with  covering  plates  (1)  and  extended  pinnulae  (2).  B,  Ambulacrum 
with  depressed  piimulic.  C,  Ambulacrum  after  removal  of  the  pinnulae  and  covering  plates.  D, 
After  the  further  removal  of  the  side  plates  and  accessory  side  plates  (except  3).  E,  After  removal 
of  the  lancet  plate  as  well.  In  the  centre  is  seen  the  mouth  with  the  spiracles  surrounding  it,  in 
the  posterior  interradius  the  anus.  1,  Covering  plates;  2,  pinnulse;  3,  deltoid  plates;  4,  their 
sloping  ambulacral  edges  ;  5,  side  plates  ;  6,  lancet  plate  ;  7,  pores  ;  8,  outer  or  accessory  side 
pieces  ;  9,  furrow  of  unknown  significance  on  each  side  plate  ;  10,  radials  =  fork  plates  ;  11,  aperture 
of  the  ambulacral  canal  ;  12,  lower  lancet  plate  ;  13,  hydrospire  folds. 

Passing  over  the  covering  plates,  which  are  very  rarely  retained,  we  have,  at  the 
centre  of  each  aml.alacrum,  a  skeletal  plate  about  half  as  wide  as  the  ambulacrum 
itself,  ami  more  or  less  similar  in  shape.  This  is  the  so-called  lancet  plate  (Fig. 


vin         ECHINODERMATA— MORPHOLOGY  OF  SKELETON        381 

331,  6).  On  its  outer,  i.e.  oral  surface,  this  plate  shows  a  more  or  less  deep  longi- 
tudinal furrow,  which  gives  off  alternating  lateral  furrows  to  right  and  left.  This 
longitudinal  furrow  of  the  lancet  plates  is  universally  considered  to  have  the  same 
significance  as  the  food  grooves  on  the  tegmen  calycis,  and  on  the  arms,  of  Crinoids. 
Each  lancet  plate  is  longitudinally  pierced  by  a  canal,  the  so-called  ambulacral 
canal. 

The  space  on  each  side  between  the  lancet  plate  in  the  middle,  and  the  lateral 
margin  of  the  ambulacrum  (which  latter  is  formed  by  the  sloping  edges  of  a  deltoid 
plate  and  of  one  limb  of  a  fork  plate  (radial)),  is  occupied  (a)  by  a  longitudinal  row 
of  large  side  plates  (5),  and  (b)  by  a  longitudinal  row  of  smaller  accessory  side 
plates  (8).  The  number  of  side  plates  and  accessory  side  plates  corresponds  with 
that  of  the  lateral  branches  of  the  median  ambulacral  furrow  on  the  lancet 
plate.  Each  side  plate  consists  of  a  narrow  portion  which  is  directed  towards  the 
edge  of  the  ambulacrum,  and  of  a  broad  portion  which  is  in  contact  with  the  lancet 
plate.  The  broad  parts  of  the  consecutive  side  plates  in  a  row  are  in  contact  with 
one  another,  but  between  the  narrow  parts  of  these  same  plates  are  spaces,  in  each 
of  which  lie  an  accessory  side  plate,  and  a  hydro-spire  pore  (7).  This  latter  leads 
below  the  surface  to  the  hydrospire  pouches  under  the  ambulacrum.  The  hydro- 
spire  pores,  the  accessory  side  plates,  and  the  narrow  ends  of  the  side  plates  follow 
one  another  regularly  in  this  order. 

The  margins  of  the  ambulacra  carry  thin  long  jointed  appendages, 
the  pinnules  (2),  which  may  be  compared  with  the  structures  of  the 
same  name  in  the  Crinoids. 

The  pinnules  are  retained  only  in  rare  cases,  and  are  then  depressed  from  the 
two  sides  orally  over  the  ambulacral  area  (ambulacrum  B,  Fig.  331).  There  can, 
however,  be  no  doubt  that  they  could  be  raised  and  opened  out  (ambulacrum  A). 
The  number  of  the  pinnules  corresponds  with  that  of  the  side  plates  in  a  longitudinal 
row,  as  also  with  that  of  the  accessory  side  plates  and  of  the  hydrospire  pores.  The 
points  of  attachment  of  the  pinnules  lie  between  the  consecutive  pores.  Each 
pinnule  consists  of  a  large  number  of  skeletal  pieces,  which,  near  the  base,  alternate 
in  two  rows,  but  above  this  are  arranged  in  one  row. 

If  the  lancet  plate  of  an  ambulacrum  is  removed  (E,  Fig.  331), 
the  (smaller  and  thinner)  lower  lancet  plate  (12),  which  lies  close  below 
it,  appears  at  the  surface. 

This  plate  resembles  the  lancet  plate  in  shape.  If  the  pinnulae,  the  side  plates, 
and  the  accessory  side  plates  be  removed,  the  edges  of  the  plates  which  border  the 
ambulacrum  (deltoid  plates,  limbs  of  the  fork  pieces)  are  seen  sloping  towards  the 
floor  of  the  ambulacrum.  These  sloping  edges  carry  a  longitudinal  row  of  transverse 
ridges,  alternating  with  depressions,  into  which  latter  the  narrower  (outer)  portions 
of  the  side  plates  fit.  On  each  side,  between  the  lower  lancet  plate  and  the  sloping 
lateral  walls  of  the  ambulacrum,  some  of  the  parallel  clefts  and  folds  of  the  hydro - 
spire  pouches  (13)  are  seen  lying  parallel  to  the  longitudinal  axis  of  the  ambulacrum. 
At  the  central  or  proximal  end  of  the  ambulacrum  (i.e.  near  the  peristome),  the 
interradial  deltoid  plates  meet,  and  are  joined  by  a  radial  suture.  An  aperture  in 
this  suture,  the  ambulacral  aperture  (11),  leads  to  the  interior  of  the  calyx.  Through 
this  ambulacral  aperture,  the  ambulacral  canal,  which  traverses  the  lancet  plate 
longitudinally,  is  connected  with  a  circular  canal  which  surrounds  the  cesophagus. 

In  closest  proximity  to  the  peristome  there  are  five  large  inter- 


COMPARATIVE  ANATOMY 


CHAP. 


382 

radial   apertures,  the   so-called   spiracles.      Each  of  these   apertures 
leads  into  the  hydrospire  pouches  in  such  a  way  that  the  halves  of 
two  adjoining  ambulacral  areas  have  one  common  spiracle  for  1 
hydrospire  pouches. 

Each  spiracle  forms  a  depression  in  the  central  part  of  the  corresponding  deltoid 
plate  and  is  further  bordered  by  the  proximal  side  plates,  and  by  the  proximal 
ends 'of  the  lancet  plates.  Occasionally,  each  spiracle  is  more  or  less  distinctly 
divided  into  two  by  a  vertical  median  ridge  (septum)  projecting  into  it 
deltoid  plate.  In  the  posterior  interradius,  the  spiracle  is  confluent  with  the 
anus. 

The   hydrospires    (Fig.    332)    are    calcareous    pouches    or    tubes 
united  in  groups. 

A  group  of  such  calcareous  pouches  is  arranged  symmetrically  on  each  side  of 
the  middle  line  of,  and  in  close  connection  with,  each  ambulacrum.     The  pouches, 


2,     3 


FIG.  33--'.— Section  through  an  ambulacrum  of  Pentremites,  diagrammatic.  1,  Deltoid,  while 
lower  down  is  the  radial ;  ~2,  hydrospire  pore  ;  3.  accessory  side  plate  ;  4,  side  plate  ;  5,  lancet  plate, 
with  its  ambulacral  canal  7  ;  6,  covering  plates  ;  8,  common  channel,  into  which  the  hydrospire 
pouches  (11)  enter  at  9  ;  10,  base  of  a  phmula  ;  11,  hydrospire  pouches  ;  12,  lower  lancet  plate. 

which  hang  down  into  the  cavity  of  the  calyx,  are  parallel  to  one  another,  and 
stretch  from  the  distal  end  of  the  ambulacrum  to  its  proximal  end,  as  far  as  to  the 
spinu'lo,  through  which  they  open  externally.  In  addition  to  this  opening,  each 
hydrospire  pouch  possesses  a  slit-like  aperture  extending  along  its  whole  length,  in 
the  ambulacral  area.  These  hydrospire  folds  are  hidden,  lying  partly  under  the  side 
plates  and  partly  under  the  lancet  plate.  After  the  removal  of  these  latter  pieces, 
at  we  have  seen,  they  appear  at  the  surface.  They  vary  in  number  from  three  to 
nine.  The  concealed  hydrospire  canal  (8),  into  which,  on  each  side,  the  hydrospire 
] touches  open  through  their  clefts,  communicates  with  the  exterior  by  means  of  the 
hydrospire  pores  already  mentioned. 

The  hydrospire  pouches  or  tubes  have  thus  a  double  manner  of  communication 
with  the  exterior,  viz.  through  the  five  or  ten  spiracles  round  the  mouth,  and 
through  the  numerous  hydrospire  pores  at  the  lateral  edges  of  the  ambulacra. 

In  certain  species,  the  peristome  was  overarched  by  a  roof  of  covering  plates, 
f.,i  th.-  most  part  irregularly  arranged  (Fig.  331,  1)  ;  at  their  centre,  five  oral  plates 


vin         E<'HIXUJ>ERM  ATA— MORPHOLOGY  OF  SKELETON         383 

can  sometimes  be  distinguished.  The  covering  plates,  which  are  rarely  retained 
complete,  are  occasionally  continued  on  to  the  food  grooves  of  the  ambulacra,  where 
they  are  arranged  in  two  longitudinal  rows.  Perhaps  they  could  be  raised  and 
depressed  ;  if  not,  it  is  difficult  to  see  how  the  food  groves  with  their  lateral  furrows 
could  function  :  the  lateral  furrows  would  then  have  to  pass  under  the  covering 
plates  so  as  to  be  in  communication  with  the  principal  groove.  In  rare  cases  the 
covering  plates  even  spread  sideways  over  the  spiracles. 

2.  Codaster  (Fig.  265,  p.  314).— The  arrangements  here  differ  considerably  from 
those  of  Pent  re  mites,  just  described.  The  food  grooves  are  deeply  sunk  into  the 
lancet  plates,  which  are  hollowed  out  on  each  side  for  the  reception  of  the  side  plates. 
Spiracles  are  wanting.  A  certain  number  of  the  hydrospire  clefts  which  run  parallel 
to  the  ambulacrum  always  appear  at  the  surface  of  the  calyx,  laterally  to  the 
ambulacrum  (Fig.  333).  These  clefts  run  at  right  angles  across  the  suture  between 


FIG.  333.— Transverse  section  through  an  ambu- 
lacrum of  Codaster  (after  Etheridge  and  Carpenter), 
diagram.  1,  Deltoid  plate  or  possibly  a  radial ;  2,  ambu- 
lacral  canal ;  3,  food  groove  ;  4,  lancet  plate  ;  5,  side  plate  ; 
<i,  apertures  of  the  hydrospire  pouches  ;  7,  lower  lancet 
plate  ;  S,  hydrospire  pouches. 


FIG.  334. — Eleutherocrinus  Cas- 
sedayi,  from  the  oral  side  (after  Ethe- 
ridge and  Carventer).  aa-bb,  Axis, 
passing  through  the  mouth  and  anus  ; 
/•,  nulials ;  ir,  interradials  ;  ?-1}  the 
radial  of  the  differently  shaped  am- 
bulacrum ;  z,  y,  the  two  larger  basals. 


the  radial  and  the  deltoid  plates.  One  or  more  hydrospire  clefts  may  be  covered  by 
the  side  plates  of  the  ambulacra.  At  those  sides  of  the  two  posterior  ambulacra 
which  are  turned  towards  the  anus,  the  clefts  are  altogether  wanting. 

3.  In  Orophocrinus  (type  0.  stclliformis.  Fig.  266,  p.  314)  there  are  no  hydro- 
spire  pores  on  the  ambulacra,  but  depressions  occur  between  the  consecutive  side 
plates  for  the  reception  of  the  bases  of  the  pinnules.     The  hydrospire  clefts  lie  quite 
hidden  below  the  surface  within  the  ambulacral  sinus,  covered  by  the  lower  lancet 
plate.     The  spiracles,  on  the  contrary,  which  are  ten  in  number,  appear  at  the  sides 
of  the  ambulacra  as  long  wavy  slits.     The  two  spiracles  of  the  posterior  interradius 
are  distinct  from  the  anus.     The  ambulacra,  round  the  mouth  at  least,  are  covered 
by  covering  plates. 

4.  The  Irregulares  (Astrocrinus  and  Eleutherocrinus}  are  chiefly  distinguished 
by  the  fact  that  one  of  the  four  ambulacra  is  developed  quite  differently  from  the 
others  (Figs.  334,  and  267,  p.  315). 


384  COMPARATIVE  ANATOMY  CHAP. 

The  bundles  of  hydrospire  tubes  or  pouches  of  the  Blastoidea 
have  been  compared  with  the  "  bursae  "  of  the  Ophiuroidea.  They  are 
said  to  have  served,  like  these  latter,  for  respiration  and  for  the 
ejection  of  the  genital  products.  The  similarity  of  these  apertures  is 
specially  marked  if  we  compare  Orophocrinus  with  an  Ophiurid. 

(b)  The  Stem. 

With  the  exception  of  the  genera  Pentephyllum,  Eleutherocrinus, 
and  Astrovrinus,  which,  at  least  in  the  case  of  all  known  adults,  were 
non-pedunculate,  the  Blastoidea  were  attached  to  the  substratum  by 
a  jointed  stem  without  cirri  (Fig.  264,  p.  314). 


VII.  Cystidea. 

The  study  of  the  skeleton  of  this  ancient  class,  which  is  limited  to 
the  palaeozoic  age,  has  no  very  great  comparative  interest.  The  organi- 
sation of  the  very  heterogeneous  groups  which  are  classed  together 
under  this  heading  can  only  to  a  small  extent  be  understood  from 
their  skeletal  remains.  According  to  the  structure  of  the  skeleton,  we 
can  perhaps  distinguish  two  principal  groups  :  the  Cystovriwidea,  whose 
skeleton  consists  of  comparatively  few  definitely  arranged  plates,  and 
which  in  some  forms  approximate  the  Crinoidea ;  and  the  Eucystidea, 
whose  skeleton  consists  of  a  very  large  number  of  plates,  showing  no 
definite  recognisable  order. 

It  is  characteristic  of  most  Cystidea,  that  all  or  some  of  the 
plates  of  the  skeleton  are  perforated  in  various  ways  by  pores,  which, 
however,  never  seem  to  establish  communication  between  the  interior  of 
the  calyx  and  the  exterior.  It  is  difficult  to  ascertain  the  significance 
of  these  pores.  They  could  not  serve  for  the  passage  of  the  ambu- 
lacral  feet,  since  the  pore  canal,  as  already  said,  does  not  stand  in 
direct  communication  with  the  interior  of  the  calyx.  It  is  now  pretty 
generally  accepted  that,  as  the  water  passed  through  them,  they  served 
for  respiration.  The  following  principal  forms  of  pores  can  be 
distinguished : — 

1.  Scattered  single  pores. 

2.  Scattered  double  pores  (the  pores  being  united  in  pairs)  (Fig. 
260,  p.  312). 

3.  Double  pores  arranged  in  rhombs.     In  this  case,  the  two  pores 
of  a  double  pore  are  found  on  two  neighbouring  plates,  and  are  con- 
nected by  a  furrow  or  a  canal,  which  sometimes  runs  at  the  outer 
and  sometimes  at  the  inner  side  of  the  plate.     This  canal  or  furrow 
lies   at    right  angles   to   the    suture    between  the    two   plates,    and 
the  suture  itself  lies  diagonally  to  the  rhomb  formed  by  the  pores. 
Such    pore -rhombs    may    occur   on    all    the    plates    of    the    Cystid 
test,  or  again  may  occur  singly.     In  the  latter  case,  the  two  halves 


vui         ECHINODERMATA—  MORPHOLOGY  OF  SKELETON^  385 


of  a  rhomb  are  not  infrequently  divided  by  a  smooth  intermediate 
region. 

Since  it  is  difficult  or  eveii  impossible  to  give  any  comprehensive  description  of 
the  perisomatic  skeleton  of  the  Cystidea,  it  is  advisable  to  treat  a  few  of  the  best 
known  forms  separately. 

Cystocrinoidea. 

Porocrinus  is  a  form  which  only  differs  essentially  from  a  simple  Crinoid  of  the 
order  Inadunata  in  the  presence  of  the  pectinated  rhombs. 

Caryocrinus.  —  The  calyx  is  almost  pear-shaped,  and  is  carried  by  a  long  stalk 
perforated  by  a  wide  axial  canal.  At  the  edge  of  the  calyx  there  are  6  to  13 
thin,  jointed,  biserial  arms,  each  provided  with  a  furrow  on  the  oral  side.  The 
hexagonal  dorsal  cup,  apart  from  two  plates  lying  interradially  in  the  circle 
of  the  radials,  consists  exclusively  of  the  already  described  plates  of  the  apical 
system  (four  infrabasals,  six  basals,  six  radials).  The  tegmen  calycis  is  formed  of  a 
great  number  of  plates,  and  at  their  centre  six  orals,  which  show  the  characteristic 
arrangement  before  described  (Fig.  294,  p.  332),  completely  cover  the  mouth.  The 
ambulacral  furrows  are  not  externally  visible.  An  excentrically  placed  aperture,  roofed 
over  by  a  pyramid  formed  of  six  triangular  plates,  is  regarded  as  the  anus.  The 
pectinated  rhombs  are  found  on  all  the  plates  of  the  dorsal  cup,  and  on  them  alone. 
The  two  pores  of  a  double  pore  are  connected  by  a  canal  on  the  inner  side  of  the  plate. 

Echinoencrinus.—  The  almost  egg-shaped  calyx  consists  of  the  already  described 
plates  of  the  apical  system  (four  infrabasals,  five  basals,  five  radials)  arranged  in  five 
rays,  and  five  other  perisomatic  plates  in  contact  with  the  circle  of  the  radials,  and 
partly  pressing  in  between  them.  At  the  oral  pole  there  is  a  depression,  round  which 
short  uniserial  tentacles  rise,  and  in  the  centre  of  which  the  star  -shaped  oral 
aperture  lies.  The  anal  aperture  has  shifted  across  the  equator  of  the  calyx 
apically,  and  lies  to  the  right  posteriorly  above  the  basals.  Three  pectinated 
rhombs  are  developed,  whose  position  is  shown,  in  Fig.  296,  p.  333.  The  calyx  is 
carried  by  a  short  thick  stem,  the  ossicles  of  which  are  perforated  by  a  wide  canal. 

Cystoblastus  (Figs.  259,  p.  312,  and  295,  p.  332)  shows  some  resemblance  to 
the  Blastoidea.  The  egg-shaped  or  spherical  calyx  consists  of  sixteen  plates  and  the 
ambulacra.  Of  these  sixteen  plates,  which  are  arranged  in  four  circles,  fourteen 
belong  to  the  apical  system  (four  infrabasals,  five  basals,  and  five  radials).  The 
radials  exactly  resemble  the  radials  (fork  plates)  of  the  Blastoidea.  Each  radial 
clasps  an  ambulacrum  between  its  two  limbs.  Between  two  neighbouring  radials, 
in  four  of  the  interradii,  a  lancet-shaped  plate,  which  is  keeled  in  the  middle, 
presses  in,  recalling  the  deltoid  plates  of  the  Blastoidea.  In  one  iuterradius  this 
plate  is  wanting,  so  that  here  the  neighbouring  radials  touch  one  another.  At  the 
centre  of  the  rosette  formed  by  the  five  ambulacra  lies  the  mouth,  and  from  it  a 
furrow  runs  to  each  ambulacrum,  passing  down  its  whole  length  and  dividing  it  into 
two  lateral  halves.  The  principal  groove  of  each  ambulacrum  gives  off  alternating 
lateral  furrows,  which  end  in  distinct  pits  (pores  ?  depressions  for  the  reception  of 
pimiulae  ?).  At  the  base  of  the  calyx  there  are  two  pectinated  rhombs  (cf.  Fig. 
295,  p.  332).  Further,  the  forks  of  the  radials  appear  transversely  striated  by 
numerous  parallel  pore  clefts,  and  a  similar  striation  also  occurs  on  each  side  of  the 
rib  or  keel  on  the  four  deltoid  plates.  Perhaps  every  two  neighbouring  rows  of  pore 
clefts,  belonging,  however,  to  different  but  adjoining  plates,  together  formed  a  kind 
of  pectinated  rhomb.  A  large  aperture  half  way  up  the  calyx  is  regarded  as  the 
anal  aperture,  and  a  smaller,  in  an  angle  between  two  ambulacra,  as  the  aperture  of 
the  water  vascular  system.  This  is,  however,  uncertain.  The  arms  are  unknown, 
VOL.  II  2  C 


386  COMPARATIVE  ANATOMY  CHAP. 

and  so  is  the  stem,  though  the  latter  was  present,  since  it  is  easy  to  distinguish  a 
depression  at  the  apex  marking  its  point  of  attachment. 

Eucystidea. 

Protocrinus  (Fig.  260,  p.  312). — The  calyx  is  non-pedunculate,  with  somewhat 
flattened  apical  side  ;  otherwise  it  is  almost  spherical.  It  consists  of  numerous 
pentagonal  or  hexagonal  bulging  plates,  each  of  which  is  provided  with  several 
double  pores.  At  the  oral  pole  lies  the  oral  aperture,  from  which  five  long  ambu- 
lacral  furrows  radiate,  giving  off,  here  and  there,  lateral  furrows.  At  the  end  of  each 
lateral  furrow  there  is  a  depression  on  a  prominence.  At  these  points,  small  arms 
or  pinnulse  were  perhaps  once  articulated.  The  ambulacral  furrow  and  the  mouth 
are  roofed  over  with  covering  plates.  The  anus  lies  excentrically  in  an  interradius, 
roofed  over  by  a  pyramid  of  valves.  Between  anus  and  moutli  there  is  a  small 
third  aperture.  The  stalked  genus  Glyptosphcerites  is  related  to  Protocrinus. 

Orocystis  (Fig.  261,  p.  313). — The  almost  egg-shaped  body  is  tessellated  with  a 
considerable  number  of  plates  which  are  usually  hexagonal,  and  are  all  provided 
with  pectinated  rhombs.  The  pores  are  on  prominences,  arranged  as  in  the  figure. 
A  stem  was  present,  but  has  never  been  met  with  attached  to  the  body.  On  the 
oral  side  of  the  body  there  are  two  principal  apertures,  the  mouth  and  anus,  lying 
on  chimney-like  prominences,  and  there  is  also  an  additional  third  aperture.  The 
area  around  the  mouth  is  never  preserved  intact ;  probably  the  mouth  was 
surrounded  by  a  few  tentacles.  In  the  genus  Echinosphcera,  very  common  in 
the  Lower  Silurian,  the  spherical  test  is  formed  by  a  large  number  of  pentagonal 
or  hexagonal  plates,  all  of  which  have  pectinated  rhombs.  In  each  rhomb  the  pores 
lying  on  the  opposite  sides  of  the  suture,  between  the  two  plates,  are  connected 
in  pairs  by  canals.  The  oral  aperture  lies  on  a  chimney-like  or  conical  promi- 
nence, surrounded  by  two  to  four  long  or  short  arms.  At  some  distance  from  the 
buccal  cone  lies  the  anus,  covered  by  a  pyramid  of  valves.  Between  the'  mouth 
and  the  anus,  but  a  little  to  one  side,  is  a  third  smaller  aperture.  In  Aristocystis 
there  are  two  smaller  apertures  between  the  mouth  and  the  anus,  one  of 
which,  near  the  anus,  perhaps  represents  the  genital  aperture  which,  in  other 
Cystids,  is  possibly  confluent  with  the  anus.  In  Ascocystis,  the  body,  which  was 
evidently  richly  plated,  is  prolonged  like  a  tube  ;  at  the  pointed  apical  pole  it  was 
attached  by  means  of  a  stem  ;  at  the  oral  side  it  is  truncated,  the  oral  disc  being 
surrounded  by  as  many  as  twenty-five  biserial,  unbranched  arms.  The  structure  of 
the  disc  surrounded  by  these  arms  has  not  yet  been  made  out  with  certainty. 

Mesites.— The  body  is  spherical,  and  was  probably  stalked.     The  test  consists 
of  numerous  plates  provided  with  double  pores,  and  showing  no  definite  arrange- 
ment     Five  furrows  run   from  the   oral  pole   in  meridians 
towards  the  apical  pole.     Each  furrow  is  covered  by  a  double 
row  of  contiguous,  so- called  ambulacral  plates,  and  is  thus 
F,,,    33o.  -Trans-    collverted  into  a  closed  canal.     Between  the  consecutive  (am- 
verse  section  through    bulacral)  plates,  pores  lead  into  this  canal,  while  on  the  plates 
an    ambulacrum    of   themselves  circular  areas  can  occasionally  be  made  out,  which 
have  been  regarded  as  points  of  attachment  of  pinnulse.     A 
groove  runs  along  the  middle  line  of  each  double  row  of  plates  • 
this  is  open  along  most  of  its  length,  but,  at  the  oral  pole,  small  plates  form  a' 
slanting  roof  over  it. 

We  thus  distinguish,  in  each  radius,  an  external  groove  which  runs  upon  the 

which  mns  below  these 


viii  ECHINODERMATA— SPINES,  ETC.  387 

In  an  interradius  on  the  oral  side  of  the  body,  nearer  one  of  the  ambulacra  than 
the  other,  lies  the  anus,  which  can  be  closed  by  valves. 

M- sites  shows  a  certain  similarity  with  the  Palseechinoidea.  By  arbitrarily 
assuming  that  an  ambulacra!  vessel  ran  in  the  canal  below  the  ambulacral  plates, 
and  that  ambulacral  feet  passed  out  through  the  pores  between  these  plates,  stress 
was  laid  upon  the  agreement  thereby  established  between  the  Echinoids  and  Mesites 
in  the  position  of  the  radial  water  vascular  trunk  on  the  inner  side  of  the  ambu- 
lacral plates.  But  (1)  it  is  quite  uncertain  that  the  ambulacral  vessel  really  lay 
in  this  canal  and  not  in  the  outer  channel,  (2)  the  ambulacral  feet,  in  Echinoids, 
pass  through  the  ambulacral  plates  and  not  between  them  as  in  Mesites,  and  (3)  it  is 
not  at  all  certain  that  the  pores  of  Mesites  really  served  for  the  passage  of  ambu- 
lacral feet. 

Agelacrinus  (Fig.  262,  p.  313). — The  body  resembles  a  more  or  less  flat,  round 
disc,  attached  to  a  firm  object  (e.g.  the  shell  of  a  Brachiopod).  The  test  is  formed 
of  numerous,  irregularly  arranged  scale-like  plates,  which  more  or  less  imbricate 
with  one  another.  In  the  middle  of  the  free  (oral)  side  of  the  disc  lies  the  mouth, 
covered  with  plates,  from  which  five  curved  ambulacral  furrows  radiate,  covered 
with  double  rows  of  alternating  plates.  The  plates  of  each  double  row  form  a 
tunnel,  raised  above  the  level  of  the  disc,  and,  between  the  plates,  apertures  may 
occasionally  be  observed  which  are  supposed  to  have  served  for  the  passage  of 
ambulacral  feet.  In  one  of  the  interradii,  between  two  ambulacra  which  converge 
so  as  to  form  a  ring,  lies  the  anus,  arched  over  by  a  pyramid  of  valves. 

Just  as  Mesites  has  been  regarded  as  nearly  related  to  the  racial  form  of  the 
Echinoidea,  so  Agelacrinus  (with  the  related  genus  Edrioaster)  was  said  to  be  nearly 
connected  with  that  of  the  Asteroidea.  But  it  appears  hardly  conceivable  that  the 
almost  rigid  skeleton  of  the  attached,  sessile  Agelacrinus  could  give  rise  to  the 
richly  jointed  and  movable  skeleton  of  the  Asteroid.  In  the  Asteroids,  it  is  not 
the  ambulacral  feet  but  the  canals  connecting  them  with  the  ampullae  which  pass 
out  between  the  ambulacral  plates,  and  the  radial  water  vascular  trunks  lie  outside 
of  the  latter.  The  double  rows  of  covering  plates  in  Agelacrinus  cannot  thus  be 
compared  with  the  double  rows  of  ambulacral  plates  in  the  Asteroid. 

In  conclusion,  it  may  here  be  remarked  that  structures  resembling  the 
pectinated  rhombs  of  the  Cystidea  occur  in  many  fossil  Crinoids  and  Echinoids. 
These  are  parallel  strife  on  the  skeletal  plates,  which  run  transversely  over  the 
suture  dividing  two  neighbouring  plates,  and  together  form  a  rhomb -like  figure. 
In  young  and  fossil  Echinoids,  it  is  principally  the  plates  of  the  apical  system 
which  are  ornamented  with  such  striated  rhombs. 


D.  The  Spines  and  their  Derivatives — The  Sphseridia  and  the 
Pedieellarise. 

1.  The  Spines. 

The  test  of  the  Echinoidea,  and  the  plated  test  of  the  Asteroidea  and 
Ojdiiurwdea  carry  large  or  small,  variously  shaped  spines  or  processes, 
which  also  vary  in  number  and  arrangement.  The  knowledge  of 
the  structure,  shape,  size,  and  arrangement  of  these  rigid  processes 
of  the  body  (aeanthology)  is  of  importance  for  classification.  We 
must  here  confine  ourselves  to  the  most  important  points,  referring 
the  reader  for  further  particulars  to  systematic  works. 

a.  The  Spines  of  the  Eehinoidea,  which  we  shall  first  consider 


388  COMPARATIVE  ANATOMY  CHAP. 

merely  as  parts  of  the  skeleton,  occur  in  all  forms.  They  are  found 
in  definite  arrangement  over  the  whole  test,  on  both  the  ambulacral 
and  the  interambulacral  plates,  but  usually  in  greater  numbers  on  the 
latter  than  on  the  former. 

The  spines  are  usually  slender  and  pointed,  but  may  also  (e.g.  the  principal 
spines  of  certain  Cidaridce)  be  club-,  egg-,  plate-,  or  oar-shaped,  etc.  ;  or  again  in 
other  cases  they  may  look  like  fine  setse.  The  skeleton  of  the  spine  shows  the 
same  microscopic  lattice-like  structure  which  characterises  all  parts  of  the  Echino- 
derm  skeleton.  Transverse  and  longitudinal  sections  of  the  spines  reveal  specific 
developments  of  this  structure,  the  lattice-work  being  closer  in  some  cases  and 
opener  in  others,  so  that  a  careful  examination  of  the  structure  of  an  isolated  spine, 
taking  into  account  certain  possibilities  of  error,  may  suffice  to  determine  the  species 
of  a  specimen.  The  spines  are  mostly  solid,  less  frequently  hollow  (e.g.  in  the 
Scutellidce). 

The  spines  are  movably  articulated  with  the  test.  Each  spine 
rises  from  a  wart-like  prominence  of  a  test  plate,  which  is  called  a 
tubercle. 

Large,  strong  spines  rise  from  large  tubercles,  and  small  spines  from  small 
tubercles,  so  that  an  examination  of  the  tubercles  on  the  test  of  an  Echinoid  which 
has  lost  its  spines  leads  to  some  conclusions  as  to  the  nature  of  its  former  spinous 
covering.  The  tests  of  the  Clypeastroida  and  Spatangoida,  for  instance,  have  very 
small  tubercles,  with  which  the  small,  inconspicuous,  seta -like  spines  of  these  orders 
correspond.  The  regular  Echinoidea  have  powerful  spines  and  large  tubercles.  In 
the  Cidaroida,  especially,  by  the  side  of  numerous  small  tubercles  carrying  small 
spines,  there  occur,  in  the  interradii,  a  smaller  number  of  remarkably  large  tubercles, 
which  carry  either  very  long  and  strong  or  else  shorter  but  very  massive  spines, 
(Fig.  336). 

Most  spines  are  in  some  way  ornamented,  by  ribs,  thorns,  etc. 

In  describing  the  various  parts  which  can  be  made  out  in  a 
spine,  and  in  that  portion  of  the  test  plate  which  surrounds  its  base, 
we  shall  select  a  principal  spine  of  Doroddaris  papillata  (Fig.  337). 
The  spine  consists  of  a  shaft  and  a  socket,  the  latter  articulating 
with  the  tubercle  of  the  test  plate.  The  shaft  thins  away  near  the 
socket  to  form  a  neck  which,  again,  is  separated  from  the  socket 
by  a  projecting  circular  ridge  or  cushion. 

Each  tubercle  rises  from  a  mound  formed  by  the  bulging  of  a 
•ound,  smooth  area,  the  edge  of  which  is  surrounded  by  a  circle  of 
smaller  tubercles,  which  carry  smaller  spines  and  pedicellariaj  (Fig. 
336). 

The  socket,  where  it  is  in  contact  with  the  tubercle,  has  a  pit, 

and  a  similar  pit  is  found  at  the  centre  of  the  tubercle  itself.     Within 

iese  two  corresponding  pits  runs  an  axial  band  consisting  of  elastic 

fibres,  which  fastens  the  spine  to  the  tubercle,  and  passes  at  its  two 

ends  into  the  organic  substance  of  the  spine  and  tubercle. 

base  of  the  spine  is  surrounded  by  a  double  fibrous  envelope, 
inner  envelope  consists  of  elastic  fibres,  the  outer  of  muscle 


v"*  ECHINODERMATA— SPINES,  ETC.  389 

fibres,  which  latter  bring  about  the  movements  of  the  spine  on  the 
tubercle.  Both  the  muscular,  and  the  elastic,  fibres  are  attached  on  the 
one  hand  to  the  socket  (below  the  circular  ridge  or  cushion),  and  on 
the  other  to  the  area  surrounding  the  tubercle.  They  pass  into  the 
organic  substance  of  the  skeleton. 

The  spine  is  covered  from  tip  to  base  (i.e.  to  the  neck)  by  a  very 
hard  and  thick  calcareous  layer,  the  cortical  layer,  which,  in  the 


0,171 


FIG.  330.— Part  of  the 
surface  of  the  test  of 
Cidaris  tribuloides,  Ag., 
near  the  ambitus,  to  show 
the  tubercles  and  ambu- 
lacral  pores  c.p.  ia,  Interam- 
bulacral  row  of  plates  ;  ant. 
ambulacral  row  of  plates. 


FIG.  337.— Large  spine  of  a  Cidaris,  dia- 
grammatic (essentially  after  Pi  ouho).  1,  Cor- 
tical layer ;  2,  middle  layer ;  3,  medulla : 
4,  neck ;  5,  integument ;  6,  pedicle ;  7,  axial 
band  ;  8,  muscle  ring ;  9,  circular  ganglion ; 
10,  ligamentous  envelope;  11,  tubercle  on 
the  test;  12,  test. 


development  of  the  test,  is  the  last  part  deposited,  and  determines 
the  ornamentation  of  the  spine. 

At  first  the  body  integument  covers  the  whole  spine,  its  epithelium 
being  provided  with  cilia.  But  when  the  spine  has  attained  its 
definitive  size,  and  the  cortical  layer  has  been  formed,  the  integument 
dies  away  on  those  parts  which  are  covered  by  that  layer,  and  is  only 
retained  round  the  base  of  the  spine. 

At  this  base,  about  half  way  up  the  muscular  envelope,  under  the 
surface  of  the  epithelium,  lies  a  nerve  ring  with  scattered  ganglion 
cells ;  this  runs  right  round  the  base  of  the  spine,  and  innervates  its 
muscles. 


COMPARATIVE  ANATOMY 


CHAP. 


390 

The  structure  of  all  Echinoid  spines  resembles  that  just  described,  except  that 
the  pits  in  the  socket  (or  acetabulum)  and  on  the  tubercle  are  usually  wanting, 
and  with  them  also  the  axial  ligament. 

The  small  spines  of  the  Cidaroida  are  protective.  They  surround  the  anal 
aperture,  the  genital  apertures,  and  the  pores  of  the  radials  (or  ocular  plates) ;  on  the 
interambulacra,  they  surround  the  bases  of  the  prin- 
cipal spines  like  a  circular  palisade,  and  on  the  am- 
bulacra they  are  arranged  in  two  longitudinal  rows. 
They  can  be  raised,  and  inclined  towards  one  another 
over  the  part  to  be  protected.  The  smaller  spines 
have  no  cortical  layer  and  no  nerve  ring  at  their 
bases.  They  are  always  covered  by  the  ciliated 
•jf  integument,  which  at  the  tip  of  the  spine  carries 
sensory  (tactile)  hairs.  Each  small  spine  carries  at 
e  its  base,  on  the  side  turned  away  from  the  part  to  be 
protected,  a  whitish,  transparent,  ampulla  -  shaped 
swelling,  which  seems  to  be  caused  by  the  presence  of 
glandular  cells  in  the  epithelium.  The  secretion  of 
this  glandular  cushion  may  perhaps  be  poisonous. 

In  Ccntrostcphanus  longispimis,  round  the  anus, 
certain  short  spines  of  a  lilac  colour  occur  ;  in  the 
living  animal,  these  constantly  rotate,  the  tips  of  the 
spines  describing  a  circle.  In  the  epithelium  of  these 
spines  there  are  sensory  prominences,  and  at  their 
bases  the  characteristic  circular  ganglion.  The  fibres 
of  the  muscular  envelope  are  transversely  striated. 

In  Podocidaris,  immovable  spines  without  any 
articulation  are  found,  principally  on  the  apical  side 
of  the  shell. 

The    poisonous    spines  of    Asthenosoma  urens 
(Echinothurid).     This   Echinoid  is   much   feared   by 
fishermen  and  divers,  on  account  of  the  acute  pain 
produced  by  contact  with  its  body.     Spines,  whose 
ends  are  swollen  into  shiny  blue  heads  (Fig.   338), 
seem  to  form  the  principal  part  of  its  poison  apparatus. 
These  poison  spines  are  arranged  in  regular  bands  on 
Kio.  33S.-Spine  with  poison   the  interambulacra,  but  in  the  larva  are  found  scattered 
head    of    Asthenosoma    urens 

(after  P.  and  F.  Sarasin),  diagram-  OVer  other  Parts  as  welL  The  axis  of  the  sPine  is 
matic.  i,  The  tip  of  the  spine ;  occupied  by  a  hollow  calcareous  rod  with  an  extremely 
2,  poison  sac ;  8,  epithelium  of  fine  point ;  this  rod,  throughout  the  greater  part  of 

int^Tr^;rf?ith0r  its 'eif hi is  perforated  ^^s  m™*°« » i°»«i- 

the  poison  head,  penetrating  the    tudmal  rows  >  at  lts  mie  ^P*  however,  there  are  only 

Hpine ;    6,   longitudinal  rows  of  a  ^ew  small  pores  or  eyes.     The  swollen  head  which 

pores  in  the  shaft  (7)  of  the  spine,    surrounds  the  tip  of  the  spine  contains  a  somewhat 

large  poison  sac,  with  an  aperture  at  its  tip,  through 

the  spine  may  protrude.      The  epithelium  lining  the  sac   passes,   at  the 

aperture,  into  the  outer  epithelium  of  the  head.     The  poison  sac  and  the  part  of 

e  spine  which  runs  through  it  are  filled  with  a  clear  fluid  with  floating  vesicles 

id  remains  of  cells),  yielded  by  the  epithelium  of  the  sac.     The  sac  and  its 

elope  of  connective  tissue  are  surrounded  by  a  powerful  muscular  capsule,  most 

fibres  are  attached  on  the  one  hand  to  the  sac,  and  on  the  other  to  the  part 

e  spine  lying  below  it.     The  contraction  of  these  muscles  causes  the  sharp  tip 

»  spine  to  protrude  through  the  aperture  of  the  sac.     Perhaps,  at  the  same 


ECHINODERMATA— SPINES,  ETC. 


391 


time,  the  poison  is  squeezed  into  the  spine  through  the  lower  pores  in  that  part  of 
it  which  lies  within  the  sac,  and  is  squirted  out  through  the  few  pores  or  eyes  at 
its  tip. 

On  the  fascicles  of  the  Spatangoida,  whose  course  has  already  been  described 
(p.  349),  there  are  exceedingly  numerous  very  small,  granular  tubercles  carrying  small 
seta-like  spines,  thickened  at  the  tip  ;  these  are  sometimes  articulated,  sometimes 
immovable.  Such  clavulae  are  covered  by  a  ciliated  integument,  which  very 
probably  contains  sensory  cells. 

I.  The  Spines  of  the  Asteroidea. — The  Asteroid  body  is  also 
usually  covered  with  spines  and  papillae.  The  form  and  arrangement 
of  these  vary  so  greatly  that  they  cannot  here  be  more  fully  described. 
We  must  refer  the  reader  to  the  principal  systematic  works  for  details. 
Their  finer  structure  is 
almost  entirely  unknown, 
and  we  have  hardly  any 
knowledge  of  the  positions 
of  the  sensory  organs  and 
glands  which  almost  cer- 
tainly occur. 


The  spines  are  often  firmly 
connected  with  the  skeletal 
plates  of  the  body  wall,  from 
which  they  rise.  Spines  occur 
most  constantly  at  the  edges  of 
the  ambulacral  furrows,  border- 
ing them  like  a  palisade.  They 
are  not  infrequently  movable  ; 
the}'  can  be  erected,  and  inclined 
over  the  furrow  for  its  protection 
(Fig.  243,  p.  298). 

Many  Phancrozonia,  and 
specially  the  Astrapectinidcc, 
are  characterised  by  short  cal- 
careous pillars  rising  from  the  integument,  which,  on  their  terminal  flat  surfaces, 
carry  a  usually  circular  group  of  small,  thickly  crowded  spines,  prominences,  or 
papillae.  These  structures  are  called  paxillae  (Fig.  309,  p.  351). 


FIG.  339. —  Three  brachial  joints  of  Ophiopteron 
elegans,  from  the  middle  part  of  the  arm,  lower  side  (after 
Ludwig).  5s,  Ventral  shields ;  te,  tentacle ;  ss,  lateral 
shields ;  1,  hook ;  2,  thorny  spine ;  3,  supporting  rods  of 

the  fins. 


c.  Spines  of  the  Ophiuridse. — In  the  Ophiuridce,  it  is  principally  or 
exclusively  the  lateral  shields  which  carry  spines,  in  a  manner  already 
described  (p.  355). 

These  spines  are  mostly  large,  slender,  and  pointed,  and  are  occasionally  provided 
with  thorns.  In  the  genera  Ophiomastix,  Astroschema,  and  Ophiocreas  club-shaped 
spines  occur,  together  with  the  ordinary  kinds.  Over  the  ends  of  these  spines  the 
epithelium  is  thickened,  and  contains  glandular  and  sensory  cells.  In  Ophiopteron 
elegans,  numerous  small  spines  of  peculiar  structure  are  found  on  the  dorsal  side  of  the 
disc.  A  short  stem  divides  into  six  long  pointed  branches,  which  are  connected 
by  a  thin,  soft  membrane  in  such  a  manner  as  to  form  a  kind  of  funnel.  The  whole 
.structure  somewhat  recalls  an  umbrella  turned  inside  out.  In  the  same  species, 
each  lateral  shield  carries,  besides  a  hook  and  a  thorny  spine,  ten  long,  slender 


C, 


392  COMPARATIVE  ANATOMY  CHAP. 

spines  arranged  in  a  row,  which  runs  up  from  the  ventral  to  the  dorsal  side  of  the 
ami ;  these  spines  are  connected  by  a  thin,  transparent  membrane  in  such  a  way  as 
to  form  a  sort  of  fin  (Fig.  339).  On  the  first  three  free  joints  of  the  arm,  the  fin  of 
one  side  of  the  arm  passes  into  that  of  the  other  side  dorsally.  We  are  justified 
in  assuming  that  the  animal  is  able  to  swim  by  means  of  these  large  fins  on 
the  arms. 

The  genera  Ophiotholict  and  Ophiohdus  are  distinguished  by  peculiar  umbrella- 
shaped  spines.  A  stem  with  a  swollen,  button -like  base,  articulating  with  a 
tubercle,  carries  at  its  tip  a  circle  of  recurved  spines,  which,  during  life,  are  covered 
by  a  common  integument.  These  are  found  either  in  groups  near  the  base  of  the 
ordinary  brachial  spines,  as  in  Ophiotholia,  Avhere  they  first  appear  at  some  distance 
from  the  disc,  or  else  replacing  the  ordinary  spines  near  the  end  of  the  arm,  as  in 
Opkiohelus. 

Function  of  the  spines. — The  fact  that  the  spines  serve  princi- 
pally for  the  protection  of  the  body  is  at  once  evident,  especially  when 
they  are  provided  with  poison  glands. 

In  response  to  stimuli,  the  spines  become  erect.  In  Diadema  setosum,  which  is 
very  sensitive  to  light,  the  long  spines  turn  threateningly  towards  a  hand  which 
approaches  them  from  any  side.  The  spines  of  most  Echinoids  further  serve  for 
locomotion,  moving  in  a  co-ordinated  manner.  This  has  been  directly  proved  in 
the  Cidaridcc,  Arbacia,  Echinus,  and  Spatangus,  etc.  In  the  first  of  these  forms, 
the  long  (principal)  spines  are  indeed  the  chief,  or  the  only,  locomotory  organs,  and 
are  used  as.  stilts.  Many  Echinoids,  e.g.,  Dorocidaris,  Arbacia,  Spatangus,  if  laid 
on  their  backs,  can  turn  themselves  over  again  by  the  help  of  their  spines. 

It  has  also  been  proved  that  the  spines  may  serve  for  seizing  prey  and  for 
forwarding  it  to  the  mouth.  Several  spines  incline  towards  the  prey,  seize  it  with 
their  tips,  and  pass  it  on  to  the  next  group  in  the  oral  direction,  and  so  on  towards 
the  mouth.  Compare  with  this  the  rise  of  pedicellarise  in  Asteroids,  p.  394. 


2.   Modified  Spines. 

a.  The  Sphseridia  of  the  Eehinoidea.— These  are  small  spherical 
or  ellipsoidal  bodies,  which,  by  means  of  a  short  stalk,  articulate  with 
a  prominence  of  the  test,  and  are  inclined  sometimes  in  one  direction, 
sometimes  in  another.  They  either  project  freely,  or  else  rise  from  the 
base  of  a  pit-like  depression  of  the  test  (Fig.  340).  This  depression 
may  more  or  less  completely  close  over  the  sphamdium.  We  are 
here  reminded  of  the  various  forms  of  acoustic  tentacles  in  the 
Medusae,  which  sometimes  rise  freely  from  the  body,  sometimes  from 
the  base  of  a  pit,  sometimes  on  the  walls  of  closed  vesicles,  which  latter 
come  into  existence  through  the  concrescence  of  the  edge  of  the  pit 
above  the  tentacle.  Here,  however,  we  have  to  do  not  with  tentacles, 
but  evidently  with  modified  spines. 

Sphoridia  occur  in  all  Echuwidea  except  the  Cidaroida.     They  are  found  only 

e  ambulacra,  and  always  on  the  peristomal  plates,  although  in  many  forms 

re  not  limited  to  these,  the  area  in  which  they  occur  stretching  out  in  the 

the  ambitus,  or  even  beyond  it.     The  number  and  arrangement  of  the 

sphaendia  vary  greatly  m  different  groups. 


VIII 


ECHINODEEMA  TA — SPH^RIDIA 


393 


Structure  of  the  Sphseridia  (Fig.  341). — The  sphseridia  consist 
(1)  of  a  very  firm  and  hard  transparent  calcareous  sphere,  which  is 
concentrically  laminated,  and  does  not  show  the  lattice-like  perforated 
structure  of  the  rest  of  the  skeleton,  and  (2)  of  the  calcareous  stem, 
which  is  perforated  like  a  sponge,  and  is  generally  continued  into  the 
interior  of  the  sphere.  The  calcareous  sphere  perhaps  answers  to  the 
cortical  layer  of  a  large  spine  of  a  Cidaroid  (cf.  Fig.  337).  Not  infre- 
quently the  head  is  traversed  by  a  canal  which  opens  at  its  free  end. 

The  sphseridium  is  covered  by  a  ciliated  epithelium  which  is  often 
pigmented  ;  the  waving  cilia  are  very  long  at  the  base  of  the  stem, 
but  gradually  diminish  in  length  towards  the  head.  The  sphseridia, 
like  the  spines,  are  surrounded  at  the  base  where  they  articulate  with 
the  tubercle,  by  a  muscular  envelope  and  by  a  circular  ganglion,  the 


FIG.  340.  —  Portion  of  an  ambulacrum 
bordering  the  peristome  in  Echinocidaris 
nigra,  Mol.  (after  Loven),  magnified.  1,  Sphseri- 
dium  in  its  niche  ;  2,  ambulacral  double  pore  ; 
3,  edge  of  the  peristome. 


FIG.  341.— Longitudinal 
section  through  a  sphaeri- 
dium,  diagrammatic.  1,  Cal- 
careous mass  of  thesphflpriclium; 
2,  epithelium ;  3,  calcareous 
stem  of  lattice-like  structure ; 
4,  muscle  envelope ;  5,  circular 
ganglion  ;  6,  tubercle  ;  7,  test. 


latter  lying  within  the  epithelium,  which  is  here  specially  thickened. 
The  hair-like  cells  of  this  circular  thickening  of  the  epithelium  are 
probably  for  the  greater  part  sensory. 

The  sphseridia  have  always  been  claimed  as  sensory  organs,  and, 
on  account  of  their  usual  position  near  the  mouth,  as  gustatory  or 
olfactory  organs.  They  have  also  been  thought  to  be  auditory,  or 
organs  for  the  appreciation  of  the  movements  of  the  water.  They 
also  remind  us  of  organs  adapted  for  appreciating  the  position  of  the 
body  in  the  water. 

b.  The  Pedieellarise. — These  are  small  seizing  organs  which  rise 
from  the  integument.  They  occur  in  all  JEchinoidea,  most  Asteroidea, 
and  a  few  Ophiuroidea  in  very  varying  number  and  arrangement,  and 
in  many  different  forms,  between  which,  again,  there  are  transition 
forms.  They  must  be  considered  as  modified  spines,  or  groups  of 
spines.  In  one  and  the  same  species  various  forms  of  pedicellarise, 


394  COMPARATIVE  ANATOMY  CHAP. 

definitely  arranged,  may  occur.  It  is  very  probable  that  many  of  the 
different  forms  of  pedicellariae,  within  certain  divisions,  have  developed 
independently  out  of  spines. 

1.  The  pedicellarise  in  the  Ophiuroidea.  —  In  Trichaster  elegans,  from  about  the 
thirty-sixth  tentacle  pore  of  an  arm  onwards,  the  two  tentacle  papillae  are  replaced 
on  the  adoral  side  of  each  pore  by  two  hooks  movably  articulated  on  a  stem.     This 
stem  also  is  articulated  with  a  ventral  lateral  process  of  the  corresponding  brachial 
vertebral  ossicle.     The  skeleton  of  this  apparatus  consists  of  three  pieces,  belonging 
to  the  stem  and  to  the  two  diverging  hooks.     The  hooks  do  not  move  against  one 
another,  the  planes  of  their  movement  being  nearly  parallel.     On  one  side  a  flexor, 
and  on  the  other  an  extensor  muscle  connects   each  hook  with  the   stem.      In 
Astropliyton  also,  similar  pedicellarise  are  found,  and  in  Ophiothrix  fragilis  the  end 
of  the  arm  is  beset  with  movable  hooks  provided  with  flexor  and  extensor  muscles. 
Similar  hooks  occur,  further,  on  the  lateral  shields  of  the  arms  in  certain  species  of 

•  Gorgonoccphalus. 

2.  The  pedicellarise  of  the  Asteroidea  (Fig.  342). — In  some  groups,  e.g.  the 
Astcrinidce,  Solastcridce,  and  Pterasteridce,  the  pedicellarise  are  altogether  wanting : 
in  the  Astropcctinidce  they  are  only  very  rarely  found.  v 

In  the  simplest  cases,  groups  of  small  spines  may  function  as  pedicellarise.  The 
spines  of  such  a  group  are  movably  connected  with  the  body,  and  may  be  arranged 
either  in  two  opposite  rows  of  four  to  five  spines  each,  these  rows  approximating  or 
diverging  ;  or  else  at  definite  points  of  the  body  three  or  four  spines  stand  close 
together,  forming,  when  they  incline  towards  one  another,  a  three-  or  four- sided 
pyramid.  Two  spines  even  may  form  a  group.  For  instance,  on  the  dorsal  surface 
of  Asterina  gibbosa,  spines  are  found  sometimes  isolated,  sometimes  united  in  larger 
or  smaller  groups.  Among  these  groups  there  are  couples  connected  at  the  base  by 
a  transverse  muscle,  and  such  spines  can  move  towards  one  another  more  energetic- 
ally than  those  of  the  other  groups  (Fig.  342,  A  to  F). 

In  the  above  cases,  we  have  to  a  great  extent  to  do  with  commencing  or  rudi- 
mentary pedicellarise,  and  we  recognise,  in  the  larger  and  smaller  groups  of  spines, 
the  material  out  of  which  pedicellarise  with  two,  three,  or  four  forceps  may  be 

r  developed.  (Of.  also  what  has  been  remarked  on  p.  392  on  the  spines  of  the  EcMnoidea 
as  organs  for  the  seizing  and  conveying  of  prey  to  the  mouth,  and  p.  390  on  the 
smaller  spines  of  the  Cidaroida.} 

The  true  pedicellariae  of  the  Asteroidea  usually  have  two  blades  or  valves,  less 
frequently  three.  Stalked  and  sessile  pedicellarise  may  be  distinguished. 

a   Sessile    pedieellariae    (Fig.    342,    G).— The   two   blades   rise 

rectly  from  the  integument.     Each  consists  of  a  calcareous  piece 

determining    its    shape,    which    may   be    long    or    short,    broad    or 

narrow,  pointed  or  blunt,  flat  or  spoon-like.     The  two  skeletal  pieces 

are  directly  articulated  with  a  skeletal  plate  of  the  integument.     In 

rymnastena  carinifem,   for   example,    numerous    double-bladed    pedi- 

llarias  rise  at  the  edge  of  the  ambulacral  furrow.     The  two  blades 

•e  connected  at  their  bases  in  a  manner  illustrated  in  the  figure,  by  a 

transverse  muscle,  the  adductor  muscle.     Further,  each  blade  at  its 

ter  side  (i.e.  at  the  side  turned  away  from  the  axis  of  the  pedicel- 

>  also  connected  with   the  subjacent  calcareous  plate  of   the 

by  an  opening  muscle  (abductor).     The   bases  of   the 


A'HI 


ECHINODERMA  TA— PEDICELLARIA 


395 


pedicellariae  are  further  attached  by  a  strong  elastic  fibrous  band  to 
this  same  plate. 

b.  Stalked   pedicellarise    (Fig.    342,    H,    K).— Each    pedicellaria 
rises  from  a  short,  soft  stalk ;  the  blades,  of  which  there  may  be  two 


FIG.  342.—  Pedicellariae  of  Asteroids.  A,  B,  C,  D,  E,  F,  Pseudo-  or  commencing  pedicellariae  of 
various  species.  G,  Sessile  pedicellaria  from  the  edge  of  the  ambulacral  furrow  of  Gymnasteria 
<:ni'(i(ifera  (after  Cuenot).  H,  Stalked  straight  pedicellaria  diagrammatised  (a^ter  Cuenot).  J, 
Basal  portion  of  a  stalked  crossed  pedicellaria  of  Asteracanthion  rubens  (after  Perrier).  K,  A 
similar  pedicellaria  of  Asteracanthion  glacialis  (after  Cuenot).  1,  Calcareous  blade  of  the  forceps  ; 
•2,  basal  piece ;  3,  occlusor  muscle  ;  3i,  axial  muscles  of  the  blades  ;  4,  opening  muscle ;  5,  axial 
band  ;  6,  epithelium  ;  7,  body  wall ;  S,  stem. 

or  three,  articulate  with  a  basal  skeletal  piece.  The  double-bladed 
(didaetyle)  pedicellariae  are  either  straight  (forfieiform)  or  crossed 
(foreipiform).  Both  kinds  may  be  found  in  one  and  the  same 
animal. 

We  select  for  description  Asterias  (glacialis),  one  of  the  Asteroids 
most  richly  provided  with  pedicellariae,  whose  arrangement  is  specially 
interesting. 

A.  glacialis  has  three  kinds  of  pedicellariae,  straight,  crossed,  and 
three-bladed. 

The  crossed  pedicellarise  are  found  in  very  great  numbers,  thickly  crowded 
together  on  a  soft  cushion,  which  surrounds  the  base  of  the  spines,  and  into  which  the 
latter  can  be  withdrawn  (Fig;  344). 


396 


COMPARATIVE  ANATOMY 


CHAP. 


The  straight  pedicellarite  are  far  less  numerous,  and  are  found  scattered  over  the 
integument  either  singly  or  in  groups. 

The  three -bladed  pedicel lari?e  are  always  found  entirely  isolated,  and  may  be 
altogether  wanting  in  some  individuals. 

Structure  of  the  straight  pedicellarise  (Fig.  342,  H).— Each  of  the  two  blades 
consists  of  a  hollow  toothed  skeletal  piece,  which  articulates  with  a  common  basal 


FIG.  343.— A  portion  of  an  arm  of  Asterias  stichantha,  Sladen,  from  the  lower  side  (after 
Sladen).  1,  2,  3,  4,  The  four  longitudinal  rows  of  ambulacral  feet ;  5,  forficiform  pedicellarht1 ;  0, 
adambulacral  spines  ;  7,  papulae ;  8,  inframarginal  spines ;  9,  forcipiform  pedicellarite  at  the 
outer  bases  of  these  latter. 

piece.  Two  muscles  serve  for  opening  the  pedicellaria,  the  outer  side  of  each  blade 
being  attached  by  a  muscle  to  the  basal  piece.  The  blades  are  closed  by  moans 
of  two  muscles  which  run  from  the  inner  sides  of  their  bases  to  the  basal  piece, 


i.  344.—  Asterias  (Stolasterias)  volsellata.—  Adambulacral  plates  and  neighbouring  portion 
f  the  oral  integument  of  an  arm.    /,  Straight ;  fc,  crossed  pedicellarite  on  a  cushion  at  the  base  of 
a  large  spine  (a<-) ;  m-lt  smaller  spine  (after  Sladen). 

and  also  perhaps   by  means  of  two  muscles  which,  lying   for   the   greater   part 
hin  the  calcareous  blades,  run  from  their  tips  to  the  basal  piece.     Each  pedi- 
211am  is  unrounded  by  a  layer  of  connective  tissue,  and  covered  by  body  epithelium, 
in  which  glandular  cells  are  scattered. 

Structure  of  the  crossed  pedicellariae  (Fig.  342,  K).— A  crossed  pedicellaria 
t  Unlike  a   forceps  with  short  handles.     It  also  consists  of  three  pieces,  the 


VIII 


ECHINODERMA  TA  —PEDICELLA  RI^ 


397 


two  limbs  of  the  forceps,  and  an  intermediate  or  basal  piece,  upoii  which  the 
blades  move.  Each  limb  of  the  forceps  consists  of  the  blade  and  the  stalk  or 
handle.  The  two  limbs  cross  at  the  two  sides  of  the  intermediate  piece  like  the  two 
parts  of  a  pair  of  pincers  or  scissors.  "When  the  two  handles  are  approximated,  the 
pincers  close,  when  they  are  drawn  apart,  they  open.  The  opening  and  closing  of 
the  pcdicellaria:  is  effected  by  means  of  six  muscles.  Two  small  muscles,  running 
from  the  outer  side  of  the  bases  of  the  blades  to  the  basal  piece,  by  their  contraction, 
open  the  forceps  ;  while  two  pairs  of  muscles  close  it.  One  of  these  pairs  runs 
within  the  blades  to  the  basal  piece,  and  the  two  muscles  of  the  other  pair  run 
transversely  from  the  handles  of  the  two  limbs  to  the  basal  or  intermediate  piece. 
An  axial  strand  of  elastic  fibres  run  from  the  stalk  of  the  pedicellaria  to  the  base 


FIG.  i>4 5.— Pedicellariae  of  Echinoids.  A,  Four-bladed  pedicellaria  of  Schizaster  canali- 
fenis  (after  KceMer).  B,  Glandular  pedicellaria  with  glandular  sacs  on  the  stalk, 
Sphaerechinus  granularis.  C,  Longitudinal  section  through  a  decalcified  tridactyle 
pedicellaria  of  Centrostephanus  longispinus  (after  Hamann).  1,  Adductor  muscle  ;  2,  nerve  ; 
3.  elastic  column  ;  4,  calcareous  rod  ;  5,  longitudinal  muscle  fibres. 

of  the  forceps.  This  strand  divides  into  two  branches,  which  embrace  its  handles. 
The  fibrous  strands  of  the  individual  pedicellarise  penetrate  the  cushion,  which  sur- 
rounds the  base  of  the  spine  (Fig.  344),  and  finally  break  up  into  fibres  which  become 
closely  matted  together.  The  whole  cushion  consists  of  thickly  interwoven  fibres  of 
connective  tissue  and  muscle.  Muscle  fibres  run  down  from  the  calcareous  piece  of 
the  spine  into  the  cushion,  in  which  they  are  lost.  By  means  of  these  muscles,  the 
cushion  can  be  drawn  up  the  spine,  like  a  sort  of  sheath.  The  pedicellarife,  like 
the  cushion  from  which  they  rise,  are  covered  by  a  markedly  glandular  epithelium. 

The  three-bladed  pedicellariae,  apart  from  the  number  of  their  blades,  agree  in 
structure  with  the  straight  two-bladed  pedicellariae. 

3.  The  pedicellariae  of  the  Echinoidea  (Figs.  345  and  346). — Pedicellarise  occur 
in  all  Echinoids  on  the  integument,  between  the  spines,  and  in  one  and  the  same 
species  two  or  more  forms  of  them  may  be  found.  The  special  arrangement  of  the 
various  forms  of  pedicellarife  on  the  body  (whether  occurring  on  the  ambulacral  or 
on  the  interambulacral  areas,  and  whether  orally  or  apically),  their  distribution, 
number,  and  very  varied  form  cannot  here  be  described  in  detail,  but  must  be 
sought  for  in  systematic  works. 


398 


COMPARATIVE  ANATOMY 


CHAP. 


The  pedicellari«  of  the  Echinoidea  are  always  stalked,  and  three-,  less  frequently 
two-   or  four-,  bladed.     Two  principal  forms  may  be  distinguished  :   seizing  pedi- 
cellarise (Fig.  345  A  and  C)  and  glandular  pedi 
cellarise  (Fig.  345  B  and  Fig.  346). 

a.  The  seizing  pedicellarise.— The  form  of  the 
blades  varies  greatly  in  details.  They  are  some- 
times long  and  slender  (p.  tridactyke,  tetradactylfe), 
sometimes  spoon-shaped  and  toothed  (p.  ophioce- 
phake,  seu  buccales,  seu  triphyllae),  or  in  other 
cases  broadened  out  like  leaves  (p.  trifoliatse). 
Each  blade  is  always '.  supported  by  a  skeletal 
piece,  which  determines  its  general  shape  and  the 
special  form  of  its  teeth,  hooks,  etc.  The  stalk 
also  is  always  supported  by  an  axial  calcareous 
rod,  which  sometimes  penetrates  the  whole  of  its 
basal  half  (p.  tridactylae),  sometimes  only  reaches 
a  short  way  into  the  base  of  the  stalk. 

The  tridactyle  pedicellarise  of  Centroste- 
phanus  longispinus  (Fig.  345  C)  will  serve  to 
illustrate  the  structure  of  the  seizing  pedicellarie. 
The  three  slender  blades  are  connected  at  their 
bases,  and  on  the  sides  turned  to  the  axis  of  the 
whole  forceps,  by  three  transverse  adductor 
muscles,  each  of  which  is  attached  on  the  inner 
(axial)  sides  of  two  neighbouring  blades.  The 
three  muscles  together  form  a  triangle.  These 
adductor  muscles  are  counteracted  by  opening 
muscles,  which  run  down  on  the  outer  sides  of  the 
bases  of  the  blades  longitudinally.  A  nerve  enters 
each  blade,  running  towards  its  tip,  and  innervat- 
ing the  musculature  and  epithelial  sensory  cells. 
The  inner  surface  of  each  blade  is  ciliated.  Within 
the  stalk,  the  supporting  calcareous  rod  reaches 
only  half  way  up,  ending  in  a  knob.  The  con- 
tinuation of  the  calcareous  rod  is  formed  by  an 
elastic  pillar,  which  consists  of  modified  connective 
tissue,  and  is  enveloped  in  a  sheath  of  longitudinal 
muscle  fibres.  This  arrangement  makes  it  possible 
for  the  distal  portion  of  the  stalk  with  the  head 
to  bend  in  various  directions,  and  even  to  bend 
right  back  upon  the  basal  portion.  When  the 

forceps  gland ;  10  and  11,  opening    muscles  which   bring   about  such  movement  are 
muscles ;  12,  nerve ;  13,  calcareous  rod    relaxed,    the    distal    part    resumes    the    upright 

position     by    means    of    the     elastic     pillar     it 
contains. 

The  adductor  muscles  of  these  pedicellarise 
consist  of  transversely  striated  muscle  fibres ; 
consequently  Uiese  tridactyle  pedicellarise  are  very  active  seizing  organs. 

b.  The  glandular  pedicellariae  have  been  carefully  investigated,  up  to  the  present 
time,  only  in  a  small  number  of  Echinoids  (Sphcerechinus  granularis,  Echinus  acutus, 
E.  melo,  Dorocidaris  papillata,  Strongylocentrotus  lividus,  Echinocardium  flavesccns], 
but  it  is  probable  that  in  time  they  will  be  found  more  widely  disputed.  In 
general  structure  they  resemble  the  ordinary  seizing  pedicellarise  possessing  three 


340.— Organisation  of  a  glan- 
dular pedicellaria  of  Sphaerechinus 
granularis,  section.  1,  Distal  tactile 
prominence ;  2,  aperture  of  the  gland 
«'C  the  forceps ;  3,  proximal  tactile 
lironiinence ;  4,  adductor  muscle ; 
•>,  skeletal  piece  of  the  forceps ;  6,  epi- 


of  the  same ;  9,  muscle  layer  of  the 


in  the  stalk  ;  14,  aperture  of  the  stalk 
gland  (16) ;  15,  epithelium  of  the  gland. 
(The  distal  tactile  prominence  here 
represented  is  wanting  in  this  species.) 


vni  ECHINODEBMA  TA— PEDICELLARIA  399 

seizing  blades,  which  are  opened  and  closed  by  special  muscles,  but  the  fibres  of  the 
adductor  muscles  are  not  transversely  striated.  In  the  stalk,  the  axial  calcareous  rod 
is  continued  as  far  as  the  three-bladed  head,  an  arrangement  which  greatly  decreases 
the  mobility  of  this  kind  of  pedicellaria. 

The  most  distinctive  characteristic  of  these  pedicellariae,  however,  is  the 
presence  of  a  large  glandular  sac  in  each  blade.  This  glandular  sac,  which,  as  is 
shown  by  recent  discoveries,  consists  of  two  fused  sacs,  causes  each  blade  to  be  pear- 
shaped.  It  is  covered  by  a  thick  glandular  epithelium,  and  has  a  muscular  wall  of 
its  own,  in  which  the  (smooth)  fibres  run  in  circular  layers.  This  muscular  wall,  no 
doubt,  serves  for  pressing  out  the  slimy,  and  probably  poisonous,  secretion,  through 
the  aperture  which  lies  near  the  tip  of  the  blade.  This  aperture  appears  in  most 
cases  to  lie  on  the  outer  side  of  the  blade. 

At  the  base  of  each  blade,  on  its  inner  side,  the  epithelium  is  thickened  to  form 
a  tactile  prominence  or  cushion,  which  (besides  cilia)  carries  immovable  sensory 
hairs.  In  Echinus  acutus,  besides  the  basal,  or  lower,  tactile  prominence,  there  is, 
on  each  blade,  a  distal  or  upper  prominence,  which  also  lies  on  the  inner  side  of  the 
blade  (Fig.  346). 

Numerous  nerves  pass  from  the  stalk  of  the  pedicellaria  into  its  head  and  its 
blades,  so  as  to  innervate  the  muscular  and  the  sensory  cells. 

In  a  few  Echinoids,  glands  also  occur  on  the  stalk  of  the  pedicellaria  ;  such 
glands  are  specially  strongly  developed  in  Sphcerechinus  granularis.  These 
glands,  three  in  number,  encircle  the  stalk  of  the  glandular  pedicellariae  (p.  gemmi- 
forrnes),  about  half  way  up.  Each  gland  is  a  large  vesicle  with  an  aperture,  through 
which,  on  stimulation,  a  slimy  secretion  is  discharged.  The  wall  of  the  vesicle  con- 
sists of  glandular  epithelium  within  a  muscular  layer.  The  three  glands  cause  large 
vesicular  swellings  on  the  stalk  of  the  pedicellariae  on  which  they  occur  ;  they  are 
covered  by  unditterentiated  outer  body  epithelium. 

If  we  imagine  that,  in  such  pedicellariae  provided  with  stalk  glands,  the  distal 
portion  of  the  stalk  above  these  glands  degenerated,  or  was  no  more  developed,  we 
should  have  the  form  of  pedicellaria  which  is  called  p.  globifer.  Such  globifers, 
occasionally  still  provided  with  rudimentary  seizing  forceps,  have  been  discovered  in 
C-  li'rostephan't'.s  lonrjispitius  and  ,V///'" -'"' ''-'hinus  granularis,  side  by  side  with  ordinary 
pedicellarise.  They  are  capable  of  pendulous  movements. 

The  function  of  the  pedicellarise  has  not  yet  been  satisfactorily  decided.     The  Xj 
view  that,   in  Echinoids,  they  play  some  part  in  locomotion,  has  recently  been  ' 
decidedly  opposed,  and  it  has  been  maintained  that  Echinoids  move  exclusively  by 
means  of  their  ambulacral  feet  and  spines.      It  has  further  been  asserted  that  the 
pedicellarue  lay  hold  of  foreign  objects,  algse,  etc.,  and  hold  them  fast  on  the  upper 
side  of  the  body  in  order  to  hide  it,  but  this  view  also  has  been  opposed,  the  function 
therein  ascribed  to  pedicellariae  being  claimed  for  the  tube-feet.    Such  a  function  could, 
in  any  case,  only  be  accessory.     Another  view  is  that  the  pedicellarise  serve  for  the"\ 
holding  of  prey,  and  for  carrying  it  to 'the  mouth.     In  the  Asteroidea,  however,/ 
which  take  the  food  in  large  pieces  (Fish,  Crabs,  Mussels,  Snails,  Echinoids,  etc.), 
they  could  not  well  play  this  part. 

The  most  probable  view  is  that  the  pedicellariae  are  protective  organs,  and  fulfil  \ 
the  function  of  cleaning  the  spine-covered  body.  They  clear  away  foreign  bodies. 
Small  animals  which  come  into  contact  with  the  body  are  seized,  and  enveloped  in 
the  slimy  secretion  of  the  epithelium,  or  in  the  possibly  poisonous  secretion  of  the 
specialised  glands  of  the  pedicellariae,  and  held  until  they  are  dead,  and  then 
"  thrown  overboard."  In  this  way  Echinoids  and  Asteroids  may  protect  themselves 
from  animal  and  vegetable  growths,  parasitic  or  otherwise.  This  would  explain  the 
astonishing  cleanness  of  most  members  of  this  group  in  spite  of  their  spinous 
covering. 


400  COMPARATIVE  ANATOMY 

E.  The  Masticatory  Apparatus  of  the  Eehinoidea. 
(Aristotle's  Lantern.) 

In  all  Eehinoidea,  with  the  exception  of  the  Spatangoida,  and  per- 
haps of  a  few  Holectypoida,  the  mouth,  which  lies  at  the  centre  of  the 
peristomal  area,  is  armed  with  five  hard  and  pointed,  interradially 
arranged  teeth.  These  teeth  are  approximated  or  moved  apart  by 
means  of  a  complicated  masticatory  or  jaw  apparatus  lying  within 
the  test,  and  resting  on  the  peristome.  This  apparatus  is  known  as 
the  lantern  of  Aristotle,  and  is  of  considerable  size ;  it  is  covered  on 
all  sides  by  a  closely  applied  integument,  the  lantern  membrane,  a 
continuation  of  the  peritoneum.  The  spaces  within  the  masticatory 
apparatus  are  completely  separated  by  this  membrane  from  the  spacious 
body  cavity  within  the  test. 

The  masticatory  apparatus  resembles  a  pentagonal  pyramid,  the 
base  of  which  is  directed  up  wards,  i.e.  projects  into  the  cavity  of  the 
test,  while  the  tip,  formed  by  the  five  teeth,  lies  in  the  mouth.  Its 
axis  is  traversed  by  the  oesophagus.  It  consists  essentially  of  skeletal 
pieces,  muscles,  and  ligaments. 

a.  The  skeleton  of  the  masticatory  apparatus  (Fig.  347)  is 
composed  of  twenty-five  pieces  (including  the  teeth),  radially  grouped 
around  the  oesophagus ;  some  of  these  pieces  have  received  very  un- 
suitable names.  There  are  five  teeth,  five  pairs  of  jaws  (alveoli), 
five  "sickles"  (falces),  and  five  radii  or  rotulse.  The  "sickles" 
may  be  named  intermediate  plates,  and  each  pair  of  jaws  forms  a 
"  pyramid." 

The  principal  part  of  the  framework  of  the  masticatory  apparatus  is  formed  by 
the  five  interradially  placed  pairs  of  jaws.  These  determine  the  conical  or  pyramidal 
form  of  the  whole  framework. ,  The  two  pieces  of  each  pair  are  firmly  connected  with 
one  another  on  the  outer  side  of  the  framework  by  a  vertical  interradial  suture,  and 
together  form  a  hollow  triangular  pyramid,  the  fifth  part  of  the  whole  pyramidal 
framework.  Each  single  pyramid  thus  has  one  outer  and  two  lateral  surfaces.  The 
five  single  pyramids  are  in  contact  with  one  another  along  these  lateral  surfaces, 
which  lie  radially  to  the  axis  of  the  whole  framework.  The  edges  along  which  the 
lateral  surfaces  come  in  contact  are  the  axial  edges,  i.e.  those  turned  towards  the  oeso- 
phagus. The  suture,  which  divides  each  single  pyramid  into  two  halves  or  jaws,  runs 
down  the  outer  surface,  exactly  halving  it.  The  walls  of  each  single  hollow  pyramid 
are,  however,  not  complete  :  (1)  the  two  lateral  surfaces  do  not  quite  meet  along 
their  inner  edges,  but  there  is  a  slit  left  between  them  (Fig.  347,  E)  ;  (2)  the  basal 
wall  (that  turned  upward)  is  wanting  ;  when  the  soft  parts  are  removed  an  aperture 
is  found  4iere,  the  foramen  basale,  which  leads  down  into  the  cavity  of  the  pyramid  ; 
(3)  a  large  incision  (foramen  externum)  is  found,  at  the  base  of  the  outer  wall,  and 
is  either  confluent  with  the  foramen  basale  or  is  separated  from  the  latter  by  an 
arch,  the  arcus. 

-in-1,-  pyramids  (or  pairs  of  jaws)  are  the  supports  and  carriers  of  the  teeth. 

ach  tooth  is  a  long,  slender,  and  hard  skeletal  piece,  curved,  so  that  its  convex  side 

«  outwards-;  it  traverses  the  cavity  of  the  pyramid,  and  projects  beyond  it  at  both 


VIII 


ECHINODERMATA—  ARISTOTLE'S  LANTERN 


401 


ends.  The  lower  end,  which  projects  beyond  the  tip  of  the  pyramid,  is  short  and 
pointed,  and  forms  the  externally  visible  part  of  the  tooth  lying  in  the  mouth.  The 
upper  end,  which  is  directed  aborally,  is  called  the  root  of  the  tooth,  and  projects 
considerably  beyond  the  foramen  basale,  it  is  usually  coiled  inwards  (towards  the 
axis  of  the  masticatory  framework).  The  growth  of  the  tooth  no  doubt  takes  place 
principally  at  this  root  end.  On  its  inner  side,  the  tooth  usually  has  a  longitudinal 
ridge,  the  carina,  and  on  its  outer  side  is  firmly  attached  to  the  outer  wall  of  the 


FIG.  347. — Masticatory  apparatus  of  an  Echinus,  original.  A,  In  profile.  B  From  the  apically 
directed  basal  side.  C,  External  view  of  a  single  pyramid.  D,  Side  view  of  the  same.  E,  Internal 
riew  of  the  same.  F,  Tooth.  1,  Arcus  ;  2,  intermediate  plate  ;  3,  freely  projecting  portion 
of  the  teeth  ;  4,  median  portion  of  a  tooth  ;  5,  upper  portion  of  the  same ;  6,  the  limbs  of  a  forked 
radius  (7)  ;  8,  single  pyramid  in  situ. 

pyramid  which  it  traverses,  in  such  a  way  that  it  cannot  move  by  itself,  but  only 
with  its  pyramid. 

The  fine  structure  of  the  teeth  differs  essentially  from  that  of  the  other  skeletal 
pieces  of  the  body  (cf.  on  this  subject  the  special  treatises  mentioned  in  the  Biblio- 
graphy). 

The  intermediate  plates  are  five  more  or  less  flat,  oblong,  skeletal  masses  lying 
on  the  base  of  the  masticatory  apparatus,  like  the  spokes  of  a  wheel  round  its  central 
axis.  Each  of  these  intermediate  plates  rests  on  the  bases  of  the  two  contiguous 
lateral  walls  of  two  pyramids  [or  pairs  of  jaws],  and  therefore  between  two  foramina 
basalia. 

Finally,  lying  apically  upon  these  falces,  are  the  five  forked  radii,  which  are  also 
arranged  like  the  spokes  of  a  wheel.  Each  radius  consists  of  a  slender  central 
stalk,  and  of  two  peripheral  diverging  prongs,  and  each  is  bent  downwards  in  such  a 
VOL.  II  2  D 


402 


COMPARATIVE  ANATOMY 


way  that  its  prongs  point  down  towards  the  peristome  over  the  edge  of  the  base  of 


of  the  masticatory  apparatus  (Fig.  848,.-We 
here'refer  back  to  what  has  been  said  of  the  perignathous  apophysial  ring,  for  the 
±^™^  and  the  apophysial  ring  are,  physiologieally,  closely  connected. 
Th  most  Important  muscles  and  bands  of  the  masticatory  apparatus  connect  its 
component  pieces  with  the  apophysial  ring,  and  the  latter  must  be  regarded  merely 
as  a  folding  inwards  of  the  edge  of  the  peristome,  which  has  come  into  existence 


Fin.  348.— Masticatory  apparatus  of  an  Echinoid  (Toxopneustes)  in  its  natural  position  at  the 
centre  of  the  oral  side  of  the  shell,  which  has  been  broken  off,  original.  1,  Root  of  the  tooth  ;  2, 
intestine ;  3,  accessory  intestine  (?)  ;  4,  axial  sinus  with  stone  canal ;  5,  forked  radii ;  6,  arcus  of  a 
single  pyramid  ;  7,  muscles  of  the  forked  radii ;  8,  perignathous  apophysis  (auricula) ;  9,  ligaments  of 
the  forked  radii ;  10,  adductor  muscles  of  the  teeth  ;  11,  opening  muscles  of  the  teeth ;  12,  radial 
canal  of  the  water  vascular  system  ;  13,  ampullae ;  im,  interambulacrum  ;  am,  ambulacrum.  The 
delicate  transparent  lantern  membrane  which  covers  the  whole  of  the  masticatory  apparatus  is 
not  represented. 

for  the  insertion  of  the  masticatory  muscles.  The  two  apparatus  are  either  absent  or 
present  simultaneously. 

Round  the  masticatory  apparatus,  ten  thin  ligaments  (9)  connect  the  forked  radii 
with  the  interradial  apophyses  of  the  perignathous  girdle.  The  two  bands  which 
belong  to  each  fork  are  attached  to  the  prongs,  and  continue  their  lines  down- 
wards to  the  apophysial  girdle  ;  they  are  inserted  into  the  two  neighbouring  inter- 
radial  apophyses  near  the  interradial  sutures. 

The  two  bands  proceeding  from  each  radial  fork  thus  diverge  downwards,  and 
the  two  proceeding  from  each  interradial  apophysis  of  the  perignathous  ring  diverge 
upwards. 

These  bands  appear  merely  to  serve  for  the  attachment  of  the  masticatory  appa- 


viir  ECHINODERMATA— CALCAREOUS  RING  403 


ratus,  and  for  its  maintenance  in  the  upright  position  over  the  oral  area.  Further 
investigations,  however,  must  decide  whether  the  bands  consist  only  of  elastic  fibres, 
or  whether  muscle  fibres  also  occur  in  them. 

The  adductor  muscles  of  the  teeth  (musculi  adductores  dentium,  10).— These 
are  present  in  five  interradial  pairs  ;  they  are  strongly  developed  as  broad  bands. 
The  two  muscles  of  a  pair  are  attached,  above,  along  the  outer  edge  of  the  arcus  of  the 
pair  of  jaws  (pyramid)  to  which  they  belong  ;  and  below,  along  nearly  the  whole 
length  of  the  corresponding  interambulacral  apophysis  of  the  perignathous  ring.  If 
these  muscles  contract,  the  upper  ends  of  the  pair  of  jaws  (or  pyramids)  are  drawn 
outwards  and  downwards,  forcing  the  lower  ends,  with  the  teeth,  inwards,  i.e. 
towards  the  centre  of  the  mouth.  In  other  words,  the  externally  visible  pointed 
lower  ends  of  the  teeth  are  pressed  together. 

The  opening  muscles  of  the  teeth  (musculi  abductores  dentium  sive  dilatatores 
oris,  11). — These  are  five  radially  arranged  pairs  of  muscles,  which  run  horizontally. 
The  two  muscles  of  each  pair  are  attached  on  the  one  side  to  the  inner  surface  of  the 
ambulacral  apophyses  (auriculae),  and,  on  the  other,  to  the  halves  of  the  jaws  nearest 
them,  close  to  the  ends  which  point  downwards.  These  muscles  counteract  the 
adductor  muscles  ;  when  they  contract,  the  lower  ends  of  the  five  pairs  of  jaws,  and 
with  them  the  tips  of  the  teeth,  are  moved,  centrifugally,  towards  the  auriculae.  The 
teeth  move  .apart,  and  the  mouth  opens. 

The  intermediate  jaw  muscles  (musculi  intermaxillares)  connect  the  apposed 
lateral  surfaces  of  the  five  pyramids  with  one  another.  The  five  pyramids  close 
firmly  together,  when  these  muscles,  which  together  act  like  a  kind  of  sphincter, 
contract. 

The  muscles  of  the  forked  radii  (7)  lie  on  the  upturned  base  of  the  masticatory 
apparatus,  forming  together  a  pentagonal  ring  by  connecting  the  five  handles  of  the 
forks  for  about  half  their  length.  As  to  the  function  of  these  muscles,  we  can 
only  imagine  that  they  depress  the  whole  masticatory  apparatus  by  their  contraction, 
and  thus  cause  the  oral  integument  to  project  conically,  especially  if  the  adductor 
muscles  of  the  teeth  contract  at  the  same  time.  It  is  well  known  that  Echiuoids  are 
assisted  in  locomotion  by  the  bulging  forward  of  the  tooth -carry ing  portion  of  the 
oral  area,  which  is  supported  by  the  masticatory  apparatus. 

In  the  Clypeastroida,  the  frequently  asymmetrical  masticatory  apparatus  is  more 
or  less  flattened,  usually  indeed  quite  flat.  The  teeth  are  not  vertical,  but  slope 
towards  one  another  quite  obliquely,  or  are  even  arranged  horizontally.  The  radii 
are  wanting,  and  the  intermediate  plates  are  rudimentary. 


r 


F.    The  Calcareous  Ring  of  the  Holothurioidea. 


In  the  Holothurioidea,  the  oesophagus  is  surrounded  by  a  circle  of 
ten  calcareous  skeletal  pieces  (Fig.  349,  3  and  13),  five  of  which  are 
radial  and  the  other  five  interradial.  This  calcareous  ring  protects 
the  nerve  ring  at  its  inner  side.  For  a  certain  distance  it  supports 
the  radial  water  vascular  trunks  and  the  tentacular  vessels,  and  may 
indeed  be  regarded  as  the  inner  skeleton  of  the  oral  region  of  the 
body.  The  five  longitudinal  muscles  or  pairs  of  muscles  of  the  body, 
and,  where  such  are  present,  the  five  retractor  muscles  of  the  oral 
region,  are  attached  to  this  ring,  i.e.  to  its  radial  portions.  The 
calcareous  ring  is  altogether  wanting  in  the  remarkable  free-swimming 
form  Pelagothnria  (Fig.  224,  p.  286). 


COMPARATIVE  ANATOMY 


CHAP. 


404 

The  form  and  size  of  the  calcareous  ring  and  its  separate  parts  vary  greatly.  The 
radials  are  often  lengthened  backwards  (apieally)  into  two  prongs  of  varying  length, 
between  which  the  radial  water  vascular  trunks  run. 

It  not  infrequently  happens  that  the  separate  parts  become  partially  or  altogether 
broken  up  into  single  pieces,  which  are  connected  together  like  a  mosaic. 

The  number  of  pieces  in  the  ring  may  increase  or  decrease.  Where  there  are 
more  or  fewer  than  ten  pieces,  it  is  always  the  interradials  which  either  increase  or 

diminish  in  number.  This  is  comprehen- 
sible when  we  remember  that  the  longi- 
tudinal muscles  of  the  body  are  attached 
to  the  radials. 

The  iriterradial  portions  are  wanting  in 
species  of  the  genera  Phylloporus,  Cucum- 
aria,  and  Trochostoma,  and  in  many  Elasi- 
poda,  especially  in  the  whole  family  of  the 
Elpidiidae. 

More  than  ten  pieces  are  found  in  many 
Synaptidce,  viz.  in  nearly  all  those  forms 
which  possess  more  than  ten  tentacles. 
The  number  of  extra  interradials  then 
usually  corresponds  with  that  of  the  super- 
numerary tentacles. 

Six  -  rayed  specimens]  of  Cucumaria 
Planci  have  been  described,  whose  cal- 
careous ring  consists  of  six  radials  and  six 
interradials. 

The  ring  which  is  originally  radiate  may 
become  bilaterally  symmetrical.    Its  plane 
of  symmetry  then  agrees  with  the  general 
plane  of  symmetry  of  the  body,  and  passes 
Fio.  34','.—  The  oesophagus  and  half  the    through  the  fifth  interradius  (the  so-called 


oral  tentacles  of  a  dendrochirote  Holo- 
thurian  (after  Lud-wig).  1,  Genital  aperture  ; 
•J,  genital  duct ;  3,  radial  pieces  of  the  cal- 
careous ring;  4,  retractor  muscles ;  Sjjnadre- 
jporite ;  6,  stone  canal ;  7,  dorsal  mesentery  ; 


dorsal  interradius  in  which  the  genital 
aperture  lies)  and  the  central  (first)  radius 
of  the  ventral  side.  The  symmetry  is 
determined  either  by  the  fact  that  the 

9,  Polian  vesicles;  10,  circular    portions  of  the  ring  on  the  ventral   side 

canal  ;,11,  continuation  of  the  radial  calcareous    differ  in  f  ^  and  manner  of  connec- 

pieces ;    12,   proximal  portions  of  the  radial    ,.        f          ,  -,  , ,       -,         ,     .  -,  -, 

canals  of  the  water  vascular  system;  13,  inter-  tlon  frOm  those  on  the  dorsal  side,  or  else 
radial  pieces  of  the  calcareous  ring  ;  14,  one  of  by  the  presence  of  a  larger  number  of  such 
the  two  small  ventral  tentacles.  portions,  in  consequence  of  an  increased 

number  of  interradials  in  definite  sym- 
metrical interradii.  For  instance,  Synapta  digitata  has  seven  interradials,  one  each 
in  the  mediodorsal  and  in  the  two  ventral  interradii,  and  two  each  in  the  dorso- 
lateral  interradii. 

The  portions  of  the  calcareous  ring  are  more  or  less  closely  united  together 
by  means  of  connective  tissue  (never  by  means  of  muscles)  ;  in  some  cases  they  are 
firmly  fused  together. 

Structures  corresponding  to  the  calcareous  ring  of  the  Holothurioidea  have  long 
been  sought  for  in  the  other  classes  of  the  Echinodermata.  It  was  thought  that 
in  the  Echinoidea  it  might  perhaps  be  represented  either  by  the  teeth  or  by  the 
perignathous  apophysial  ring,  or  in  certain  parts  of  the  masticatory  apparatus. 

The  horaology  of  the  teeth  of  the  Echinoidea  with  the  calcareous  ring  of  the 


viii  ECHINODERMATA—ENDOSKELETON,  ETC.  405 

Holothurioidea  is  no  longer  maintained.  The  two  structures  are  altogether  differently 
related  to  the  nervous  and  water  vascular  systems. 

The  hornology  of  the  calcareous  ring  with  the  perignathous  apophysial  ring  of 
the  Echinoidea  is  equally  doubtful.  The  radials  of  the  calcareous  ring  were  in  this 
case  compared  with  the  auriculae  (ambulacral  apophyses).  But  each  auricule  is 
paired  and  consists  of  two  processes/or  folds,  of  the  edge  of  the  peristome,  which 
may  or  may  not  be  connected  together  by  an  arch  ;  the  radials,  however,  are 
from  the  first  unpaired.  Only  the  arches  of  the  auricula?  could  be  compared  with 
the  radials  ;  the  arch,  however,  is  not  a  single  plate,  but  is  formed  by  apposition 
of  the  two  neighbouring  ambulacral  apophyses  of  one  and  the  same  ambulacrum. 

The  comparison  of  the  calcareous  ring  with  the  masticatory  apparatus  or 
Aristotle's  lantern  of  the  Echinoidea  still  remains.  The  five  radials  have  been 
compared  with  the  five  fork  pieces,  and  the  five  iuterradials  with  the  five  arches 
of  the  pairs  of  jaws  (pyramids)  of  the  lantern.  This  comparison  is  in  many  ways 
plausible,  but  here,  as  before,  many  difficulties  appear  when  the  subject  is  carefully 
investigated.  The  arches  of  the  jaws  are  paired  structures,  and  cannot  therefore  be 
compared  with  the  iuterradials,  which  are  from  the  first  unpaired.  Moreover,  it 
is  very  doubtful  whether  they  represent  independent  skeletal  pieces  ;  they  appear 
rather  to  be  merely  muscular  processes  of  the  halves  of  the  jaws.  Further,  the 
sinews  which  proceed  from  the  forks  of  the  radial  fork  pieces  are  attached  to  the 
perignathous  apophysial  ring  interradially  (i.e.  to  the  interambulacral  apophyses), 
while  the  muscles  which  are  attached  to  the  radials  of  the  calcareous  ring  of  the 
Holothurioidea  run  strictly  radially. 

G.    Further  Deposits  of  Calcareous  Matter. 

Deposits  of  calcareous  corpuscles  and  masses  may  occur  in  the 
connective  tissue  of  the  walls  of  various  internal  and  external  organs, 
especially  in  the  ambulacral  and  alimentary  systems.  These  will  be 
considered  in  connection  with  the  systems  to  which  they  belong. 

We  shall  here  only  mention  certain  calcareous  deposits  in  the 
Cl-ypeastroida.  An  endoskeleton  is  here  formed.  On  the  oral,  as  well 
as  on  the  apical,  inner  surface  of  the  test,  needles,  pillars,  lamellae, 
etc.  rise,  sometimes  only  at  the  edge,  sometimes  over  large  areas. 
These  may  traverse  the  whole  depth  of  the  test,  connecting  its  oppo- 
site walls.  They  more  or  less  completely  separate  the  ambulacral 
structures  from  the  other  internal  organs,  such  as  the  intestine,  the 
genital  organs,  etc.,  and  may  in  some  cases  attain  such  great  develop- 
ment that,  as  in  Encope,  they  form  a  sponge-like  or  cellular  calcareous 
framework  throughout  the  whole  interior  of  the  test,  in  which  larger 
spaces  are  left  for  the  masticatory  apparatus,  the  intestine,  the 
ambulacra,  etc.  Xot  infrequently,  the  ambulacral  vessels  are  completely 
vaulted  over  b}r  deposits  of  calcareous  matter. 

H.    Concluding  Remarks  on  the  Section  on  the  Skeletal  System. 

In  the  above  section,  I  have  adopted  the  views  of  those  investigators  whose  wide 
and  for  the  most  part  difficult  researches  have  convinced  them  that  at  least  the 
plates 'of  the  apical  and  oral  systems  (the  central,  infrabasals,  basals,  radials,  and 
orals)  are  homologous  throughout  the  whole  group  of  the  Echinodermata.  These 
plates  therefore  must  be  ascribed  to  the  common  racial  form.  But  these  plates  are 


406  COMPARATIVE  ANATOMY  CHAP. 

in  reality  only  characterised  according  to  their  position  in  the  adult  animal,  whether 
radial  or  interradial,  apical  or  oral,  and  according  to  the  place  of  their  first  appear- 
ance (above  one  or  the  other  ccelomic  vesicle).  They  have  no  other  distinctive 
characteristic  by  which,  for  example,  a  radial  could  be  recognised  throughout  the 
class  of  the  Echinodermata.  It  is  therefore  still  possible  that  such  correspondence 
may  be  merely  superficial,  merely  the  expression  of  the  radiate  structure  so  common 
among  Echinoderms.  There  is  nothing  astonishing  in  the  fact  that  the  skeleton 
of  a  radiate  animal  commences  at  the  poles  either  with  radially  or  with  interradially 
arranged  plates.  Such  correspondence,  then,  as  far  as  it  goes,  is  described  as  homo- 
logy.  But  nothing  is  really  gained  by  insisting  that  such  and  such  Ophiurids 
"  possess  infrabasals,"  because  the  system  of  plates  commences  at  the  apex  with  five 
radial  plates,  which  are  followed  by  another  outer  row  of  radial  plates.  Is  it,  after 
all,  certain  that  the  infrabasals  are  wanting  when  the  skeletal  system  at  the  apex 
begins  with  interradial  plates  (which  are  on  that  account  called  basals)  ? 

III.  The  Outer  Morphology  of  the  Holothurioidea. 

The  Holothurioidea  form  an  exception  to  the  rule  which  applies  to  all  other 
Echinoderms  that  the  outer  form  of  the  body  is  accurately  reproduced  in  the  test  of 
skeletal  plates.  This  test  gives  us,  as  a  rule,  exact  information  as  to  the  position 
of  the  outer  apertures  of  the  internal  organs,  and  as  to  the  relation  of  the  radii 
or  ambulacra,  to  the  interradii  or  interambulacra.  But  in  the  Holothurioidea,  in 
whose  integument  only  microscopically  small  and  isolated  calcareous  bodies  occur, 
this  is  not  the  case.  Having  treated  of  the  external  morphology  of  the  Echinoidea,  the 
Asteroidea,  the  Ophiuroidea,  and  the  Pelmatozoa  in  the  section  on  the  skeletal  system, 
we  must  now  give  some  account  of  the  outer  morphology  of  the  Holothurioidea. 

We  shall  begin  with  those  forms  in  which  the  body,  elongated  in  the 
direction  of  the  principal  axis,  is,  in  section,  circular  or  pentagonal  with 
rounded  edges  (cf.  for  example,  Cucumaria  Planci,  Fig.  226,  p.  287).  At 
the  oral  pole  of  the  principal  axis  (i.e.  in  the  Holothurioidea,  anteriorly) 
lies  the  mouth,  surrounded  by  feelers ;  at  the  opposite,  apical  (posterior) 
pole,  the  anus.  Along  the  body,  from  before  backward,  run  five  ridges, 
corresponding  with  the  radii,  and  causing  the  pentagonal  form  of  the 
transverse  section.  On  each  edge  there  are  two  longitudinal  rows  of 
tube-feet. 

Careful  examination  shows  that  the  radiate  structure  of  Cucumaria 
is  even  externally  disturbed  by  certain  characters  which  make  it  bi- 
laterally symmetrical.  There  is  only  one  genital  aperture,  at  the  oral 
margin  of  an  interradius,  which  we  will  arbitrarily  call  the  dorsal 
interradius.  Further,  of  the  ten  oral  feelers,  two  adjacent  feelers 
are  much  smaller  than  the  rest.  They  lie  exactly  opposite  the  genital 
aperture,  and  distinguish  the  middle  ventral  radius.  The  plane  which 
passes  through  the  dorsal  interradius  and  the  middle  ventral  radius,  in 
the  direction  of  the  principal  axis  (i.e.  longitudinally  through  the  body) 
is  the  plane  of  symmetry. 

If  the  animal  is  opened,  it  is  seen  that  this  external  symmetry 

corresponds  with   an   internal  symmetry  ;   the  anterior  limb  of  the 

A_intestme  is  attached  by  a  mesentery  to  the  body  wall  in  the  dorsal 

X   !  The  stone  canal  and  the  genital  glands  lie  in  the  dorsal 

interradius,  and  the  Polian  vesicle  in  the  middle  ventral  radius. 


vni  ECHINODERMATA— MORPHOLOGY  OF  HOLOTHURIOIDEA  407 

The  application  of  the  terms  "ventral"  and  "dorsal,"  in  very 
many  Holothurioidea,  is  fully  justifiable,  since  there  arises,  parallel 
to  the  principal  (longitudinal)  axis,  a  flattened  creeping  sole,  along 
the  middle  of  which  the  above-named  ventral  radius  runs,  while  the 
middle  of  the  vaulted  dorsal  surface  opposed  to  this  creeping  sole  is 
occupied  by  the  middle  dorsal  interradius. 

The  radii  and  interradii  now  become  arranged  in  such  a  way  that 
three  radii  (one  middle  and  two  lateral)  belong  with  their  ambulacral 


rdd 


Trimum 


FIG.  350.— Diagrammatic  section  illustrating  the  symmetry  of  the  Holothurioidea.  De- 
velopment of  the  bivium  and  the  trivium  (mainly  after  Ludwig).  imd,  Medio-dorsal  interradius  ; 
ids,  left  dorsal  ditto ;  isr,  left  ventral ;  idv,  right  ventral ;  idd,  right  dorsal  interradius ;  rds,  left 
dorsal  radius ;  rst;  left  ventral  ditto ;  rmv,  medio-ventral ;  rdv,  right  ventral ;  rdd,  right  dorsal 
radius ;  mi,  anterior  or  dorsal  mesentery ;  m^,  middle  or  left ;  7/13,  posterior  or  right  mesentery ; 
i i,  io,  13.  tirst.  second,  and  third,  or  anterior,  middle,  and  posterior  limbs  of  the  intestine  ;  vd  and 
vv,  dorsal  and  ventral  intestinal  vessels ;  bd  and  bs,  right  and  left  branchial  tree  (aquatic  lung) ; 
go,  gonad ;  dg,  genital  duct ;  a,  body  cavity. 

feet  to  the  creeping  sole,  and  form  the  trivium ;  while  on  the  dorsal 
surface  two  radii  (one  right  and  the  other  left)  form  the  bivium.  Two 
interradii,  on  the  other  hand,  belong  to  the  creeping  sole,  and  three 
to  the  dorsal  surface  (cf.  the  diagram,  Fig.  350). 

The  creeping  sole  usually  runs  along  over  the  whole  length  of  the  body,  less  fre- 
quently (Psolus,  Psolidium)  it  is  limited  to  a  circumscribed  region  between  the 
anterior  and  posterior  ends. 

The  difference  between  the  ventral  and  the  dorsal  side  (the  trivium  and  the 


408 


COMPARATIVE  ANATOMY 


CHAP. 


bivium)  becomes  still  more  accentuated  by  the  different  development  of  the  ambu- 
lacral  feet  in  the  two  regions.  On  the  ventral  side,  these  feet  are  altogether  or  princi- 
pally locomotory  tube-feet  (ending  in  suckers),  on  the  dorsal  side  (the  bivium)  they  are 
exclusively  or  chiefly  non-locomotory  papillae  (with  more  or  less  pointed  ends).  This 
difference  between  the  dorsal  and  ventral  tube-feet  is  found  both  in  those  forms  in 
which  the  ainbulacral  feet,  limited  to  the  radii,  are  arranged  in  one  or  more  longi- 
tudinal rows,  and  in  those  in  which  they  are  also  found  in  the  interradii  and  arranged 
irregularly. 

In  the  genus  Psolus,  the  distinction  between  dorsal  and  ventral,  and  consequently 
the  bilateral  symmetry  of  the  body,  becomes  still  more  marked  by  the  entire  absence 


TVS  •' 

rmv 

Fio  351  -Derivation  of  Bhopalodina  (A)  from  an  ordinary  Holothurian  (B)  (after  Ludwig). 
n.'elio?  In1ter?iedi?te  form'    **•  "»•  ™,  left  dorsal,  left  ventral,  medioventral  radius;  imd, 
.1  interradms  ;  o,  mouth  ;  an,  anus  ;  go,  genital  aperture  ;  ««,  water  vascular  ring. 

of  ambiilacral  appendages  on  the  bivium.     The  tube-feet  of  the  middle  ventral  radius 

o  be  wanting  in  some  species  of  this  genus. 

Where  ventral  and  dorsal  are  sharply  distinguished,  the  mouth  and  the  anus 
tend  to  shift  on  to  the  ventral  side. 


P 


genUS  Rh°Palodi™  (Fig-  351)  is  quite  peculiar. 

th  '  and  produced  into  a  lon§  stalk-  At  ^  end 

betwee    tti  ,°l!e    °  '"i  ™0t^  He  the  m°Uth  and  the  anus>  and 
odv  UP  f  ^al  apertUre'     °n  the  swolle11  P°rtion  ^  the 

I'    ^  fte^dou^.lonSitudinal  ^ws  of  ambulacral  feet,  so  that 

onlvfiv      PT  MT  -P°SSeSSeS  ten  radli'  Whereas  {i  ^  reality 
only  five      To  obtain  this  condition  we  have  to  imagine  (1) 

n    ?       ^  dejldrochirot«  Holothurian  bent  upward  an- 
b    t  Tl  y'  and  (2)  the  W^imation  of  the  anus  and 

by  the  great  shortening  of  the  dorsal  interradius.     The  accom- 


VIII 


EGHINODERMATA— RADIAL  ORGANS 


409 


panying  diagram  will  help  to  make  this  clear.  The  genus  Ypsilo- 
thurin  seems  to  have  become  fixed  while  in  the  act  of  being  similarly 
modified. 

In  the  genus  Psychropotes  (Fig.  223,  p.  285)  the  dorsal  surface  is 
prolonged  beyond  the  anus  into  a  long  caudal  appendage  directed 
posteriorly.  Peniagone  is  distinguished  by  an  anteriorly  inclined  comb, 
rising  transversely  from  the  neck.  On  the  swimming  disc  of  Pelago- 
thurio,  cf.  Figs.  224  and  225,  p.  286. 


IV.  Position  and  Arrangement  of  the  Most  Important  Organs 
in  the  Radii. 

The  position  and  arrangement  of  the  organs  in  the  radii  can  best 
be  explained    by  describing  cross -sections.      In    the  Asteroidea,  the 


. 

FIG.  352.— Transverse  Section  of  a  radial  region  of  the  body  wall  of  a  Holothurian, 
partly  diagrammatic.  1.  Eudothelinm  of  the  body  cavity ;  2,  circular  musculature  ;  3,  longitudinal 
musculature ;  4,  motor  nerve ;  5,  radial  water  vascular*  canal ;  6,  radial  blood  lacuna ;  7,  radial 
ridge  of  the  deeper  oral  nervous  system ;  S,  ampulla  ;  9,  cutis ;  10,  epidermis ;  11,  tube-foot  canal 
of  the  vascular  system ;  12,  tube-foot ;  13,  nerve  of  the  same ;  14,  vessel  of  the  same ;  15,  radial 
nerve  strand  of  the  superficial  oral  nervous  system  ;  16,  epineural  canal ;  17,  peripheral  nerve  ;  18, 
pseudohsemal  canal. 

Ojtl'iuroidea,  and  the  Crinoidea,  in  which  the  body  is  produced  radially 
into  arms,  the  sections  to  be  described  will  be  those  of  the  arms ;  in 
the  Holothurioidea  and  Echinoidea  the  sections  are  of  a  radial  region  of 
the  body  wall. 

Holothurioidea    (Fig.   352). — In  a   transverse  section  through  a 


410  COMPARATIVE  ANATOMY  CHAP. 

radial  region  of  the  body  wall  of  an  actinopodan  Holothmian  we  find, 
proceeding  from  without  inwards  :— 

(a)  The  outer  body  epithelium  (10). 

(b)  The  cutis,  or  the  connective  tissue  layer  of  the  body  wall,  wit 

the  calcareous  corpuscles  (9). 

(c)  The  epineural  canal  (16). 

(d)  The  radial  nerve  trunk  of  the  superficial  oral  system  ( 

(e)  The  radial  nerve  trunk  of  the  deeper  oral  system  (7). 
(/)  The  subneural  pseudohsemal  canal  (18). 

(a)  The  radial  blood  lacuna  (radial  blood  vessel)  (6). 


FIG.  353.— Transverse  Section  through  a  radial  region  of  the  body  wall  of  an  Echinoid,  partly 
diagrammatic.  1,  Ampulla,  traversed  by  muscle  filaments  ;  2  and  3,  the  two  canals  traversing  the 
test  and  connecting  the  ampulla  and  the  tube-foot  canal  (5) ;  4,  circular  nerve  in  the  terminal  disc 
of  the  tube-foot ;  5,  tube-foot  canal ;  6,  nerve  of  the  tube-foot ;  7,  integumental  nerve  ;  8,  calcareous 
substance  of  the  ambulacral  plate  ;  9,  nerve  plexus  in  the  depths  of  the  body  epithelium ;  10,  suture 
between  two  plates  of  the  two  contiguous  rows  of  ambulacral  plates ;  11,  body  epithelium ;  12, 
epineural  canal ;  13,  endothelium  of  the  body  cavity ;  14,  pseudohsemal  canal ;  15,  radial  blood 
vessel ;  16,  radial  canal  of  the  water  vascular  system  ;  17,  radial  nerve  strand  ;  18,  lateral  canal  of 
the  radial  canal  of  the  water  vascular  system  to  the  ampulla. 

(h)  The  radial  canal  of  the  water  vascular  system  (5),  and  the  tube- 
foot  canal  branching  from  it  transversely  (11),  and  finally 
also  the  ampulla  of  the  tube-foot  (8). 

(i)  The  circular  musculature  of  the  body  (2). 

(k)  The  longitudinal  musculature  of  the  body  (3). 

(1)  The  endothelium  of  the  body  cavity  (1). 

The  figure  also  illustrates  the  relation  of  a  tube-foot  to  its  canal 
and  ampulla. 

This  description  does  not  apply  to  the  Paradinopoda  (Synaptitto)  in 
so  far  as,  in  these  latter,  the  radial  canals  of  the  water  vascular  system 
are  altogether  wanting. 


VIII 


ECHINODERMATA— RADIAL  ORGANS 


411 


Echinoidea  (Fig.  353). — In  a  transverse  section  of  an  anibulacral 
area  we  find  : — 

(a)  The  outer  body  epithelium  (11). 

(b)  The  cutis,  almost  entirely  calcified,  as  ambulacral  plates  (8). 

(c)  The  epineural  sinus  (12). 

(d)  The  radial  nerve  trunk  (17). 

(e)  The  subneural  sinus — pseudohaBmal  canal  (14). 


iz 


ti 


10 


21 


Fi< -.  :;;>4.—  Transverse  Section  through  the  arm  of  an  Asteroid,  <tiagraimuatic.  1,  Ridges  of 
the  deeper  oral  nervous  system  ;  2.  radial  canal  of  the  water  vascular  system ;  3,  continuation  of  the 
axial  organ  in  the  arm  ;  4,  radial  nerve  ridge  of  the  superficial  oral  system  ;  5,  pseudohsemal  canal ; 
6  and  7,  branches  of  the  pseudohaemal  system  running  to  the  tube-feet ;  8,  pedicellaria  ;  9,  spine ; 
10,  genital  aperture  ;  11,  branchial  vesicle  (papulla) ;  12,  sessile  pedicellaria  ;  13,  continuation  of  the 
body  cavity  into  the  branchial  vesicle  ;  14,  brachial  diverticulum  of  the  stomach  ;  15,  circular  sinus 
of  the  schizocrel  round  the  branchial  vesicle  ;  16,  supramarginal  plate  ;  17,  infrainargiual  plate ;  18, 
adambulacral  plate  ;  19,  marginal  canal  of  the  pseudohsemal  system  ;  20,  canal  connecting  it  with 
the  body  cavity ;  21,  endothelium  of  the  body  cavity  ;  22,  genital  sinus  of  the  ccelom ;  23,  gonad 
(ovariuin) ;  24,  mesenteries  of  the  diverticula  of  the  stomach  ;  25,  ampulla  canal  of  the  water  vas- 
cular system  ;  26,  ampulla  ;  27,  tube-feet  canal ;  28,  upper  and  lower  transverse  muscles  of  the 
ambulacral  skeleton  ;  29,  motor  branches  of  the  deeper  oral  nervous  system  ;  30,  ambulacral  plates  ; 
31,  brachial  cavity  (ccelom)  ;  32,  apical  longitudinal  muscle  of  the  arm  ;  33,  nerve  ridge  of  the  apical 
nervous  system. 

(/)  The  radial  blood  vessel  (15). 

(g)  The  radial  canal  of  the  water  vascular  system  (16). 

(h)  The  endothelium  of  the  body  cavity  (13). 

The  figure  at  the  same  time  illustrates  the  relation  of  the  tube-feet 
to  their  ampullae,  the  double  pores,  etc. 

Asteroidea  (Fig.  354). — In  a  transverse  section  through  the  lower 
Coral)  wall  of  an  Asteroid  arm  we  find,  from  without  inwards : — 


412 


COMPARATIVE  ANATOMY 


CHAP. 


(a)  That  the  body  epithelium  stretching  across  the  ambulacral  fur- 

row is  thickened  into  the  longitudinal  ridge  which  stands  up 
in  the  base  of  the  furrow,  and  here  contains  :    _ 

(b)  The  radial  nerve  strand  (lying  in  the  epithelium  itself)  (4). 
We  find  further :  (c)  below  the  latter,  to  the  right  and  left,  the 

trunks  of  the  deeper  oral  nervous  system  (1). 
(d)  The  radial  pseudohaemal  canal  (5),  which  is  divided  into  two 
lateral  portions  by  a  vertical  septum. 


FIG.  355.— Transverse  Section  through  the  arm  of  an  Ophiuroid,  diagrammatic.  1,  Ambu- 
lacral tentacle ;  2,  its  water  vascular  canal ;  3,  epineural  circular  canal  at  the  base  of  the  tentacle  ; 
4,  circular  ganglion  at  the  base  of  the  tentacle ;  5,  ventral  shield  ;  6,  radial  epineural  canal ;  7,  radial 
nerve  trunk  of  the  superficial  oral  nervous  system  ;  8,  continuation  of  the  axial  organ  in  the  arm  (?) ; 
f>,  radial  trunk  of  the  deeper  oral  nervous  system;  10,  radial  pseudohsemal  canal;  11,  peripheral 
branch  of  the  radial  nerve  trunk  ;  12,  spine  ;  13,  lower  (oral)  intervertebral  muscle  cut  across  ;  14, 
lateral  shield  ;  15,  vertebral  ossicle ;  16,  upper  (apical)  intervertebral  muscle ;  17,  dorsal  canal  of 
the  brachial  cavity  (coeloin)  ;  18,  ciliated  strip  of  endothelium  ;  19,  dorsal  shield  ;  20,  radial  canal  of 
the  water  vascular  system ;  21,  lateral  portions  of  the  brachial  cavity,  which  are  segmentally  re- 
]>«at."l ;  22,  branch  of  the  .water  vascular  system  running  to  the  tentacle  ;  23,  ganglion  at  the  base 
of  the  spine ;  24,  motor  branch  of  the  nerve  (of  the  deeper  oral  system). 

(e)  The  radial  canal  of  the  water  vascular  system  (2),  with  the 
canals  of  the  tube-feet  branching  from  it.  (All  these  are 
separated  from  one  another  by  thin  layers  of  connective 
tissue.) 

(/)  The  ambulacral  plates  (30),  with  the  transverse  muscles  which 
connect  them  (28). 

((])  Still  further  in,  and  projecting  into  the  body  cavity,  are  the 
ampullae  (26)  of  the  tube-feet. 


VIII 


ECHINODERMATA—  RADIAL  ORGANS 


413 


(/>)  The  endothelium  of  the  body  cavity  (21). 

The  figure  also  shows  the  relations  of  the  ampullae  to  the  tube-feet 

and  their  canals,  and  the  organs  of  the  apical  side  of  the  arm 

of  an  Asteriod. 

Ophiuroidea  (Fig.  355). — In  a  section  through  an  arm,  proceeding 
from  the  lower  (oral)  to  the  upper  (apical)  side,  we  find : — 

(a)  The  body  epithelium. 

(b)  The  ventral  shield  (5). 


Q 


FIG.  35<5.— Transverse  Section  through  the  arm  of  a  Crinoid,  diagrammatic.  1,  Radial  nerve 
trunk  of  the  superficial  oral  nervous  system  ;  2,  radial  pseudohsemal  canal ;  3,  radial  canal  of  the 
water  vascular  system  ;  4,  the  paired  deeper  longitudinal  nerves  of  the  arms  ;  5,  7,  and  11,  the  three 
radial  sinuses  of  the  brachial  coelom ;  6,  genital  sinus  with  genital  rachis ;  7  (see  5) ;  8,  nerve 
trunk  of  the  apical  nervous  system ;  9,  end  of  the  nerves  at  the  surface  ;  10,  branch  connecting  4 
and  8  ;  11  (see  5) ;  12,  tentacle  nerve  ;  13,  tentacle  canal  of  the  water  vascular  system ;  14,  sensory 
cone  on  the  tentacle  ;  15,  food  groove  of  the  arm. 

(c)  The  radial  epineural  canal  (6). 

(d)  The  radial  nerve  trunk  of  the  superficial  oral  nervous  system  (7). 

(e)  The  radial  nerve  trunk  of  the  deeper  oral  system  (9). 
(/)  The  (subneural)  radial  pseudohsemal  canal  (10). 

(g)  The  radial  canal  of  the  water  vascular  system  (20). 

(h)  The  calcareous  mass  of  the  vertebral  ossicle   (15),  which   is 

traversed  by  the  tentacle  canals  (22),  and  the  intervertebral 

musculature  (16  and  13). 


414  COMPARATIVE  ANATOMY  CHAP. 

(i)  The  endothelium  of  the  body  cavity. 

(k)  The  much  diminished  body  cavity  itself  (enterocoel,  17  and  21). 

(I)  The  dorsal  (apical)  body  wall,  which  in  this  connection  is  of  no 
further  interest. 

Crinoidea  (Fig.  356). — On  the  section  of  the  arm  of  a  Crinoid, 
proceeding  from  the  oral  to  the  apical  side,  we  find  : — 

(a)  The  body  epithelium  covering  the  food  groove. 

(b)  Deep  in  this  epithelium,  the  radial  nerve  trunk  of  the  super- 

ficial oral  system  (1). 

(c)  Below  the  epithelium  (not  invariably  present)  a  small  schizo- 

ccel  canal  (pseudohremal  canal  (2). 

(d)  The  radial  canal  of  the  water  vascular  system  (3). 

(e)  At  its  two  sides,  the  paired  subepithelial  longitudinal  nerves  of 

the  arms  (4). 

(/)  The  three  radial  sinuses;  viz.  two  paired  sinuses  (5  and  11), 
separated  by  a  vertical  septum  (the  so-called  ventral  or  sub- 
tentacular  canals),  and  a  third  unpaired  sinus  (7),  the 
dorsal  canal,  separated  from  the  first  two  by  a  horizontal 
(transverse)  septum. 

All  these  parts  lie  embedded  in  somewhat  sparse  connective  tissue. 
In  the  middle  between  them  run  : — 

(g)  The  narrow  genital  sinus  (6),  with  the  genital  tube  (rachis) 
within  it. 

(h)  The  skeletal  ossicle  of  the  arm,  or  (according  to  the  plane  of 
the  section)  the  apical  and  oral  muscles  and  bands,  uniting 
the  ossicles. 

(i)  In  the  centre  of  the  joint  we  find  the  section  of  the  nerve  canal 
(axial  canal)  with  the  radial  trunk  of  the  apical  nervous 
system  (8)  which  it  encloses. 

The  figure  also  shows  the  tentacles,  and  the  nerves  which  connect 
the  paired  radial  nerves  of  the  oral  with  the  radial  trunk  of  the  apical 
nervous  systems. 

V.  The  Integument. 

The  integument  of  the  Echinodermata  consists  of  (1)  the  uni- 
laminar  body  epithelium  which  covers  the  whole  body  with  its  processes 
and  appendages,  and  (2)  a  strong  subjacent  connective  tissue  layer 
(the  cutis  or  corium)  of  mesenchymatous  origin,  in  which  the  various 
skeletal  structures  develop.  The  cutis  forms  by  far  the  largest  part 

the  body  wall.  Internally,  it  is  either  directly  lined  by  the  endo- 
thelium of  the  body  cavity  or  else  is  separated  from  the  endothelium 
by  musculature  (Holothuria,  Asteroidea). 

(1)  The  body  epithelium.  — (a)  This  is  distinct  from  the  subjacent  cutis  in  the 
ulca,  Asteroidea,  many  Holothurioidea,  and  on  the  oral  surface  of  the  disc  and 
arms  of  the  Crinoidca ;  also  in  the  Euryalce. 

In  the  Ophiuroidea  (excluding  Eurydlas),  and  on  the  apical  side  of  the  disc  and 


ECHINODERMATA— THE  INTEGUMENT  415 

anus  of  the  Crinoidea,  there  is  no  sharp  line  of  distinction  between  the  body 
epithelium  and  the  cutis.  Such  a  distinction  is,  however,  demonstrable  in  very  young 
stages.  In  later  stages  of  development  the  elements  of  the  two  forms  of  tissue  seem 
to  mingle,  and  skeletal  substance  forms  right  up  to  the  surface  of  the  integument. 

In  many  Holothurioidea  also,  the  body  epithelium,  as  such,  is  very  indistinct. 
In  Cucuinaria,  for  example,  the  cutis  appears  at  the  surface  of  the  integument,  and 
the  body  epithelium  is  found  in  the  form  of  nests  of  cells  scattered  within  the  peri- 
pheral layer  of  the  cutis.  Each  cell  sends  a  thin  process  to  the  surface  of  the 
integument. 

(&)  The  body  epithelium  is  usually  covered  by  a  cuticle  of  varying  thickness. 

(c)  The  body  epithelium  is  ciliated  over  the  whole  surface  of  the  body  in  the 
Axtd-oidea  and  Echinoidea,  but  in  the  Crinoidea  only  in  the  food  grooves. 

The  integument  of  the  Ophiuroidea,  Crinoidea  (with  the  exception  of  the  food 
grooves),  and  Holothurioidea,  is  non-ciliated. 

(d)  The  body  epithelium  of  the  Asteroidea  is  rich  in  glands.     The  glands  are 
usually  unicellular  (goblet  glands,  granular  glands,  etc. ),  and  remain  on  the  level  of 
the  epithelium.     In  Echinastcr  sepositus,  large  multicellular  glands  are  also  found, 
whose  pear-shaped  or  spherical  bodies  dip  down  into  the  cutis.     In  the  integument 
of  the  Holothurioidea  also  glands  have  been  described,  and  it  will  probably  be  dis- 
covered that  certain  epithelial  cells  of  the  Echinoidea  are  of  a  glandular  character. 

(e)  The  integumental  pigment  may  belong  to  the  epithelium  as  well  as  to  the 
cutis  ;  it  not  infrequently  occurs  in  both  layers. 

(/)  Epithelial  sensory  cells,  ganglion  cells,  and  nerve  fibres  will  be  described  in 
another  place. 

(2)  The  cutis  of  the  Echinodermata  is  always  very  thick,  although  it  shows 
extraordinary  variations  in  this  respect  according  to  the  genus  and  species.  It 
everywhere  consists  (a)  of  a  ground-  or  intercellular  substance  of  gelatinous  or  carti- 
laginous consistency,  and  (ft)  of  the  nucleated  connective  tissue  cells  which  secrete 
this  ground- substance  and  are  embedded  in  it ;  these  cells  are  spindle-shaped,  star- 
shaped,  etc.  There  are.  further,  (c)  in  all  Echinodermata,  granulated  plasm  cells 
or  wandering  cells  (amoebocytes)  similar  to  those  which  are  to  be  found  in  different 
body  fluids.  These  can  move,  like  amcebfe,  in  and  through  the  different  tissues. 

In  Ilolothurioidca,  these  wandering  cells  may  collect  in  such  quantities  in  the 
deep  looser  layer  of  the  cutis,  as  to  form  a  distinct  layer  (Wanderzellenschicht). 

The  calcareous  skeleton  of  the  body  wall  of  the  Echinodermata  always  lies  in 
the  cutis,  whether  it  consists,  as  in  the  Holothurioidea,  of  isolated  calcareous  cor- 
puscles, or,  as  in  other  Echinoderms,  of  larger  plates  of  lattice-like  or  spongy  struc- 
ture. In  sections  through  the  decalcified  body  wall,  the  spaces  in  which  the  skeleton 
lay  are  visible.  In  other  Avords,  the  connective-tissue  fills  up  all  the  spaces  in  the 
spongy  calcareous  skeleton.  Since  the  wandering  cells  can  travel  to  the  surface 
through  these  spaces,  they  may  play  an  important  part  in  the  nutrition  of  the  soft 
parts  which  lie  at  the  surface  of  the  skeleton,  especially  in  Asteroids  and  Echinoids. 

It  appeal's  that  even  the  intercellular  substance  may  occasionally  become  differ- 
entiated into  fibres,  which,  however,  are  difficult  to  distinguish  from  the  fibrous 
processes  of  the  connective-tissue  cells. 

Where  two  skeletal  plates  are  united  by  a  suture,  this  suture  is  formed  of  thickly 
crowded  parallel  fibres,  which  connect  the  ground -substance  of  one  plate  with  that 
of  the  other. 


416  COMPARATIVE  ANATOMY  CHAP. 

VI.  The  Water  Vascular  System. 
(System  of  the  Ambulacral  Vessels  :  Hydroccel.) 

This  is  a  system  of  canals  filled  with  fluid,  the  arrangements  of 
which  may  be  generally  described  as  follows. 

An  outer  aperture,  the  madreporite,  leads  first  into  a  vesicular 
section  of  the  coelom,  the  madreporitie  ampulla.  This  again  is  con- 
nected by  means  of  a  stone  eanal  (so  called  because  that  portion  of  its 
wall  which  consists  of  connective  tissue  is  often  calcified)  with  a  ring 
canal  which  surrounds  the  oesophagus.  Into  the  madreporitie  ampulla 
there  opens,  further,  the  axial  sinus  of  the  body  cavity,  which  follows 
the  stone  canal  in  its  course,  and  surrounds  a  lymphatic  gland,  the 
axial  organ. 

The  water  vascular  ring  may  carry  various  accessory  structures, 
whose  principal  function  seems  to  be  that  of  lymphatic  glands,  and 
which  are  known  as  Polian  vesicles,  Tiedemann's  bodies,  etc. 

From  the  ring  canal  there  run  out  into  the  radii  of  the  body, 
either  in  the  body  wall  or  in  close  contact  with  it,  as  many  radial 
canals  as  there  are  radii  (usually  therefore  five).  The  radial  canals 
send  off,  on  each  side,  tube-feet  canals,  which  run  into  outer  append- 
ages of  the  body  wall,  ending  blindly  at  their  tips.  These  extensible 
appendages  are  usually  present  in  great  numbers,  and  serve  either  as 
tube-feet  for  locomotion  (Holothurioidea,  some  Echinoidea,  Asteroidea), 
and  are  then  provided  with  a  terminal  sucker,  or  as  tentacles,  ten- 
tacular gills,  etc.,  for  tactile  purposes,  for  respiration,  and  for  conduct- 
ing food  (some  Echinoidea,  Ophiuroidea,  Crinoidea).  In  connection  with 
the  tube-feet  canals,  tube-feet  ampullse  are  very  often  found  (Holo- 
thurwidea,  Echinoidea,  Asteroidea)  ;  these  are  accessory  contractile 
vesicles,  which  serve  for  the  swelling  of  the  tube- feet.  Special 
valves  are  so  arranged  as  to  prevent  the  flowing  back  of  the  water 
vascular  fluid  into  the  radial  canals  (Fig.  352,  p.  409). 

The  chief  departures  from  this  general  description  met  with  in 
the  five  classes  of  Echinoderms,  affect  the  madreporite,  the  madre- 
poritie ampulla,  and  the  stone  canal.  These  will  be  described  in 
detail  later  on. 

Structure  of  the  wall  of  the  water  vessels.— Lining  the  lumen  of  the  vessels, 
there  is  generally  found,  first  of  all,  a  ciliated  epithelium.  This  is  followed,  in 
most  parts  (always  in  the  ambulacral  appendages),  by  a  longitudinal  muscle  layer. 
Outside  this  latter  lies  a  layer  of  connective  tissue,  and,  outermost  of  all,  there  is 
almost  always  an  external  ciliated  epithelium.  On  the  ambulacral  appendages  (the 
tube-feet  and  tentacles)  this  last  is  nothing  more  than  the  external  body  epithelium. 
But  in  those  parts  of  the  water  vascular  system  which  project  into  or  lie  in  the 
body  cavity,  it  is  the  endothelium  of  the  ccelom.  This  outer  epithelium  of  the 
water  vascular  system  is  rarely  altogether  wanting  ;  it  is,  however,  absent  in  such 
parts  of  the  systeir  as  run  embedded  in  the  body  wall.  A  circular  musculature  is 
seldom  found  ;  it  only  occurs  locally. 


viii         ECHINODERMATA— WATER  VASCULAR  SYSTEM         417 

Calcareous  corpuscles  may  be  formed  in  the  connective  tissue  layer  of  the  wall  in 
other  parts  of  the  water  vascular  system  besides  the  stone  canal.  Such  calcification 
always  takes  place  in  locomotory  tube-feet. 

The  fluid  contained  in  the  water  vascular  system  is  sea-water 
with  traces  of  albumen  (in  a  0 '5  -  2  per  cent  solution).  Floating  in 
this  fluid  are  found  amoeboid  cells  (lymph  bodies)  and  coloured  cor- 
puscles often  united  into  small  lumps.  The  fluid  occasionally  appears 
of  a  pale  yellow,  or  reddish,  colour. 

The  origin  of  this  fluid  is  a  question  of  frequent  recurrence. 
The  view  which  still  appears  best  supported  is  that  sea-water  flows  in 
through  the  madreporite  and  the  stone  canal,  but  an  exactly  opposite 
view  has  also  been  maintained.  The  observations  made  on  this 
subject  appear  to  contradict  one  another,  it  being  very  difficult  to 
carry  on  investigations  in  a  decisive  and  satisfactory  manner. 

A.  Madreporite  and  Stone  Canal. 

1.  Holothurioidea  (Fig.  357). — The  condition  which  must  be  con- 
sidered as  the  original  is  that  in  which  only  one  stone  canal  occurs ; 
this  is  attached  to  the  dorsal  mesentery  (cf.  p.  406),  and  its 
madreporite  lies  mediodorsally  in  the  integument,  and  its  pore 
canal  or  canals  open  outward  direct. 

Such  a  condition  is  found  in  the  adult  only  in  certain  Elasipoda 
and  in  Pelagothuria. 

In  the  large  majority  of  Holothurioidea,  the  stone  canal  loses  all 
direct  communication  with  the  exterior,  while  at  its  distal  end, 
which  now  lies  in  the  body  cavity,  a  new  inner  madreporite  forms, 
through  whose  canals  communication  is  established  between  the 
stone  canals  and  the  body  cavity. 

In  a  comparatively  small  number  of  Holothurioidea  (never  in 
Molpadiidce  and  JElasipoda)  the  number  of  stone  canals  increases  (the 
single  canals  usually  shortening  at  the  same  time),  and  may  finally 
become  very  great  (over  160). 

The  inner  madreporite  is  found  in  the  form  of  a  variously  shaped  swelling  on 
the  stone  canal,  which  is  often  S-shaped  or  spirally  coiled.  Only  the  primary  stone 
canal  is  connected  with  the  dorsal  mesentery  ;  this  is  never  the  case  with  accessory 
canals.  These  latter  float  freely  in  the  body  cavity,  and  this  is  also  the  condition 
of  the  primary  stone  canal  of  the  Aspidochirotce,  which  has  lost  its  connection  with 
both  the  body  wall  and  the  mesentery. 

More  than  one  canal  is  found  in  only  a  very  small  number  of  forms  even  among 
the  Synaptidce,  the  Dendrochirotce,  and  Aspidochirotce.  The  number  of  accessory 
canals  varies  greatly  in  different  forms  ;  it  does  not  seem  to  be  of  systematic  import- 
ance, since  it  varies  in  individuals  of  one  and  the  same  species.  It  is  probable  that 
the  accessory  canals,  ontogenetically,  bud  off*  secondarily  from  the  water  vascular 
system,  whereas  the  dorsomedian  stone  canal  arises  primarily  from  the  canal  which, 
in  the  larva,  connects  the  hydroccel  with  the  exterior. 

Branched  stone  canals,  with  a  madreporite  at  the  distal  end  of  each  branch, 
occur  in  Synapta  Beselii,  Jag  ;  and  Thyone  chilensis,  Semp. 

VOL.  II  2  E 


418 


COMPARATIVE  ANATOMY 


CHAP. 


The  madreporite  of  the  primary  (mediodorsal)  stone  canal     The  simplest,  and 
the  most  primitive  condition  is  found  in  Pelagothuria  and  m  certain 

' 


^  sp  >  , 

ITd  zLhodytes.  In  these  the  stone  canal  opens  simply  through  a  single 
m  Idorsal  pore,  which  lies  in  front  of  the  genital  aperture  (Fig.  37  A).  In  other 
species  of  these  genera  and  in  species  of  Psychropotes,  Lcetmogone  llyodcemon,  more 
than  one  madreporite  pore  is  found,  their  number  varying,  according  to  the  ^species 
from  twoorthree  to  fifty  or  more  (Fig.  357,  B).  In  other  cases  (species  of  the  Elasipod 
zenera  Irpa  Elpidia,  Oneirophanta,  Orphnurgus,  Benthodytcs,  and  the  Molpadndan 
genera  Trochostww  and  Ankyroderma]  the  distal  end  of  the  stone  canal  still  remains 


Fio.  357.— Diagrams  illustrating  the  various  relations  existing  between  the  stone  canal 
and  the  madreporites  in  the  Holothurioidea.  1,  Body  wall ;  2,  commencement  of  the  radial 
canal ;  3,  oesophagus  ;  4,  dorsal  mesentery ;  5,  stone  canal ;  6,  outer  madreporite  ;  61,  inner  madre- 
porite ;  7,  genital  aperture ;  8,  genital  duct ;  9,  water  vascular  ring  ;  10,  Polian  vesicle. 

embedded  in  the  body  wall,  but  it  has  lost  the  pore  or  pores  which  formed  the  com- 
munication between  it  and  the  exterior.  New  pores  therefore  arise  laterally  at  the 
distal  portion,  which  still  lies  in  the  body  wall,  and  these  now  open  communication 
between  the  lumen  of  the  stone  canal  and  the  body  cavity,  and  make  this  widened 
part  of  the  stone  canal  into  an  inner  madreporite  (Fig.  357,  C).  Other  Molpadiidce 
and  the  Synaptidce  and  Dendrochirota  differ  from  these  last  only  in  the  fact  that  in 
them  the  stone  canal  has  become  entirely  detached  from  the  body  wall  (Fig.  357,  D). 
In  the  Aspidochirota,  which  also  possess  an  inner  madreporite,  the  latter  appears 
complicated,  in  that  its  pore  canals  do  not  open  direct  into  the  lumen  of  the  stone 
canal,  but  first  into  a  collecting  cavity,  which  in  its  turn  communicates  by  means  of 
an  aperture  (occasionally  through  several)  with  the  lumen  of  the  stone  canal. 

2.  Eehinoidea  (Fig.  358,  33).— In  the  Echinoidea,  so  far  as  is 


viii         ECHINODERMATA— WATER  VASCULAR  SYSTEM         419 


FIG.  358.— Diagram  of  the  organisation  of  a  regular  Echinoid.  Section  in  the  direction  of 
the  principal  axis.  The  surface  of  the  section  lies  interradially  on  the  left  and  radially  on  the 
right.  The  left  half  is  incomplete.  1,  External  gill  (this  would  not  exactly  come  into  this  section, 
since  there  are  five  pairs  of  interradially  placed  gills) ;  2,  seizing  pedicellaria  ;  3,  oral  integument ; 
4,  tooth ;  5,  mouth ;  6,  cushion  of  connective  tissue ;  7,  nerve  ring  of  the  superficial  system ;  8, 
deeper  oral  nervous  system ;  9,  radial  epineural  canal ;  10,  radial  blood  vessel ;  11,  arch  of  the 
ambulacral  apophysis  (auricula) ;  12,  sphseridium  in  its  niche  ;  13,  radial  canal  of  the  water  vascular 
system  ;  14,  radial  nerve  trunk  (of  the' superficial  oral  system);  15, 'radial  pseud  ohsemal  canal ;  16, 
circular  ganglion  at  the  base  of  the  spine ;  17,  spine ;  18,  glandular  pedicellaria ;  19,  ambulacral 
tube-feet  with  terminal  disc ;  20,  21,  ambulacral  tentacles  (without  terminal  disc) ;  22,  terminal 
feeler  or  tentacle  emerging  through  the  pore  in  the  radial  (ocular)  plate  ;  23,  apical  (genital)  ring 
sinus ;  24,  perianal  sinus  of  the  coelom  ;  25,  anus  ;  26,  sinus,  into  which  a  process  (27)  of  the  axial 
organ  projects  ;  27,  aboral  process  of  the  axial  organ  ;  28, 'madreporite  ;  29,  genital  aperture  on  the 
genital  papilla  ;  30,  genital  duct ;  31,  madreporitic  ampulla,  into  which  the  stone  canal  and  axial 
sinus  enter  from  below ;  32,  axial  organ  ;  33,  stone  canal ;  34,  part  taken  by  the  blood  lacuna  in 
the  formation  of  the  Polian,  vesicle  ;  35,  root  of  the  tooth  ;  36,  muscle  of  the  forked  radii  (Fig.  348,  7) 
cut  through  ;  37,  arch  (arcus)  of  a  jaw  pyramid  of  the  masticatory  apparatus  ;  38,  lantern  mem- 
brane;  39,  ligament  of  a  forked  radius;  40,  adductor  muscle  of  the  teeth;  41,  interambulacral 
apophysis  ;  42,  general  body  cavity  (coelom) ;  43,  pyramid  ;  44,  peripharyngeal  sinus,  lantern  sinus 
of  the  coelom  ;  45,  part  taken  by  the  water  vascular  system  in  the  formation  of  the  Polian  vesicle  ; 
46,  circular  vessel  of -the  blood  lacunar  system  ;  47,  water  vascular  ring  ;  48,  oesophagus ;  49,  axial 
sinus  of  the  coelom  ;  50,  hind-gut ;  51,  perirectal  sinus  of  the  coelom  ;  at  52  and  54  the  section  is  not 
quite  radial,  so  that  it  does  not,  as  at  22  and  56,  take  in  the  radial  canal  of  the  water  vascular 
system,  but  passes  transversely  through  its  lateral  canals  which  lead  to  the  ampullae ;  in  53  the 
plane  of  the  section  lies  still  more  to  the  side,  so  that  the  ampulla  is  taken  in  (cf.  Fig.  353) ;  57, 
abductor  muscle  of  the  teeth  ;  58,  Stewart's  organ  ;  59,  muscles  between  the  pyramids  ;  60,  inter- 
mediate plate  ;  61,  forked  radius  ;  62  and  65,  intestinal  vessels  ;  63,  accessory  intestine ;  64,  prin- 
cipal intestine.  The  accessory  intestine  in  reality  runs  on  the  axial  side  of  the  principal  intestine. 


420  COMPARATIVE  ANATOMY  CHAP. 

yet  known,  there  is  always  only  one  stone  canal,  and  it  always  com- 
municates with  the  exterior  by  means  of  the  pores  of  the  madreporite. 
This  communication  is,  however,  by  no  means  direct.  The  pores  of 
the  madreporite  first  lead  into  a  small  cavity  lying  below  it,  the 
madreporie  ampulla,  into  which  opens,  on  the  one  hand,  the  ascend- 
ing stone  canal,  and,  on  the  other  hand,  the  axial  sinus  of  the  entero- 
coel,  to  be  described  later.  The  stone  canal,  on  leaving  the  ampulla, 
traverses  the  body  cavity,  following  the  axial  sinus  with  its  lymph 
gland,  and  runs  down  to  the  water  vascular  ring,  which  in  the 
Cidaroida  and  Clypeastroida  encircles  the  oesophagus  immediately  above 
the  masticatory  framework  (Fig.  358),  but,  in  the  Spatangoida,  imme- 
diately above  the  mouth.  In  both  the  former  groups  the  stone  canal 
is  short  and  more  or  less  straight,  but  in  the  Spatangoida  it  is  very 
long  and  runs  in  coils. 

On  the  possibly  great  morphological  importance  of  the  ampullae,  cf.  the  section 
on  Ontogeny. 

Echinocyamus  pusillus,  a  Clypeastrid,  shows  an  embryonic  condition  in  the 
adult  in  that  the  madreporite  has  only  one  pore.  All  other  Echinoids,  examined 
with  reference  to  this  point,  possess  as  adults  several  or  numerous  pores.  The 
number  of  pores  increases  with  age  and  growth. 

The  pore  canals  which  traverse  the  madreporites  may  anastomose  with  one 
another.  They  may  enter  the  ampulla  through  several  inner  pores  or  else  through 
one  common  inner  aperture.  In  the  Spatangidce,  they  traverse  the  substance  of  a 
large  skeletal  process  (apophysis)  of  the  madreporite,  which  projects  into  the  cavity 
of  the  test. 

The  condition  of  the  stone  canal  in  the  Spatangoida  deserves  further  investiga- 
tion, since  the  observations  hitherto  recorded  contradict  one  another.  According  to 
one  account,  the  stone  canal  (in  Echinocardium)  breaks  up  into  branches  on  its  way  to 
the  water  vascular  ring,  these  branches  communicating  with  the  axial  blood  lacunar 
system.  According  to  another,  it  ends  blindly  (in  Spatangus  purpureus),  and  the 
water  vascular  ring  is  said  in  no  way  to  communicate  openly  with  the  apical  stone 
canal.  A  canal,  however,  runs  from  the  water  vascular  ring  towards  the  stone  canal, 
without  reaching  it.  The  existence  of  any  kind  of  communication  with  the  lacunar 
system  is  emphatically  denied  by  those  who  hold  this  latter  view. 

3.  Asteroidea.  —  In  all  Alferoids  the  madreporite  is  external, 
and  takes  the  form  of  a  skeletal  plate,  which  is  perforated  by  many 
pores,  and  always  lies  on  the  apical  side  of  the  disc  interradially. 
The  stone  canal,  within  the  axial  sinus  and  attached  by  a  band  to  its 
wall,  descends  direct  to  the  water  vascular  ring  which  surrounds  the 
oasophagus,  and  enters  this  ring  interradially.  The  wall  of  the  stone 
canal  is  generally  highly  calcified,  and  its  lumen  is  divided  in  a  more 
or  less  complicated  manner  into  shelves,  niches,  etc.,  by  projecting 
folds  which  frequently  branch.  It  not  infrequently  happens  in 
Asteroids  that  there  is  more  than  one  stone  canal  and  madreporie 
plate.  For  example,  all  Asteroids  which  reproduce  asexually  (i.e.  by 
division)  possess  more  than  one  canal. 

The  relations  of  the  madreporite  to  the  axial  sinus  are  interesting. 
Not  all  the  pores  of  the  madreporie  plate  open  into  the  stone  canal ; 


Till 


ECHINODERMATA— WATER   VASCULAR  SYSTEM 


421 


some  of  them  open  direct  into  the  axial  sinus.  No  direct  communica- 
tion between  the  stone  canal  and  the  axial  sinus  is  found  in  adult 
animals. 

The  madreporite  appears  externally  marked  by  furrows  radiating  from  the  centre 
to  the  periphery  (Fig.  359).     In  the  bases  of  these  furrows  lie  the  apertures  of  the 
pores.     The   pore  canals,  which  run  through  the  substance  of  the 
madreporite  to  the  stone  canal,  anastomose  in  definite  ways,  which 
cannot  here  be  described. 

The  increase  of  surface  of  the  inner  wall  of  the  stone  canal 
(Fig.  360)  is  of  some  interest.     As  the  middle  layer  of  connective 
tissue  takes  part  in  the  formation  of  the  folds  projecting  into  the 
lumen,  these  folds  may  calcify.     The  simplest  condition  is  found  in 
the  Echin  aster  idee  and  Astcrias  tenuispina,  where  a  projecting  longi- 
tudinal ridge  is  formed  on  the  inner  wall  of  the  stone  canal  (Fig.    quarter  of  the 
360,  A).     In  Asterina  the  free  edge  of  this  fold  splits  up  into  two    niadreporic 
diverging  lamellae,  in  such  a  way  that  the  transverse  section  is  Y-  or   Plate    °5h.As' 
anchor -shaped  (B).      The   lamellae   may  become   coiled   (species   of  m^ng    (after 
Asterias,  Pentaceros,  Grymnasteria,  C).    Occasionally  the  ridge  traverses    Ludwig). 
the  whole  lumen  of  the  canal  as  a  septum  (D),  and  then  carries  on 
each  surface  a  coiled  lamella  (species  of  Astropccten) .     The  whole  lumen,  further, 
may  be  traversed  by  septa  which,  in  transverse  section,  form  a  network  (Luidia, 
Culcita,  species  of  Astropecten  and  Ophidiaster,  F). 

Number  of  the  stone  canals  and  madreporitic  plates. — Several  madreporites 
and  stone  canals  (two  to  five  and  more)  are  not  infrequently  found  in  individuals 


FIG.  359.— A 


FIG.  360.— A-F,  Transverse  sections  through  the  stone  canal  of  various  Asteroids.  1,  Sus- 
pensor  of  the  stone  canal  to  the  wall  of  the  axial  sinus  ;  2,  endothelium  of  the  axial  sinus  ;  3,  inner 
epithelium  of  the  stone  canal ;  4,  connective  tissue  portion  of  the  wall. 

with  six,  seven,  or  more  arms,  belonging  to  species  which  normally  have  five  arms. 
There  are,  however,  some  species  (having  normally  five  or  more  arms)  which  habitu- 
ally possess  more  than  one  madreporite  (Asterias  capensis,  A.  polyplax,  Ophidiaster 
Gen/iani,  Acanthaster  echinites  and  A.  Ellisii}.  On  the  other  hand,  the  species  of  the 
genera  Solaster,  Heliaster,  and  Luidia,  which  normally  have  numerous  arms,  possess 
only  one  madreporite.  When  more  than  one  madreporite  is  present  they  lie,  as  a 
rule,  in  different  interradii.  Cases  have,  however,  been  observed  in  which  two  stone 
canals  occurred  in  one  and  the  same  interradius,  and  even  in  one  and  the  same  axial 
sinus. 


422 


COMPARATIVE  ANATOMY 


CHAP. 


4.  Ophiuroidea. — In  this  class,  as  a  rule,  one  single  madre- 
porite  with  one  pore  aperture  and  a  single  stone  canal  are  present. 
The  pore  aperture  is  not  found,  as  in  Asteroids  and  JEchinoids,  on  the 
apical  side  of  the  body,  but,  in  adult  Ophiuroids,  on  the  oral  side  of 
the  disc,  asymmetrically  in  an  interbrachial  area,  and  on  that  edge  of 
the  oral  shield  which  is  turned  to  the  bursal  aperture.  This  oral 
shield  thus  becomes  the  madreporic  plate.  The  pore  aperture  leads 
first  into  an  ampulla  (Fig.  361,  3),  which  probably  corresponds  with 
the  axial  sinus  of  the  Askeroidea  and  Echinoidea.  Into  this  ampulla 
the  stone  canal  which  descends  from  the  water  vascular  ring  opens. 


FIG.  361. —Stone  canal  and  neighbouring  parts  of  Amphiura  squamata,  diagrammatic 
vertical  section  through  the  madreporic  interradius  of  the  disc.  1,  Water  vascular  ring ;  2, 
stone  canal ;  3,  ampulla ;  4,  madreporic  canal ;  5  and  7,  axial  sinus  (?) ;  6,  circular  genital  sinus ; 
8,  axial  organ  ;  9,  genital  rhachis  ;  10,  bursal  pouch  ;  11,  oral  wall  of  the  intestine  ;  12,  peristomal 
sinus  ;  13,  irtterradial  muscle  ;  14,  circular  nerve  ;  15,  teeth ;  16,  mouth  ;  17,  oral  surface  of  the  disc. 

A  large  part  of  the  ampulla  lies  on  that  side  of  the  stone  canal  which 
is  turned  towards  the  mouth.  In  consequence  of  the  position  of  the 
pore  aperture,  the  stone  canal,  which  rises  out  of  the  water  vascular 
ring  interradially,  runs  in  a  downward  (oral)  direction. 

The  diagram  (Fig.  361)  illustrates  in  detail  (1)  the  relation  of  the  stone  canal  to 
the  axial  sinus  ;  (2)  the  manner  in  which  the  former  enters  the  madreporic  ampulla, 
the  cylindrical  epithelium  of  the  former  being  directly  continued  into  the  tessellated 
epithelium  of  the  latter  ;  (3)  the  opening  outward  of  the  ampulla  through  a  madre- 
poric canal. 

It  appears  that  in  many  species  of  the  genera  Amphiura,  Ophiolepis,  OpJdo- 
plocus,  Ophionereis,  and  Ophiocnida,  several  or  many  pore  apertures  occur  at  the 
edge  of  the  oral  shield.  This  is  certainly  the  case  in  many  Astrophytidce.  In 
Trichaster,  however,  only  one  pore  aperture  is  present ;  but  this  and  the  stone  canal 
belonging  to  it  are  repeated  in  each  interradius. 

-  In  Ophiactis  viretis  also,  which  reproduces  itself  asexually  by  division,  several 
(as  many  as  five)  stone  canals  occur  in  the  adult  in  different  interradii.  In  young 
individuals  only  one  is  found. 


vin         EGHINODERMATA—  WATER  VASCULAR  SYSTEM         423 

5.  Crinoidea. — Adult  Crinoids  have  at  least  five,  and  usually 
many  more  or  even  very  numerous  stone  canals,  all  of  which  open 
into  the  body  cavity.  Communication  between  the  exterior  and  the 
body  cavity  is  brought  about  by 
at  least  five  ciliated  pores  (Kelch- 
poren)  in  the  tegmen  calycis ; 
their  number  is  generally  far 
greater,  and  may  mount  up  to  a 
thousand.  Each  single  pore 
corresponds  with  a  madreporite 
with  one  pore  canal.  We  must 
not  therefore  compare  the  calyx 
pores  of  a  Crinoid  collectively 
with  the  numerous  pores  of  a 
madreporic  plate.  Originally,  ^  ^  _A  st<me  ^  and  pore  of  ^  ^ 

the  number  Of  pores  on  the  calyx  of    RMzocrinus    lofotensis,    diagrammatic    (after 

no  doubt  agreed  with  the  number  Ludwig).    Interradial  section  in  the  neighbourhood 

of    srnnp     rarmk         Tn     rxw*     in  of  the  mouth-    l>  Tegmen  calycis ;  2,  calyx  pore ; 

tone     CanalS.        in     Case.,     in  3>  aperture  of  tiie  stone  canal  into  the  body  cavity  ; 

which    both    Structures    are  Very  4,    intestinal    epithelium;     5,    intestinal    cavity; 

numerous,      however,      no       SUch  6,  ccelom  ;  7,  stone  canal ;  8,  ring  canal ;  9,  circular 

-,,.  t  i   i  T  r    j  nerve":  10,  oesotfhageal  epithelium;  11,  oesophagus; 

relation  can  be  established.  12>  coimective  tissue. 

In  many  inadunate  Crinoids 

(cf.  p.  303)  a  madreporite  occurs  in  the  posterior  interradius  of  the 
tegmen. 

Pihizocrinus  lofotensis  and  Adinocrinus  verneuilianus  have  only  five 
interradial  stone  canals  and  five  interradial  pores  in  the  calyx.  The 
openings  of  the  stone  canals  into  the  body  cavity  lie  directly  below 
the  pores  belonging  to  them. 

For  the  number  and  arrangement  of  the  calyx  pores,  cf.  the  section  on  the  Tegmen 
Calycis  of  the  Crinoids,  p.  377. 


B.  The  Water  Vascular  Ring  and  its  Appendages. 

1.  Holothurioidea. — The  water  vascular  ring  always  encircles  the 
oesophagus  behind  (i.e.  apically  to)  the  calcareous  ring.  In  all  Holo- 
thurioidea without  exception  it  carries  Polian  vesicles.  As  a  rule, 
only  one  Polian  vesicle  is  present. 

These  pear-shaped  or  tubular  cteca  of  the  water  vascular  ring,  which  project 
freely  backward  into  the  body  cavity,  vary  greatly  in  size.  In  extreme  cases  they 
may  be  half  as  long  as  the  body. 

In  the  Molpadiidce,  and  among  the  Elasipoda  in  the  Psychropotidce  and  Deimatidce, 
more  than  one  vesicle  has  never  yet  been  observed,  and,  in  the  Elpadiidce,  there 
is,  normally,  only  one.  In  other  divisions,  a  varying  number  of  species,  greatest  in 
the  Synaptidcc,  have  more  than  one  Polian  vesicle.  In  all  such  species,  however, 
there  was  originally  only  one  vesicle.  Where  accessory  vesicles  occur  they  vary 
greatly  in  number,  and  appear  to  have  very  slight,  if  any,  systematic  significance. 


OF  THR 

UNIVERSITY 


424  COMPARATIVE  ANATOMY  CHAP. 

Where  only  one  Polian  vesicle  occurs,  it  lies  in  the  left  ventral 
interradius,  very  seldom  in  the  left  dorsal  interradius. 

Where  two  or  more  vesicles  occur,  they  are  also  mostly  found  in  the  ventral 
region  of  the  circular  canal. 

The  walls  of  the  Polian  vesicles  correspond  in  structure,  essentially,  with  those  of 
the  ring  canal.  Cells  belonging  to  the  inner  epithelium  become  amoeboid  and  break 
away  from  the  wall.  These  are  said  to  become  the  lymph  cells  of  the  water  vascular 
system. 

2.  Eehinoidea. — In  the  Spatangoida  (which  have  no  masticatory 
apparatus)  the  ring  canal  encircles  the  oesophagus  immediately  above 
the  mouth.     In  other  Eehinoidea,   however,  it  is  pushed  up  by  the 
masticatory  apparatus  which  intervenes  between  it  and  the  mouth. 
The  canal  therefore  surrounds  the  oesophagus  at  the  point  where  this 
latter  emerges    from   the    lantern.     The   ring   canal,   as  well    as  its 
accessory  structures,  nevertheless,  lie  within  the  lantern  membrane, 
which  envelops  the  whole  masticatory  apparatus.     The  circular  vessel 
(the  lacunar  ring)  is  in  close  contact  with  the  canal  (Fig.  358), 

In  the  Spatangoida  and  some  Clypeastridce  (Echinocyamus  pusillus), 
the  ring  canal  has  no  accessory  structures.  In  the  Stereosomata,  on  the 
contrary,  it  has,  in  each  interradius,  a  small  outgrowth,  which  ramifies 
and  intertwines  with  similar  ramifications  of  the  circular  blood  vessel 
to  form  together  a  spongy  body,  which  is  known  as  the  Polian  vesicle, 
and  is  regarded  as  a  lymph  gland.  This,  which,  in  the  Stereosomata,  is 
confined  to  certain  localised  interradial  points,  occurs  in  the  Cidaroida, 
certain  Clypeastroida  (e.g.  Peronella  orbicularis),  and  the  Streptosomata, 
along  the  whole  course  of  the  canal,  so  that  the  intertwining  of  the 
appendages  of  the  ring  canal  and  of  the  circular  blood  vessel  gives 
rise  to  a  spongy  ring. 

An  intermediate  stage  is  found  in  Echinodiscus  biforis  (Clypeastroid),  in  which 
the  interradial  spongy  bodies  of  the  circular  canal  are  longer  than  in  the  Stereosomata, 
but  long  radially  arranged  tracts  are  still  left  free,  the  canal  at  these  parts  retaining 
its  simple  lumen. 

3.  Asteroidea. — The  circular  canal  which  surrounds  the  mouth, 
following  the  inner  outline  of  the  oral  skeleton,  here  has  two  kinds 
of  appendages :    Tiedemann's  bodies  and  the  Polian  vesicles,  both 
of  which  lie  interradially.     Tiedemann's  bodies  appear  to  occur  in  all 
Asteroids,  whereas  the  Polian  vesicles  are  wanting  in  some  families, 
e.g.  the  Asteriidce,  Echinasteridce,  and  Linckiidce. 

Tiedemann's  bodies  (Fig.  363,  7)  are  small  tufts  of  tubules,  closely  crowded 
together,  their  walls  of  connective  tissue  being  fused  with  one  another.  These 
tubules,  which  open  into  the  circular  canal,  are  lined  internally  with  a  cubical 
epithelium,  and  contain  within  their  lumen  bundles  of  cells  which  have  broken 
away  from  the  wall.  These  cells,  the  protoplasm  of  which  contains  pigmented 
concretions,  become  the  amoeboid  lymph  cells  which  float  in  the  fluid  of  the  water 
vascular  system.  They  give  the  Tiedemann's  bodies  their  more  or  less  distinct 
coloration. 


VIII 


EGHINODEEMATA— WATER  VASCULAR  SYSTEM         425 


Two  Tiedemann's  bodies  usually  occur  in  each  interradius  ;  the  interradius 
containing  the  stone  canal  not  infrequently,  however,  forms  an  exception  to  this 
rule,  only  one  such  vesicle  being  present  in  it  (Asteriidce,  Echinasteridce,  Linckiidoe, 
Aster  inidce,  Culcitidof}.  If  the  circular  canal  is  viewed  internally  in  the  position 
shown  in  Fig.  363,  this  body  lies  to  the  right  of  the  stone  canal. 

The  Polian  vesicles  (Fig.  363,  6)  are  large  structures  with  long  stalks,  and  to 
them,  as  to  Tiedemann's  bodies,  the  function  of  lymph  glands  has  been  attributed. 
In  the  Asterinidce,  Culcitidce,  Luidia,  and  several  species  of  Astropecten,  one 


10. 


FIG.  363.— Circular  canal,  Polian  vesicles,  Tiedemann's  bodies,  and  ampullae  of  the  water 
vascular  system  of  Asterina  gibbosa  (after  Cue'not).  Seen  from  within,  i.e.  from  the  body  cavity. 
1,  Mouth  at  the  centre  of  the  oral  membrane  ;  2,  stone  canal ;  3,  axial  sinus  ;  4,  transverse  muscles 
of  theambulacral  plates ;  5,  ambulacral  plates  ;  0,  Polian  vesicles ;  7,  Tiedemann's  bodies  ;  8,  circular 
canal ;  9,  blood  vascular  ring(?)  ;  10,%ampullae. 

vesicle  is  found  in  each  interradius.  Only  in  the  interradius  containing  the  stone 
canal  is  it  wanting,  or  else  (in  species  of  Astropecten)  two  are  here  found  instead  of 
one.  Astropecten  aurantiacus  has  two  to  four  (usually  three)  Polian  vesicles  in  each 
interradius  (even  in  the  stone  canal  interradius).  The  wall  of  this  vesicle,  proceeding 
from  without  inwards,  consists  of:  (1)  the  ciliated  endothelial  covering  ;  (2)  a  layer 
of  connective  tissue  in  which  run  the  longitudinal  muscle  fibres ;  (3)  a  circular 
muscle  layer,  and  (4)  the  inner  epithelium,  whose  cells  lie  in  the  interstices  of  a  net- 
work of  connective  tissue. 

4.  Ophiuroidea. — The  water  vascular  ring  here  possesses  one 
Polian  vesicle,  which  functions  as  lymph  gland  in  each  interradius 
except  that  of  the  stone  canal.  The  structure  of  the  wall  of  this 
vesicle  resembles  that  in  the  Asteroidea,  the  longitudinal  musculature, 
however,  seems  always,  and  the  circular  musculature  frequently,  to  be 
wanting.  The  canals  to  the  first  two  tube-feet  arise  directly  from  the 
circular  canal,  commencing  usually  as  a  common  canal  which  forks 
later,  but  occasionally  the  canals  are  distinct  from  the  first. 

Ophiactis  virens  (Fig.  364)  occupies  an  exceptional  position  among  the  Ophiuroidea, 
being  capable  of  asexual  reproduction  by  means  of  fission.  This  form  not  only  has, 
as  already  mentioned,  several  stone  canals,  but  in  each  interradius  two  to  three 


426 


COMPARATIVE  ANATOMY 


CHAP. 


Polian  vesicles  and,  besides  (an  altogether  unique  condition),  six  to  fifteen  long  thin 
accessory  vessels  in  each  interradius,  which  are  hollow  and  end  blindly  ;  these 
encircle  the  intestine  and  in  sexually  mature  animals  penetrate  between  the  genital 
organs.  The  walls  of  these  vessels,  which  are  filled  with  blood  and  lymph  corpuscles, 
and  communicate  with  the  circular  canal,  consist,  from  without  inwards  of:  (1)  the 


FIG.  364.— A  portion  of  the  disc  of  OpMactis  virens  in  horizontal  section,  somewhat  diagram- 
matic (after  Cue*not).  1,  Oral  tentacles ;  2,  tooth  ;  3,  circular  canal ;  5,  section  of  the  stomachal 
sac ;  6,  Polian  vesicles  ;  7,  accessory  vessels  of  the  circular  canal ;  8,  stone  canal. 

endothelium  of  the  body  cavity  ;  (2)  a  thin  layer  of  connective  tissue  ;  (3)  the  inner 
epithelium.  This  altogether  peculiar  development  of  the  water  vascular  system  in 
OpMactis  virens  is  considered  to  be  connected  with  the  absence  of  bursse  which  serve 
for  respiration,  OpMactis  standing  alone  among  the  Ophiuroidea  in  having  no  such 
structures.  This  peculiar  development  of  the  water  vascular  system  is  said  to  be  a 
supplementary  means  of  respiration. 

5.  Crinoidea. — The  circular  canal  which  surrounds  the  mouth  has 
here  no  accessory  structures  except  the  stone  canals.  It  is  provided 
with  longitudinal  muscle  fibres,  which  are  connected  with  the  epithelial 
cells  (epithelial  muscle  cells).  As  in  the  radial  canals,  muscle  cells 
also  occur  transversely  traversing  the  lumen  of  the  canal.  The 
circular  canal  gives  off  canals  direct  to  the  five  groups  of  tentacles 
which  surround  the  mouth. 


C.  The  Radial  Canals,  the  Canals  of  the  Tentacles  and  Tube-feet ; 
the  Tentacle  and  Tube-feet  Ampullae. 

1.  Holothurioidea.  —  The  Holothurioidea  fall  into  two  very 
distinct  groups,  the  Synaptidce  being-  distinguished  from  all 
other  members  of  the  class  by  the  fact  that,  in  adults,  neither 
tube-feet,  tube -feet  canals,  ampullae,  nor  any  traces  of  radial 
vessels  are  found.  The  Synaptidce  (Paractinopoda)  have  only  oral 


viii         ECHINODERMATA— WATER  VASCULAR  SYSTEM         427 

tentacles  and  tentacle  canals,  the  latter  springing  directly  out  of 
the  circular  canal. 

The  arrangement  in  all  other  Holothurioidea  (Adinopoda)  may  be 
described  as  follows.  There  are  five  radial  canals,  and  never  more. 
The  tentacle  canals  never  spring  directly  from  the  circular  canal,  but 
arise  out  of  the  radial  canals.  The  tentacles  are  to  be  regarded  as  the 
first  (modified)  tube-feet,  and  the  tentacle  canals  as  the  first  tube-feet 
canals. 

The  canals  of  the  tube-feet  and  tentacles  are  usually  connected 
with  ampullae. 

Actinopoda  (Fig.  365). — From  the  circular  canal  the  radial  canals  run  forward 
(anteriorly)  along  the  oesophagus  towards  the  mouth,  passing  the  axial  surface  of 
the  calcareous  ring  (i.e.  between  it  and  the  oesophagus).  They  then  pass,  together 
with  the  radial  nerves  on  whose  inner  side  they  lie,  through  the  incisions  or  apertures 
belonging  to  them  in  the  ring,  and  run  backwards  (aborally)  in  the  body  wall, 
outside  the  circular  musculature,  and  end  blindly  near  the  anus. 

In  some  rare  cases,  where  the  ventral  surface  is  sharply  distinguished  from  the 
dorsal,  and  the  dorsal  ambulacral  appendages,  i.e.  those  of  the  bivium,  have  entirely 
disappeared,  the  corresponding  dorsal  radial  canals  are  said  also  to  be  wanting.  In 
a  few  isolated  forms  the  central  radial  canal  of  the  ventral  side  (i.e.  of  the  trivium) 
is  also  said  to  be  wanting. 

The  tentacle  canals  branch  off  from  their  radial  canals  just  above  the  calcareous 
ring.  Their  number  corresponds  with  that  of  the  tentacles  to  which  they  run. 
These  canals  are  often  connected,  at  the  anterior  edge  of  the  ring,  with  tentacle 
ampullae  (Fig.  365,  18).  These  latter  are  tubular  outgrowths,  which  vary  greatly  in 
size,  stretching  back  over  the  outer  surface  of  the  calcareous  ring,  and  for  the  most 
part  projecting  freely  into  the  body  cavity.  Where  such  ampullse  occur,  all  the 
canals  without  exception  are  provided  with  them.  They  are  entirely  wanting  in  the 
families  of  the  Elasipoda  and  Dendrochirota,  but  occur  normally  in  the  Synaptidce, 
MolpadtidtBi  and  Aspidoclnrota.  In  Pelagothuria,  branches  run  through  the  peculiar 
swimming  disc  (cf.  p.  286),  radially,  and  reach  even  to  the  tips  of  its  processes. 
They  are  evidently  to  be  regarded  as  modified  tentacle  ampullae. 

The  canals  of  the  tube-feet  branch  off  alternately  from  the  radial  canals.  As  a 
rule,  a  separate  canal  runs  from  the  radial  canal  to  each  foot ;  but  in  some  cases 
(Holothuria  tubulosa}  one  canal,  by  branching,  runs  to  several  (4-6)  tube-feet.  In 
the  Molpadiidee,  and  the  above-named  Holothurian,  it  is  said  that  there  are  tube-feet 
canals  which  end  blindly,  and  thus  have  no  tube-feet  answering  to  them.  Except 
in  the  footless  Molpadiidee  and  the  Psychropotidce,  the  tube-feet  canals  are  connected 
with  egg-shaped,  often  somewhat  long  and  occasionally  branched  ampullae.  These 
either  lie  as  covered  ampullae  outside  of  the  circular  musculature  of  the  body  wall  or, 
as  free  ampullae,  press  in  between  the  transverse  musculature  into  the  body  cavity. 

At  the  point  where  the  ampulla  opens  into  the  tube-foot  canal,  but  in  that  part 
of  the  latter  which  comes  from  the  radial  canal,  there  is  a  valve,  similar  to  that 
found  in  Asteroids,  which  will  be  described  later.  This  valve  is  arranged  in  such  a 
way  as  to  prevent  the  return  of  the  fluid  into  the  radial  vessel,  either  from  the  foot 
or  from  the  ampullae.  Valves  are  also  found  in  the  tentacle  canals. 

The  walls  of  the  ampullae  resemble  those  of  the  Polian  vesicles  in  structure. 
The  radial  canals  and  their  branches  are  chiefly  distinguished  by  the  fact  that  the 
longitudinal  musculature  is  only  developed  in  the  outer  part  of  the  walls. 

Paractinopoda. — The  tentacle  canals  here  spring  directly  out  of  the  circular 
canal,  and  nearly  always  agree  in  number  with  the  tentacles.  At  the  level  of  the 


428 


COMPARATIVE  ANATOMY 


CHAP. 


calcareous  ring,  in  each  tentacle  canal  a  muscular  membrane   forms  a  semilunar 
valve  which  projects  from  the  wall  with  its  concave  side  directed  forwards  (orally). 


FIG.  365.— Section  through  the  oral  region  of  an  Actinopod,  in  the  direction  of  the  principal 
(longitudinal)  axis.  On  the  right,  the  plane  of  the  section  is  radial ;  on  the  left,  almost  interradial. 
1,  Cutis  ;  2,  body  epithelium  ;  3,  oral  tentacle,  cut  oft';  4,  water  canal  of  the  oral  tentacle  ;  5,  blood 
vessel  of  the  oral  tentacle  ;  6,  tentacle  nerve  ;  7,  circular  nerve  ;  8,  oral  portion  of  the  coelomatic 
perioesophageal  sinus ;  9,  mouth ;  10,  cesophagiis ;  11,  perioesophageal  sinus ;  12,  interradial 
portion  of  the  calcareous  ring ;  13,  water  vascular  ring ;  14,  blood  vascular  ring ;  15,  ventral 
intestinal  vessel ;  16,  intestinal  epithelium ;  17,  Polian  vesicle  ;  18,  ampulla  of  the  oral  tentacle  ; 
19,  endothelium  of  the  body  cavity;  20,  circular  musculature  of  the  body  wall ;  21,  body  cav.ity  ; 
22  and  26,  radial  blood  vessels  ;  23,  radial  nerve  trunk  of  the  superficial  system  ;  24,  radial  epineural 
canal ;  25,  radial  perihsemal  canal ;  27,  radial  canal  of  the  water  vascular  system  ;  28,  longitudinal 
muscles  ;  29,  commencement  of  the  radial  canal  of  the  water  vascular  system  ;  30,  radial  portion  of 
the  calcareous  ring  ;  31,  retractor  muscle  ;  32,  dorsal  intestinal  vessel. 


This  valve  prevents  the  water  vascular  fluid  flowing  back  out  of  the  tentacles  into 
the  circular  canal. 

The  wall  of  the  tentacle  canals  consists,  from  without  inwards,  of :  (1)  the  endo- 
thelium of  the  body  cavity';  (2)  a  longitudinal  muscle  layer ;  (3)  a  layer  of  connective 
tissue  ;  (4)  a  circular  muscle  layer  ;  (5)  an  inner  epithelium. 


vni         ECHINODERMATA— WATER  VASCULAR  SYSTEM         429 

2.  Eehinoidea  (Fig.  358,  p.  419). — In  the  Spatangoida,  where  a 
masticatory  apparatus   is   wanting,  the   radial   canals,  on  leaving   the 
circular    canal    which    surrounds    the   mouth,    are    already   in   their 
respective  radii,  and  commence  at  once  to  send  out  branches  right 
and  left  to  the  tube-feet.     In  other  Eehinoidea,  however,  the  radial 
canals  have  to  descend  from  the  circular  canal  which  encircles  the 
oasophagus  above  the  lantern  to  the  peristome,  and  thus,  after  rising 
out  of  the  circular  canal,  have  first  to  run  under  the  intermediate 
plates  and  over  the  intermaxillary  musculature.     They  emerge  at  the 
periphery  of  the  lantern  and  then  descend  on  its  outer  side,  i.e. 
outside  the  intermaxillary  musculature  to  the  peristome.     Having 
reached  this  latter,  they  first  give  off  a  branch  which  runs  in  the  oral 
region  towards  the  mouth.     They  then  pass  through  the  auriculae, 
in  order  to  run  up  radially  towards  the  apex,  on  the  inner  side  of 
the   test,   and   in    the  middle    lines    of   the  ambulacra.      They  end 
blindly  in  the  pores  of  the  radial  plates  of  the  apical  system. 

While  running  up  on  the  inner  side  of  the  test,  the  radial  canals 
give  off  alternating  lateral  branches,  each  of  which  enters  an  ampulla 
(cf.  Fig.  353,  p.  410).  The  ampulla,  which  projects  into  the  body 
cavity,  is  itself  connected,  by  means  of  one  or  two  canals,  with  the 
cavity  of  a  tube-foot  or  tentacle,  which  latter  projects  freely  on  the 
outer  side  of  the  test.  The  ambulacral  plate  at  such  a  point  is  per- 
forated by  either  a  single  or  a  double  pore,  according  as  the  canal  to  the 
tube-foot  is  single  or  double  (cf.  the  section  on  the  Skeletal  System). 

In  all  Eehinoidea,  the  tube-feet  in  young  animals  are  all  alike,  and  each  is 
connected  with  its  ampulla  by  a  single  pore  through  the  test.  This  may  be 
considered  to  be  the  primitive  arrangement.  Tube-feet  with  single  pores  are  found 
in  adults  in  a  few  Spatangoida :  in  the  Pourtalesiidce,  in  the  Ananchytidan  genera 
Urcchinus,  Cystcchinus,  Calymne,  in  the  Spatangoid  genus  Palaeotropus  and  the 
Cassidulid  genus  Neolampas. 

In  all  other  Echinoids,  the  tube-feet  or  tentacles  have  double  pores.  In  the 
regular  Echinoids  (Cidaroida,  Diadematoida),  only  double  pores  are  found  :  but  in 
the  Clypeastroida  and  the  Spatangoida,  only  the  pores  of  the  petaloids  are  double  : 
those  on  the  remaining  ambukcral  regions  being  single. 

The  ampullae  are  delicate  structures  which  vary  in  shape.  In  cases  in  which 
they,  like  the  tentacles  to  which  they  belong,  stand  at  some  distance  from  one 
another,  they  are  pear-shaped  or  spherical ;  but  where  they,  like  the  tube-feet,  stand 
in  compact  rows  in  the  ambulacral  meridians,  as  in  the  regular  Echinoids  and  in  the 
petaloids  of  the  irregular  forms,  they  are  lengthened  out  horizontally  and  flattened 
vertically  (dorso  -  ventrally ).  The  walls  of  the  ampullae,  from  without  inwards, 
consist  of:  (1)  a  ciliated  endothelium  ;  (2)  a  layer  of  connective  tissue,  containing 
occasional  embedded  calcareous  corpuscles  ;  (3)  a  circular  muscle  layer  ;  (4)  an  inner 
ciliated  epithelium.  The  lumen  is  traversed  from  wall  to  wall  by  fibres,  which  are 
probably  muscular.  In  several  Echinoids,  at  the  points  where  the  lateral  canals  of 
the  radial  canals  open  into  the  ampullae,  valves  have  been  observed. 

The  branches  of  the  radial  canal  which  run  in  the  oral  integument  supply  the 
tube-feet  or  tentacles  occurring  in  this  region. 

3.  Asteroidea. — The  radial  canals,  in  this  class,  run  along  the 


430  COMPARATIVE  ANATOMY  CHAP. 

bases  of  the  ambulacral  furrows  of  the  arms,  outside  the  ambulacra! 
plates.  At  the  tips  of  the  arms  they  end  blindly  in  the  terminal 
ocular  tentacles.  In  their  courses,  consecutive  widenings  and 
narrowings  are  not  infrequently  found,  these  correspond  with  the 
segmentation  of  the  arm,  but  are  never  very  marked.  Each  radial 
canal  gives  off — at  regular  intervals,  which  correspond  with  the 
skeletal  segments,  and  at  opposite  points  to  right  and  left — canals  to 
the  tube-feet.  At  the  point  where  such  a  canal  opens  into  the  tube- 
foot,  a  second  canal,  the  ampulla  canal,  branches  off  from  it.  This 
canal  rises  up  between  two  consecutive  ambulacral  plates  to  widen  out 
above  these  latter  into  an  ampulla  which  projects  freely  into  the  body 
cavity  (Fig.  354,  p.  411). 

This  ampulla  is  single  in  all  young  Asteroids  and  many  adults  (LincJciidce, 
Echinasteridce,  Asteriidce,  Luidia).  In  other  Asteroids  (Astropectinidce  excluding 
Luidia,  Asterinidce,  Pentacerotidce,  e.g.  Culcila)  two  separate  ampullae  occur  to 
each  tube-foot  in  the  adult. 

Valves  are  found  at  the  points  where  the  canals  of  the  tube-feet 
open  into  the  radial  canal.  A  muscular  membrane,  resembling  a 
truncated  cone  with  the  base  attached  horizontally  round  the  wall  of 
the  canal,  projects  into  the  lumen  directed  towards  the  foot.  This 
valve  prevents  the  fluid  pressed  out  of  the  ampulla  from  returning 
into  the  radial  canal,  either  because  the  membrane  is  able  by  muscular 
action  to  close  the  aperture,  or  because  the  pocket  surrounding  this 
projecting  membrane  is  swelled  up  by  pressure  of  water  from  the 
foot  or  ampulla,  and  so  closes  the  valve. 

4.  Ophiuroidea. — The  first  point  to    be   noted   with  regard    to 
the  Ophiuroidea  is  that  they  have  no  tube-feet  ampullae. 

The  radial  trunks  of  the  water  vascular  system  run  in  the  arms 
between  the  ventral  shields  and  the  vertebral  ossicles.  At  the  tip 
of  the  arm  each  trunk  ends  in  a  small  terminal  tentacle.  Regularly 
consecutive  and  distinct  widenings  are  found  in  their  courses 
corresponding  with  the  regular  segmentation  of  the  arms.  Between 
every  two  of  these  consecutive  widenings,  the  radial  canal  is  provided 
with  a  single  layer  of  band-like  circular  muscle  fibres.  A  narrow 
tube-foot  canal  runs  off  to  right  and  left  from  each  widening,  running 
either  straight  into  its  tentacle  or  first  forming  a  V-shaped  loop,  which 
ascends  apically  into  the  calcareous  mass  of  the  vertebral  ossicle.  At 
the  point  where  the  tentacle  canal  enters  the  tentacle,  the  lumen  of 
the  former  becomes  much  widened,  and  a  valve  occurs  (similar  to  that 
described  in  the  Asteroids),  which  prevents  a  flowing  back  of  the 
water  vascular  fluid  out  of  the  tube-foot  into  the  radial  canal. 

The  first  two  pairs  of  canals  to  the  tube-feet  or  tentacles  (the  so- 
called  oral  tentacles)  come  direct  from  the  circular  canal. 

5.  Crinoidea.  —  Tentacle  ampullae  are  wanting1.     The  radial 
canals  lie  close  under  the  food  grooves  of  the  disc,  of  the  arms,  and  of 
the  pinnulse,  whose  courses  they  exactly  follow,  so  that  they  branch  just 


vni         ECHINODERMATA—  WATER  VASCULAR  SYSTEM         431 

as  often  as  do  the  arms  and  their  food  grooves.  Their  course  is  more 
or  less  markedly  zigzag,  and  they  give  off  at  the  angles  thus  formed 
(i.e.  alternately)  lateral  tentacle  canals.  Each  of  these  latter  runs  to 
a  group  of  three  small  tentacles  at  the  edge  of  the  food  groove,  and 
here  divides  into  three  canals,  which  enter  the  three  tentacles  and 
form  their  cavities. 

Tentacle  canals  are  wanting  in  all  cases  where  food  grooves  are 
wanting,  which  is  the  case  in  Actinometra  over  a  great  part  of  the 
arms,  and  in  some  species  of  Antedon  in  certain  proximal  pinnulse  of 
the  arms. 

All  authors  agree  in  maintaining  that  the  inner  epithelium  of  the  water  vascular 
system  in  the  Crinoids  differs  from  that  in  all  other  Echinoderms  in  not  being 
ciliated.  A  band  of  longitudinal  muscle  fibres  runs  in  the  wall  of  the  canals  along 
the  side  turned  to  the  food  groove.  The  lumen  of  the  canals  is  at  certain  points  (i.e. 
where  the  tentacle  canals  branch,  or  at  the  commencement  of  these  canals)  traversed 
by  muscular  fibres.  This  arrangement  perhaps  fulfils  the  function  of  the  valves 
found  in  other  Echinoderms. 


D.  The  Ambulaeral  Appendages. 
(Tube-feet,  Tentacles,  Feelers,  Ambulacra!  Papillae,  etc.) 

1.  Holothurioidea. — The  following  facts  require  first  of  all  to  be 
emphasised. 

a.  In  all  Holothurioidea,  a  smaller  or  greater  number  of  ambulacral 
appendages  (10-30)  are  developed  as  tentacles  near  the  mouth. 

b.  The  Synaptidce  and  Molpadiidce  have  no  ambulacral  appendages 
except  these  tentacles. 

c.  In    all    other   Holothurioidea  besides   the  tentacles  there   are 
tube-feet   (and    papillae)    varying   greatly    in    number    (often    very 
numerous),  in  structure,  and  in  arrangement. 

d.  These   tube-feet  (and  papillae)   are  found  either  only  on  the 
radii,  one  or  two  or  more  longitudinal  rows  being  arranged  in  each 
radius,  or  else  they  are  distributed,  usually  in  an  irregular  manner, 
over  some  or  all  of  the  interradii.     The  arrangement  of  the  tube-feet 
is  not  of  great  systematic  importance,  since  even  within  one  and  the 
same  genus   (e.g.  Cmumaria),  all  the  intermediate  stages  between  a 
strictly   radial    and    an    altogether    scattered    arrangement   can    be 
observed. 

e.  Where  the  ventral  and  the  dorsal  surfaces  are  distinctly  differ- 
entiated, the  ambulacral  appendages  are  developed  on  the  ventral  side 
(in  the  trivium),  normally  as  loeomotory  tube-feet  with  sucking  discs 
supported  by  perforated  plates :   on  the  dorsal  side,  on  the  other 
hand,  they  take  the  form  of  conical  non-locomotory  papillae,  which 
have  either  a  rudimentary  perforated  plate  at  the  narrow  tips  or  none 
at  all. 


432  COMPARATIVE  ANATOMY  CHAP. 

No  very  sharp  distinction  between  tube-feet  and  papillae  is,  however,  possible, 
either  with  regard  to  their  distribution,  their  form,  or  their  structure. 

With  regard  to  the  number  of  the  tentacles,  the  following  numbers  seem  to 
prevail  in  the  different  families :  20  in  the  Aspidochirotce,  20  in  the  sub-family 
Deimatidce  of  the  Elasipoda,  15  in  the  Molpadiidcc,  13-16  in  the  Pelagothuridce,  12 
in  the  Synaptidce,  and  10  in  the  Dendrochirotce  and  in  the  sub-family  Elpidiidce- 
of  the  Elasipoda. 

With  regard  to  form  :  the  tentacles  are  feathered  (Molpadiidce  Synaptidce,  Fig. 
229,  p.  288),  dendriform  (Dendrochirotce,  Fig.  226,  p.  287),  and  shield  -  shaped 
(Aspidochirotce,  Elasipoda}.  In  the  latter,  the  disc  or  shield,  the  edge  of  which  may 
be  more  or  less  deeply  indented,  is  carried  by  a  stalk. 

The  size  of  the  tentacles  has  already  been  sufficiently  indicated  in  the  systematic 
review. 

The  relation  between  the  arrangement  and  size  of  the  tentacles  on  the  one  hand 
and  the  symmetry  of  the  rest  of  the  body  on  the  other  is  interesting.  In  the 
Dendrochirotce,  (cf.  Fig.  226,  p.  287),  of  the  ten  tentacles,  the  two  ventral  are  almost 
always  distinguished  by  being  much  smaller  than  the  rest. 

In  many  species  of  Myriotrochus,  Synapta,  and  CMrodota  with  twelve  tentacles, 
these  are  distributed  symmetrically  as  follows  :  three  occur  in  each  of  the  two 
dorsal  interradii,  and  two  in  each  of  the  three  ventral  interradii. 

The  tentacles  may  be  swelled  and  extended  ;  and^on  the^  other  hand  they  can  be 
withdrawn  into  the  body  cavity  together  with  the  surrounding  anterior  part  of 
the  body,  although  not  invaginated  like  the  tentacles  of  a  Gastropod. 

2.  Eehinoidea. — Ambulacral  feet  are  developed  in  all  Echinoids 
without  exception.  In  early  youth,  they  are  always  found  to  resemble 
one  another,  and  in  both  the  Echinidce  and  the  Pourtalesiidce  this 
is  still  the  case  in  adults,  the  former  having  tube-feet  with  terminal 
sucking  discs  and  the  latter  tube-feet  with  rounded  ends.  In  most 
Eehinoidea,  on  the  contrary,  more  or  less  marked  polymorphism 
occurs,  division  of  labour  taking  place  between  the  ambulacral 
appendages  of  one  and  the  same  individual. 

This  polymorphism  is  not  very  striking  in  the  regular  Eehinoidea, 
e.g.  the  Cidaroida,  Echinothuridce,  Diadematidce,  Arbaciidce,  Echinometridce, 
etc.  In  these,  the  ambulacral  appendages  appear,  as  a  rule,  in  three 
different  forms  :  (1)  as  loeomotory  tube-feet  with  terminal  or  sucking 
discs ;  (2)  as  tactile  or  branchial  tentacles  without  terminal  sucker ; 
and  (3)  as  oral  or  sensory  feet  with  bi-lobate  terminal  disc. 

All  these  tube-feet  are  connected  by  means  of  double  pores  with  their  ampullae, 
which  lie  writhin  the  test.  Without  detriment  to  their  principal  function,  they 
may  all  act  as  respiratory  organs,  since  the  presence  of  the  double  pore  allows  of 
a  circulation  of  the  ambulacral  fluid  between  the  inner  ampulla  and  the  outer 
ambulacral  foot ;  the  fluid  in  the  foot  takes  in  oxygen,  carries  it  back  into  the 
ampulla,  and  gives  it  off  through  the  wall  of  the  ampulla  to  the  fluid  in  the  body 
cavity. 

The  loeomotory  tube-feet  are  found  on  the  oral  hemisphere  of  the  body,  but 
may  also  occasionally  occur  on  the  apical  hemisphere  as  well. 

The  tactile  or  branchial  tentacles  are  limited  to  the  apical  hemisphere.  They 
are  specially  suited  for  respiratory  purposes  when  the  ampullae  are  large,  and  have 
thin  and  delicate  walls  containing  no  calcareous  corpuscles. 


vin         ECHINODERMATA—  WATER   VASCULAR  SYSTEM 


433 


The  oral  tube-feet  (always  ten  in  number  ?)  surround  the  mouth,  and  especially 
when  food  is  being  taken  in,  are  subject  to  active  swinging  or  pulsating  movements; 
without,  however,  touching  the  food.  They  have  been  regarded  as  olfactory  or 
gustatory  organs.  They  seem  to  be  wanting  in  the  Cidaroida  and  the  Uchinothuridce  ; 
on  the  other  hand  they  occur  in  those  Eehinidce  which  otherwise  possess  only  one 
sort  of  tube-feet,  viz.  those  with  sucking  discs. 

The  polymorphism  of  the  ambulacral  appendages  is  much  more 
marked  in  the  Cbjpeastroida  and  the  Spatangoida.  It  must  first  be 
noted  that  the  ambulacral  appendages,  in  those  apical  regions  of  the 
ambulacra  which  are  known  as  petaloids  (cf.  p.  347),  serve  for  respira- 
tion (ambulaeral  gills).  They  seem  to  be  peculiarly  fitted  for  this 
activity  by  the  delicacy  of  their  walls,  the  want  of  calcareous  corpuscles, 
the  increase  of  surface  obtained  by  branching,  the  possession  of 
double  pores  (whereas  the  ambulacral  appendages  in  other  parts  of  the 
body  have  single  pores)  and  by  the  size  of  their  ampullae. 

In  the  Clypeastroidat  besides  the  ambulacral  gills  of  the  petaloids,  three  kinds  of 
appendages  have  been  observed  :  (1)  the  ordinary  slender  tube-feet,  with  rounded 
terminal  knobs,  scattered  on  the  test ;  (2)  sessile  knobs,  with  deep  sensory 


FIG.  366.— Longitudinal  section  through  an  ambulacral  brush  of  a  Spatangoid  (after  Loven 
and  Hamann).     1,  Body  epithelium  ;  2,  supporting  rod  ;  3,  supporting  plate  of  the  terminal  disc  ; 
ta  ;  5,  canal  of  the  water  vascular  system ;  6,  longitudinal  muscles ;  7,  nerve ;  8,  circular 
muscle  fibre*. 

epithelium  (sensory  tentacles)  ;  (3)  short,  thick  tube-feet  with  truncated  ends  :  these 
occur  between  the  ordinary  feet  on  the  oral  side,  and  are  perhaps  locomotory. 

Among  the  Spatangoida,  the  polymorphism  of  the  ambulacral  appendages  is  very 
marked  in  all  the  divisions  except  in  the  Echinoncidce  ;  it  reaches  its  highest  point 
in  the  families  of  the  Spatanyidcc  and  Apetala. 

The  ambulacral  gills  of  the  four  paired  petaloids  have  been  described  above. 
We  note  first  the  characteristic  ambulacral  brushes  which  occur  in  the  Spatangoida 
more  or  less  near  the  mouth  and  the  anus,  and  in  the  Cassiduloida  on  the  phyllodes 
(cf.  p.  347).  The  terminal  plate  or  disc  of  an  ordinary  tube-foot  (Fig.  366)  is  here 
extraordinarily  widened,  and  carries  a  number  (usually  large)  of  club-shaped  or 
conical,  solid  appendages,  each  of  which  is  supported  by  a  calcareous  rod.  These 
ambulacral  brushes  are  said  to  play  an  important  part  in  the  taking  in  of  food  by 
VOL.  II  2  F 


434  COMPARATIVE  ANATOMY  CHAP. 

stirring  up  the  sand.  On  other  parts  of  the  ambulacra,  slender  tentacles  without 
prehensile  discs  occur,  to  which  a  tactile  function  has  been  ascribed.  Still  more 
interesting  are  the  ambulacral  appendages  of  the  anterior  unpaired  ambulacrum, 
which  are  certainly  to  a  still  higher  degree  tactile  organs.  These  vary  in  shape  ;  in 
all  young  Spatangoida  and  many  adults  they  are  distinguished  by  their  remarkable 
size,  and  help  to  emphasise  the  bilateral  symmetry  of  the  whole  body.  In  Spatangi'n 
and  other  genera  they  end  in  a  flat  disc,  the  edge  of  which  is  drawn  out  into  short, 
solid,  knobbed  processes,  which  are  supported  by  calcareous  rods.  The  whole 
terminal  disc  thus  looks  like  a  beautiful  rosette. 

As  compared  with  the  ordinary  tube-feet,  truly  gigantic  proportions  are  attained 
in  the  genera  Aceste  and  JErope  by  the  ambulacral  appendages,  which  lie  in  the 
depressed  anterior  ambulacrum  within  the  peripetaloid  fasciole.  They  are  found 
only  in  small  numbers,  yet  in  their  contracted  condition  they  almost  completely  fill 
the  depression  from  whose  base  they  rise.  Their  ends  are  provided  with  large  discs. 

Turning  to  the  finer  structure  of  the  ambulacral  appendages  of  the  Echinoidea, 
their  wall  is  found  to  consist  of  the  typical  layers.  In  the  locomotory  tube-feet  of 
the  regular  Echinoids,  the  inner  layer  of  the  connective  tissue  is  specially  modified 
as  an  elastic  membrane,  with  circular  fibres.  Calcareous  corpuscles  are  wanting 
only  in  the  respiratory  tentacles  of  the  apical  surface  of  the  body.  In  all  other  parts 
of  the  body  they  are  found  in  great  numbers  in  the  stalk,  while  in  the  terminal  discs 
of  the  tube-feet  they  take  the  form  of  delicate,  circular,  terminal  plates  usually  com- 
posed of  several  pieces.  The  whole  surface  of  the  tentacles,  except  that  of  the  terminal 
discs,  is  ciliated.  A  nerve  enters  each  tube- foot,  running  in  the  epithelium  until 
near  the  tip,  Avhere  it  forms  a  lateral  ganglion,  which  can  be  externally  recognised  as 
a  swelling.  From  this  ganglion,  the  terminal  apparatus  of  the  tube-foot  is  innervated. 
In  tube-feet  with  terminal  discs,  the  epithelium  at  the  edge  of  the  disc  is  differen- 
tiated as  a  deep  sensory  epithelium,  and  within  it  runs  a  basal  nerve  ring,  connected 
with  the  lateral  ganglion  by  two  nerves.  Where  the  tentacles  end  in  knobs  (tactile 
tentacles  and  sessile  knobs  of  the  Clypeastroida)  or  carry  knobbed  processes  on  their 
terminal  discs  (ambulacral  brushes,  rosette-like  tube-feet  of  the  anterior  unpaired 
ambulacrum  of  the  Spatangoida]  these  knobs  are  caused  by  the  great  thickening  of 
the  sensory  epithelium.  In  the  ambulacral  gills  of  the  Clypeastridce  (Echinocyamus, 
Echinodiscus)  the  epithelium  occasionally  thickens  to  form  sensory  papillae.  The 
sensory  epithelia  appear  everywhere  to  carry  stiff  sensory  setfe  or  hairs. 

The  lumen  of  the  tube-feet  at  its  central  and  basal  parts  is  not  infrequently 
traversed  by  transverse  muscle  fibres  ;  occasionally  it  appears  to  be  double — a  septum 
consisting  of  transverse  bands  continues  for  some  distance  into  the  tube-foot,  the 
partition  in  the  test  between  the  two  apertures  of  the  double  pore.  The  cavity  of  the 
large  terminal  discs  of  the  paint-brush  tentacles  in  the  Spatangoida  is  traversed  by 
concentric,  much-perforated  septa  (Fig.  366).  The  ambulacral  gills  and  their  ampullae 
are  traversed  by  bands  arranged  radially  around  an  axis,  in  such  a  way  that  the 
fluid  contained  is  forced  to  circulate  at  the  periphery. 

3.  Asteroidea. — The  ambulacral  appendages  always  take  the  form 
of  tube-feet,  and  stand  in  two  or  four  longitudinal  rows  in  the  ambu- 
lacral furrows  which  run  from  the  mouth  to  the  tips  of  the  arms.  It 
has  already  been  pointed  out  (p.  353)  that  the  tube-feet,  even  when 
apparently  present  in  four  rows,  in  reality  belong  only  to  two.  That 
there  are  only  two  rows  is  very  clear  in  young  animals.  In  young 
Asteroids  all  the  tube-feet  are  alike ;  all  have  conical  ends,  with 
rounded  tips.  This  is  still  the  case  in  many  adult  Asteroids  (Astro- 


VIII 


ECHIXODERMATA  —  WATER  VASCULAR  SYSTEM 


435 


pecten,  Luulin,  etc.),  while  in  very  many  other  genera  (e.g.  Asterias, 
Solaster)  a  slight  difference  in  shape  occurs.  Only  the  tube-feet  near 
the  ends  of  the  arms  retain  the  primitive  shape,  while  a  well-developed 
disc  is  found  on  all  the  rest.  The  former  then  function  chiefly  as 
tactile  tentacles. 

The  wall  of  the  tentacles  shows  the  typical  layers.  In  the  tactile  tube-feet,  the 
epithelium  at  the  conical  end  is  much  thickened,  and  contains  very  numerous 
sensory  cells.  Within  the  epithelium,  a  layer  of  nerve  fibres  is  developed  ;  these 
run  from  the  base  to  the  tip  of  the  foot.  The  layer  is  specially  strongly  developed 

i 


10 


FIG.  367.— Portion  of  the  disc  of  Hemipholis  cordifera,  from  the  oral  side  (after  Lyman).  1, 
oral  tube-feet ;  2,  buccal  shields  ;  3,  jaw  =  oral-angle  plates ;  4,  lateral  buccal  shields  ;  5,  first  veutral 
shield  ;  6,  oral  integument,  lip  ;  7,  spines  on  the  marginal  plates  ;  8,  retracted  tentacle  ;  9,  tentacle 
scale  ;  10,  ventral  shields  ;  11,  extended  tentacle  ;  12,  tentacle  pore  ;  13,  tonis  angularis  ;  14,  teeth. 

within  the  terminal  sensory  epithelium.  A  similar  deep  sensory  epithelium,  consist- 
ing of  sensory,  supporting,  and  glandular  cells,  also  covers  the  sucking  discs  of  the 
other  tube-foot  ;  these  discs  have  a  depression  at  their  centres,  while,  round  their 
edges  the  nerve  tissue  lying  within  the  epithelium  becomes  thickened  into  a  nerve 
ring. 

From  the  centre  of  each  sucker,  radial  muscle  fibres  run  out  towards  the  periphery 
and  are  attached  round  the  ambulacral  canal  which  ends  below  the  sucker.  These 
muscles,  by  their  contraction,  cause  the  sucker  to  adhere.  They  are  entirely  dis- 
tinct from  the  longitudinal  muscles  of  the  appendage,  which  explains  the  fact  that 
a  tube-foot  sticking  to  an  object  may  be  cut  off  without  becoming  detached. 

This  applies  almost  equally  well  to  the  sucking  discs  of  the  tube-feet  of  the 
Echinoids. 


436  COMPARATIVE  ANATOMY  CHAP. 

4.  Ophiuroidea  (Fig.  367). — In  the  Ophiuroidea,  the  ambulacral 
appendages  have  no  locoraotory  significance ;  they  resemble  tentacles, 
and  never  have  suckers.     Locomotion  is  caused  by  the  jointed  arms 
themselves.    The  tentacles  are  always  strictly  segmentally  arranged,  i.e. 
one  pair  of  tentacles  occurs  on  each  brachial  segment.      Each  of  these 
emerges  through  an  aperture  between  the  ventral  shield  and  the  lateral 
shield  of  a  segment.      The  tentacles  are  not  infrequently  covered  with 
a  large  number  of  sensory  papilla.     A  nerve,  coming  from  the  basal 
circular  ganglion  and  running  in  the  deeper  portion  of  the  layer  of 
connective  tissue,  traverses  the  tentacle  from  base  to  tip. 

It  has  already  been  mentioned  above  that  the  first  ten  pairs  of 
tentacles  (i.e.  the  first  two  pairs  of  each  arm)  have  shifted,  as  oral 
tentacles,  to  a  position  round  the  mouth,  and  receive  their  canals  direct 
from  the  circular  canal. 

5.  Crinoidea. — The   small   tentacles  which   form  the   ambulacral 
appendages  of  this  class  have  already  been  sufficiently  noticed  (p.  414). 
They  never   possess   suckers,  and   have   no  locomotory  function,  but 
simply  serve  for  respiration,  and  for  conducting  food  to  the  mouth. 


VII.  The  Ccelom. 
(The  Enterocoel.  the  true  or  secondary  Body  Cavity.) 

All  those  cavities  of  the  body  which  are  derived  from  the  entero- 
eoelomie  vesicles  of  the  larva  are  considered  to  belong  to  the  coelom, 
which  is  lined  throughout  with  endothelium,  usually  developed  as 
ciliated  epithelium.  The  ecelomie  fluid  exactly  resembles  in  constitu- 
tion the  water  vascular  fluid  already  described.  The  ccelom  is,  how- 
ever, except  at  one  single  point  to  be  mentioned  later,  altogether 
separate  from  the  ambulacral  vascular  system. 

The  coelom  is  never  found  as  a  single  cavity,  but  is  always  divided 
into  several  cavities,  which  may  be  entirely  distinct  one  from  the 
other.  The  largest  of  these  cavities  is  that  which  contains  the  viscera, 
and  which  may  be  termed  simply  the  body  cavity. 

The  body  cavity  is  most  spacious  in  the  Echinoidea  and  the  Holothu- 
rioidea  ;  in  these  forms  it  occupies  almost  the  whole  cavity  of  the  test, 
or  the  sac-  or  tube-shaped  body.  In  the  disc  of  the  Asteroidea  it  is 
somewhat  less  spacious,  and  is  very  limited  in  that  of  the  Ophiuroidea. 
In  the  Crinoidea,  it  is  traversed  by  a  more  or  less  strongly  calcified 
network  of  connective  tissue. 

Where  the  body  is  drawn  out  into  arms  in  the  radii,  the  body 
cavity  runs  into  these,  and  forms  the  braehial  cavities.  The  brachial 
cavities  in  the  Asteroidea  are  very  spacious,  but  are  much  narrowed  in 
the  Ophiuroidea  and  the  Crinoidea,  owing  to  the  great  development  of 
skeletal  plates  (vertebral  ossicles,  joints)  in  the  arms. 

A  special  section  of  the  ccelom,  the  pericesophageal  sinus  (peri- 
pharyngeal  sinus)  encircles  the  oesophagus  or  pharynx.  In  the  Echi- 


ECHINODERMATA—THE  CCELOMIC  CAVITIES  437 

f,  this  is  quite  cut  off  from  the  body  cavity.  The  membrane 
which  separates  the  body  cavity  from  the  perioesophageal  sinus  is 
called,  in  those  Echinoids  which  are  provided  with  a  masticatory  frame- 
work (Cidaroida,  Diadematoidu,  and  Clypeastroida),  the  lantern  mem- 
brane. This  membrane  entirely  covers  the  lantern  on  the  side  turned 
to  the  body  cavity. 

In  many  Echinoids,  this  part  of  the  ccelom  protrudes  externally 
at  the  edge  of  the  peristome,  forming  the  outer  gills  :  in  others, 
the  lantern  membrane  bulges  out  into  the  body  cavity,  and  forms 
Stewart's  organs. 

In  the  Holothurioidea  and  Echinoidea,  the  hind-gut  is  surrounded  by 
a  small  ccelomic  sinus,  the  perianal  sinus. 

In  the  E'-hinoidea,Asteroidea,  and  Ophiuroidea,  a  part  of  the  coelom, 
cut  off  from  the  rest,  runs  from  the  region  of  the  madreporite  inter- 
radially  to  the  circular  canal  of  the  water  vascular  system.  This  is 
the  axial  sinus  in  which  the  stone  canal  runs.  It  contains  also  a 
lymph  gland,  the  so-called  ovoid  gland  or  axial  organ. 

The  axial  sinus,  in  the  Echinoids,  is  in  open  communication  with 
the  ampulla  which  lies  beneath  the  madreporite.  Recent  ontogenetic 
researches  have  shown  that  this  ampulla  also  is  of  enteroccelomic  origin, 
and  is  thus  a  section  of  the  coelom.  Since  the  stone  canal  opens  into 
the  ampulla,  an  open  communication  exists  at  this  point,  and  at  this 
alone,  between  a  closed  division  of  the  coelom  (the  axial  sinus)  and  a 
section  of  the  water  vascular  system  (the  stone  canal). 

A.  The  Body  Cavity. 

1.  Holothurioidea. — The  spacious  body  cavity  of  the  Holothurioidea 
is  divided  up  by  the  mesentery  which  attaches  the  intestine  to  the 
body  wall.  This  mesentery  may  be  described  as  having  three  parts 
corresponding  with  the  three  sections  of  the  intestine,  viz.  the  dorsal 
mesentery,  belonging  to  the  first  section  of  the  intestine  which  runs 
backward ;  the  left  dorsal  mesentery,  belonging  to  the  second 
section,  which  bends  forward ;  and  the  right  ventral  mesentery, 
belonging  to  the  third  section,  which  runs  backward  to  the  cloaca. 
All  three  parts  of  the  mesentery  lie  interradially. 

The  so-called  ciliated  urns  or  funnels  (Fig.  368)  are  found  only 
in  the  Synaptida.  These  are  funnel-,  cup-,  or  slipper-shaped  organs, 
each  of  which  is  attached  to  the  body  wall  or  the  mesentery  by  a  stalk, 
and  hangs  down  freely  into  the  bod}T  cavity.  They  are  specially 
numerous  to  the  right  and  left  of  the  dorsal  mesentery.  In  Chtrodota, 
many  funnels  have  one  common  stalk,  and  so  form  trees  of  ciliated 
funnels. 

The  funnel  consists  of  three  layers,  an  outer  endothelial  tessellated  epithelium,  a 
middle  and  extremely  thin  layer  of  connective  tissue,  and  an  inner  layer  of  columnar 
epithelium,  which  lines  the  lumen  and  carries  long  cilia.  Towards  the  stalk 
the  lumen  is  closed,  but  it  is  open  towards  the  body  cavity. 


438 


COMPARATIVE  ANATOMY 


CHAP. 


The  vigorous  movements  of  the  ciliated  urns  no  doubt  serve  to  promote  the 
streaming  and  circulation  of  the  fluid  in  the  body  cavity. 

2.  Eehinoidea. — In  the  Echinoids,  the  body  cavity  is  partitioned 
in  a  manner  similar  to  that  described  for  the  Holothurioidea,  by  mesen- 
teries which  follow  the  intestine 
in  its  windings  (see  p.  480),  and 
attach  it  to  the  inner  surface  of 
the    test.       The    genital    organs 
are  also  attached  to  the  test  by 
mesenteries.     In  regular   Echin- 
oids, the  mesenteries  are   much 
perforated,  but  are  only  slightly, 
if  at  all,  broken  through  in  the 
Spatangoida. 

In  this  latter  order,  where 
the  mesenteries  have  to  carry 
the  heavy  intestine,  filled  with 
sand,  they  are  specially  strong 
and  tough.  The  coils  of  the 
intestine  are  here  also  united 
inter  se  by  mesenteries.  Special 
bands  attach  the  intestine  to  the 
apical  and  oral  poles  of  the  test, 

Fir  368 -Ciliated  urns  of  a  Synaptid  (after  internal    processes    or    apophyses 

Cuenot).     1,  mesentery ;  2,  circular  muscle  layer  ,     .  .  ,         i         j     * 

of  the  body  wall ;  3,  ciliated  inner  epithelium  of  being    Sometimes    developed     lor 

the    urn;   4,  endothelium    of  the   body   cavity;  the     attachment     of      the     bands. 

Two  ^h  apophyses  are  found 
at  the  apical  pole,  at  the  end  of 
the  stone  canal,  and  a  third  not  infrequently  occurs  at  the  peristome, 
in  an  interradius. 

The  axial  sinus,  with  the  axial  organ  and  the  stone  canal,  is  at- 
tached by  bands  on  the  one  hand  to  the  apical  pole,  and  on  the 
other  to  the  oesophagus. 

For  a  description  of  the  calcareous  pillars,  septa,  etc.,  which,  in  the  Clypeastridce, 
traverse  the  cavity  of  the  test,  see  p.  405. 

In  the  fluid  of  the  body  cavity  in  Echinoids  there  are  found,  besides  blood  cor- 
puscles, great  numbers  of  spermatozoa-like  cells,  with  long  flagella  in  vigorous  move- 
ment. These  may  set  up  currents  in  the  fluid  of  the  body  cavity. 

3.  Asteroidea. — The  body  cavity  of  the  disc  is  not  spacious,  the 
greater  part  of  it  being  filled  by  the  large  digestive  sac.     Mesenteries 
are  wanting  in  the  greater  part  of  the  intestine,  or  are  only  developed 
as  isolated  filaments  or  strands  of  connective  tissue.     In  the  peripheral 
portion  of  the  disc,  radially  placed  bands  or  septa  traverse  the  body 
cavity  vertically  in  the  interradii,  connecting  the  dorsal  (apical)  body 
wall  with  the  ventral  (oral)  wall. 


UNI 

ECHINOD£RMATA—THE  CCELOMIC  CAVITU 

The  lymph  gills,  branchial  vesicles  or  papulae,  which  are  only 
found  in  the  Asteroids,  deserve  attention.  These  are  small  vesicular 
bulgings  of  the  body  wall,  which  occur  in  great  numbers  between  the 
skeletal  plates.  On  these  bulgings,  the  body  wall  is  very  thin,  and,  in 
order  to  facilitate  osmosis,  devoid  of  calcareous  deposits.  It  consists 
of  layers  similar  to  those  found  in  other  parts  of  the  body :  an  outer, 
strongly  ciliated  and  glandular  epithelium  ;  a  middle  layer  of  con- 
nective tissue,  containing  longitudinal  and  circular  muscle  fibres ;  and 
an  inner  ciliated  epithelium,  which  is  nothing  else  than  the  endothelium 
of  the  body  cavity.  The  cavities  of  the  papulae  are  merely  diverticula 
of  the  body  cavity  which  bulge  out  the  much-thinned  body  wall. 

The  papulae  are  sensitive,  and  contract  at  the  slightest  touch. 

In  Asteroids  with  very  thick  body  wall,  diverticula  of  the  body  cavity  force  their 
way  into  it,  branching  on  their  way  to  the  surface.  On  reaching  this  latter,  each 
branch  enters  a  branchial  vesicle. 

In  certain  forms,  a  single  diverticulum  traversing  the  body  wall  supplies  a  whole 
group  of  branchial  vesicles. 

A  constant  streaming  to  and  fro  of  the  body  fluid  can  easily  be  observed  in  the 
branchial  vesicles. 

Each  branchial  diverticulum  of  the  body,  cavity  is  surrounded,  in  the  connective 
tissue  layer  of  the  body  wall,  by  a  circular  lacuna. 

The  branchial  vesicles  occur  both  011  the  arms  and  on  the  disc.  In  the  Phanero- 
:<niia  they  are  found  only  on  the  upper  side  of  the  body  ;  in  the  Cryptozonia,  "on  the 
contrary,  they  occur  on  the  sides  of  the  arms  as  well,  and  on  the  lower  (oral)  side  of 
the  body. 

4.  Ophiuroidea. — The  body  cavity  of  the  disc  is  much  limited 
by  the  digestive  sac  and  the  bursa?.  Filaments  and  bands  of  con- 
nective tissue,  covered  with  endothelium,  traverse  it  at  irregular 
intervals,  and  connect  the  viscera  with  the  body  wall. 

o.  Crinoidea. — The  body  cavity  of  the  calyx  is  almost  completely 
filled  with  bands,  trabeculas,  filaments,  etc.,  of  connective  tissue  covered 
with  endothelium,  which  together  form  a  spongy  network,  and  often 
become  calcified.  In  this  network  a  sac -like  membrane  becomes 
differentiated,  which  divides  the  body  cavity  into  a  central  and  a 
peripheral  space.  The  central  space,  which  contains  the  intestine,  is 
known  as  the  peri-intestinal  cavity ;  the  peripheral,  as  the  subtegu- 
mentary  cavity.  The  peri-intestinal  cavity,  again,  contains  another 
separate  portion  of  the  coelom,  the  axial  body  cavity,  round  which  the 
intestine  coils  itself.  This  latter  cavity  encloses  the  genital  stolon 
and  communicates,  on  the  one  hand,  with  the  five  chambers  of  the 
chambered  organ  which  lies  in  the  apex  of  the  calyx,  and  through  it 
with  the  coelomic  canals  of  the  stalk  and  the  cirri ;  and  on  the  other, 
with  the  oral  or  subtentacular  canals  of  the  arms.  The  peri-intestinal 
cavity,  on  the  contrary,  is  continued  into  the  dorsal  or  apical  brachial 
canals. 


440  COMPARATIVE  ANATOMY  CHAP. 


B.  The  Braehial  Cavities. 

1.  Asteroidea. — The  body  cavity  of  the  disc  is  produced  in  the 
form  of  large  cavities  which  run  along  the  arms  to  their  tips.     The 
body  cavity  is  thus  found  throughout  the  whole  of  that  part  of  the 
arms  which  is  enclosed  by  the  skeletal  plates,   and  contains  (1)  the 
ampullae  of  the  water  vascular  system,  (2)  the  two  radial  caeca  of  the 
stomach,  and   finally  (3)  a   part   of   the  genital  glands  as  well.     The 
radial  caeca  of  the  stomach  (two  of  which  occur  in  each  arm)  are  each 
attached  to  the  dorsal  wall  of  the  arm  by  two  suspensors,  which  run 
in  the  longitudinal  direction,  so  that,  above  each  caecum  there  lies  a 
ccelomic  canal  whose  walls  are  formed  (1)  dorsally  by  the  brachial 
wall,  (2)  ventrally  by  the  Avail  of  the  caecum,  and  (3)  laterally  by  the 
two  suspensors  (cf.  Fig.  354,  p.  411). 

2.  Ophiuroidea  (Fig.  355,  p.  412). — The  vertebral  ossicles  occupy 
so  large  a  part  of  the  transverse  section  of  the  arm,  that  only  a  very 
small  space  is  left  for  the  brachial  ccelom.     This  latter  is  found  as  a 
flat  cavity  below  the  dorsal  wall  of  the  arm.     It  is  divided  into  con- 
secutive chambers,  which  agree  in  number  with  the  segments  of  the 
arm ;  these  chambers  are  incompletely  separated  from  one  another  by 
transverse,    vertical,    calcareous    septa,   which    connect    the    vertebral 
ossicles  with  the  outer  skeletal  plates  of  the  arm.     These  septa  leave 
a  mediodorsal  space  free,  through  which  all  the  chambers  are  in  open 
communication  with   one   another.     This    is    the    "dorsal   canal"   of 
authors. 

The  endothelium  of  the  brachial  cavity  is  thickened  in  the  dorsal  middle  line, 
and  carries  specially  strong  cilia.  There  is  thus  in  each  arm  a  longitudinal  ciliated 
band  or  streak,  which,  when  it  reaches  the  disc,  passes  into  the  ordinary  endo- 
thelium. The  activity  of  these  strong  cilia  originates  and  maintains  the  circulation 
of  the  body  fluid  in  the  brachial  cavity.  Occasionally  the  ciliated  streak  is  deepened 
into  a  groove. 

3.  Crinoidea  (Fig.  356,  p.  413). — In  this  class  also  the  brachial 
cavity  is  much  reduced  by  the  strong  development   of  the  skeletal 
joints  of  the  arms;  unlike  that  of    the  Ophiuroidea,   however,  it  is 
found  on  the  ventral  side  of  the  arm.     The  cavity  is  divided  by  a 
horizontal  longitudinal  septum  into  two  canals,  one  lying  above  the 
other ;  both  of  these  run  through  the  arm  and  its  branches  as  far  as 
to   the   tips   of   the   pinnulae.      The   dorsal  (apical)  ccelomic  canal    is 
known  as  the  dorsal,  and  the  ventral  (oral)  canal  as  the  ventral  or 
subtentaeular  eanal.     The   latter,  on  reaching  the  disc,  enters  the 
axial  ccelom,  the  former  enters  the  peri-intestinal  cavity.     The  ventral 
canal  itself,  again,  is  divided  by  a  vertical  longitudinal  septum  into  two 
lateral  canals. 

The  endothelium  of  the  dorsal  canal  occasionally  (especially  in  the  pinnule) 
shows  small  sac-like  bulgings,  which  are  known  as  ciliated  baskets  or  sacs.  The 


vin 


ECHINODERMATA  —  THt:  CCELOMIC  CAVITIES 


441 


floors  of  these  sacs  consist  of  flat  cells,  but  the  epithelium  round  their  openings  into 
the  dorsal  canal  is  much  thickened,  and  provided  with  large  cilia.  These  ciliated 
sacs  undoubtedly  serve  the  same  purpose  as  the  ciliated  bands  in  the  brachial 
cavities  of  the  Ophiuroidea  and  the  urns  of  the  Synctptidce. 


C.  The  Pericesophageal  Sinus. 

1.  Holothurioidea  (Fig.  365,  p.  428). — Between  the  mouth  and 
the  water  vascular  ring  the  oesophagus  is  surrounded  by  a  membrane- 
like  sheath,  in  such  a  way  that  between  it  and  this  membrane  a 
narrow  cavity  is  left ;  this  is  the  perioesophageal  sinus,  which  is  a 
section  of  the  ccelom.  The  radial  canals  of  the  water  vascular  system 
run  along  its  outer  side,  and  in  the  radial  direction  it  is  traversed  by 
numerous  bands  and  filaments  which  are  attached  on  the  one  hand  to 
the  cesophageal  wall,  and  on  the  other  to  the  outer  membrane  of  the 


10  9 

FIG.  369.— Median  section  through  the  oral  region  of  Spatangus  purpureus  (after  Cue'not). 
Ig,  Posterior  unpaired  interradius  ;  RHI,  anterior  unpaired  radius.  1,  Radial  canal  of  the  water 
vascular  system  ;  2,  radial  blood  vessel ;  3,  radial  pseudohfemal  canal ;  4,  radial  nerve  trunk  ;  5, 
radial  epineural  canal ;  6,  test ;  7,  body  epithelium  ;  S,  mouth  ;  9,  epineural  circular  canal ;  10, 
circular  nerve  ;  11,  endothelium  of  the  body  cavity  ;  12,  pericesophageal  sinus ;  13,  water  vascular 
ring  ;  14,  blood  vascular  ring  ;  15,  oesophagus  ;  1(5,  membrane  which  separates  the  perioesophageal 
sinus  from  the  body  cavity ;  17,  septum,  which  separates  the  pseudohfemal  canal  from  the  perioeso- 
phageal sinus. 

sinus.  These  are  only  wanting  in  the  part  near  the  mouth,  which  is 
thus  distinct,  and  is  known  as  the  peribuceal  sinus.  The  periceso- 
phageal  sinus  is  usually  in  open  communication  with  the  common  body 
cavity  by  means  of  a  varying  number  of  apertures  in  its  outer  mem- 
brane. In  Ciiai.mo.ria  there  are  five  such  apertures,  which  are  large 
and  interradial  in  position.  In  the  Elasipoda  alone  is  the  periceso- 
phageal sinus  completely  separated  from  the  general  body  cavity  by 
an  uninterrupted  outer  membrane,  which  runs  from  the  water  vascular 
ring  direct  to  the  body  Avail. 

2.  Echinoidea. — In  the  Spatungoida  (Fig.  369)  a  membrane, 
which  runs  out  horizontally  from  the  commencement  of  the  intestine 
to  surround  the  circular  canal  of  the  water  vascular  system,  completely 
separates  a  very  small  perioesophageal  sinus  of  the  coelom  from  the 
spacious  body  cavity.  In  those  Echinoids  which  are  provided  with  a 
masticatory  framework,  this  latter  develops  within  this  sinus,  which 


442  COMPARATIVE  ANATOMY  CHAP. 

thus  attains  a  large  size.  The  membrane  which  completely  separates 
the  perioesophageal  sinus  from  the  general  body  cavity  then  becomes 
the  lantern  membrane,  surrounding  the  masticatory  apparatus  on  all 
sides,  from  the  point  where  the  intestine  leaves  the  lantern  down  to 
the  perignathous  apophysial  ring.  The  greater  part  of  the  perioeso- 
phageal sinus  (Fig.  358,  44,  p.  419)  is  filled  by  the  masticatory  frame- 
work, the  remaining  space  being  traversed  by  trabeculse,  bands,  etc. 
All  the  radial  organs  arise  from  within  the  perioesophageal  sinus, 
running  in  it  as  far  as  to  the  auricula?. 

The  outer  gills  and  Stewart's  organs  of  the  Echinoids  are  develop- 
ments from  the  perioesophageal  sinus. 

The  outer  gills  (Fig.  358,  1)  consist  of  five  pairs  of  branched 
appendages,  which  rise  at  the  periphery  of  the  oral  region,  at  the 
inner  edge  of  the  oral  integument,  and  project  freely  outward.  One 
pair  of  such  gills  occurs  on  each  interradius.  The  peristomal  edge  of 
the  disc  is  indented  at  certain  points  for  the  reception  of  the  gills,  so 
that  their  presence  or  absence  may  be  determined  by  the  presence 
or  absence  of  these  incisions  on  the  test  of  either  extant  or  fossil 
Echinoids.  The  gills  are  hollow  outgrowths  of  the  oral  integument, 
their  cavities  being  direct  prolongations  of  the  pericesophageal  sinus, 
and  thus  in  open  communication  with  this  latter ;  the  body  fluid  of 
the  sinus  can  thus  enter  the  gills  and  flow  back  into  the  sinus  again. 
The  wall  of  such  a  gill  consists  of  a  deep  outer  epithelium  provided 
with  long  cilia,  a  central  layer  of  connective  tissue  with  calcareous 
corpuscles  and  lacunae,  and  an  inner  ciliated  covering  of  endothelium. 

External  gills  occur  in  most  endocyclic  (regular)  Echinoids.  They 
seem  to  be  wanting  only  in  the  Cidaroida. 

Stewart's  organs. — Just  as  external  bulgings  of  the  oral  integu- 
ment lead  to  the  formation  of  external  gills,  internal  bulgings  of  the 
lantern  membrane  into  the  body  cavity  give  rise  to  Stewart's  organs. 
These  are  delicate-skinned  vesicles  or  tubes,  which  vary  greatly  in  size. 
Five  are  usually  present,  projecting  into  the  body  cavity  from  the 
edge  of  the  (apically  directed)  base  of  the  masticatory  framework, 
immediately  below  the  fork  plates  or  radii  between  these  and  the 
falces  or  intermediate  plates  (cf.  p.  400).  The  cavity  of  the  vesicle 
is  a  diverticulum  of  the  pericesophageal  sinus. 

The  Stewart's  organs  of  the  Cidaroida  are  large,  and  are  usually  beset  with 
secondary  outgrowths.  Since  the  Cidaroida  possess  no  gills,  it  has  been  conjectured 
that  Stewart's  organs  fulfil  the  functions  of  the  absent  external  gills,  and  they  have 
therefore  been  called  internal  gills.  It  is,  however,  very  difficult  to  demonstrate 
with  any  certainty  the  respiratory  significance  of  these  organs. 

In  the  Echinothuridce,  although  external  gills  are  present,  Stewart's  organs  may 
attain  a  gigantic  size  (Astlienosoma  iirens,  Fig.  370).  They  here  fill  the  greater 
part  of  the  body  cavity,  and,  it  has  been  conjectured,  serve  in  this  case  to  prevent  the 
collapse  of  the  flexible  test  at  the  time  when  the  genital  products  are  ejected.  On 
the  other  hand  they  may  be  quite  vestigial,  or  even  altogether  absent  (Phormosoma).1 

1  Of.  Bell,  Ann.  Mag.  N.  II .  vol.  iv.  1889,  p.  437. 


VIII 


ECHINODERMATA  —  THE  CCELOMIC  CAVITIES 


443 


In  certain  Clypeaitroida  (Echinodiscus  biforis,  Peronella  orbicularis)  small  inter- 
radial,  thin -walled,  vesicular  outgrowths  of  the  lantern  membrane,  on  the  base  of 
the  lantern  itself,  have  been  called  Stewart's  organs.  Two  of  these  occur  in  each 


FIG.  370.— Viscera  of  Asthenosoma  (after  F.  and  P.  Sarasin).  1,  Gonads  ;  2,  constriction  in  a 
Stewart's  organ ;  3,  Stewart's  organ  ;  4,  muscle  lamellae ;  5,  radial  canal  of  the  water  vascular 
system  ;  6,  tip  of  a  Stewart's  organ  ;  7,  forked  radius  of  the  masticatory  framework  ;  8  and 
'.'.  upper  and  lower  coils  of  the  intestine ;  10,  Polian  vesicle  ;  11,  intestine. 

interradius,  but  are  generally  wanting  in  that  interradius  in  which  the  intestine  in 

ascending  lies  upon  the  lantern. 

3.  Ophiuroidea. — Two  circular  membranes,  one  above  the  other, 
found  round  the  oesophagus,  and  traversing  the  body  cavity,  connect 
the  oesophagus  with  the  oral  skeleton.  They  thus  cut  off  two  very 
small  pericesophageal  sinuses  from  the  general  body  cavity. 


444  COMPARATIVE  ANATOMY  CHAP. 


D.  The  Perianal  Sinus. 

In  the  Holothurioidea  (with  the  exception  of  the  Synaptida?)  and  in 
the  Echinoidea  the  end  of  the  hind-gut  is  connected  with  the  neigh- 
bouring body  wall  by  means  of  a  circular  membrane,  which  cuts  off 
a  small  perianal  sinus  from  the  general  body  cavity.  If  the  circular 
muscle  fibres,  with  which  the  walls  of  this  sinus  are  abundantly  sup- 
plied, contract,  they  act  as  sphincters,  and  close  the  anus.  In  the 
regular  Echinoids,  below  the  perianal  sinus,  there  is  a  second  closed 
sinus  surrounding  the  hind-gut ;  this  is  the  periproetal  sinus. 


E.  The  Axial  Sinus. 

1.  Asteroidea. — In  the  madreporitic  interradius  the  general  body 
cavity  is  traversed  by  a  large,  vertical,  flattened  tube,  with  tough,  flat, 
radially  arranged  lateral  walls.      This  tube  connects  the  region  of  the 
madreporite  with  the  ventral  body  wall.     The  cavity  of  the  tube  is  an 
enclosed  portion  of  the  true  body  cavity,  and  in  a  young  stage  is  in 
open  communication  with  the  enterocoel ;   it  is  known  as  the  axial 
sinus  (sac-  or  tube-like  canal,  sac  hydrophorique).     It  surrounds  and 
contains  (1)  the  stone  canal,  which  runs  down  from  the  madreporitic 
plate  to  the  circular  canal ;    and  (2)  the  axial   organ  (dorsal  organ, 
heart,  pseudo-heart),  which    is   attached   to   its  wall   by   means   of   a 
mesentery.     The  tough  wall  of  the  axial  sinus  consists  of  the  follow- 
ing layers:   (1)  the  ciliated  endothelium  of  the  body  cavity  (on  the 
side  facing  that  cavity) ;  (2)  longitudinal  muscle  fibres  ;  (3)  connective 
tissue ;  (4)  the  inner  ciliated  epithelium  lining  the  axial  sinus.     The 
axial  sinus  opens  dorsally  into  the  aboral  circular  canal  (circular 
sinus)  of  the  genital  system. 

2.  Ophiuroidea  (Fig.  361,  p.  422). — In  consequence  of  the  shifting 
of  the  madreporite  on  to  the  oral  side,  the  stone  canal  which  springs 
out  of  the  circular  canal  bends  outward  and  downward.      At  its  distal 
end  it  is  connected  with   a  small    ccelomic    sinus,  usually  called  an 
ampulla,  which  lies  on  the  side  turned  to  the  centre  of  the  disc,  and 
itself  opens  outward  through  the  water  vascular  pore.      This  sinus  no 
doubt  corresponds  with  the  axial  sinus  of  Asteroids.     Another  sinus 
accompanies  the  stone  canal  on  the  side  turned  to  the  periphery  of  the 
disc,  and  opens  into  the  circular  canal  of  the  genital  system.     That 
portion  of  the  wall  of  this  second  canal  which  is  in  contact  with  the 
stone  canal  is  developed  as  the  ovoid  gland. 

3.  Eehinoidea  (Fig.  358,  p.  419). — The  axial  sinus,  which  runs 
up  from  the  circular  canal  to  the  apex  and  is  accompanied  by  the 
stone    canal,    is    here    almost    completely   filled    by   the   large    axial 
organ.     It  is  completely  cut  off  by  a  septum  from  a  spacious  sinus 
which  lies  near  the  ampulla,  and  into  which  a  process  of  the  axial 
organ   projects.      The   two   communicate   only  in    an   early  stage  of 


vin  ECHINODERMATA—THE  AXIAL  ORGAN  445 

development.  Further,  the  communication  which  always  primitively 
exists  between  the  axial  sinus  and  the  aboral  circular  sinus  of  the 
genital  system  is  interrupted  in  adult  Echinoids,  the  only  known 
exception  to  this  rule  being  Eddnocymniis  pusillus. 

4.  Crinoidea. — In  the  Comatulidce,  an  axial ,  section  of  the  body 
cavity  round  which  the  intestine  becomes  coiled  is  said  to  exist.      In 
other  Crinoids,  such  an  axial  sinus  seems  to  be  wanting,  or  else  is  filled 
with  connective  tissue.     There  is,  in  adults,  no  connection  between  this 
sinus  and  the  stone  canals.     In  the  direction  of  the  principal  axis, 
however,  the  axial  sinus  is  traversed  by  a  dorsal  (glandular)  organ, 
which,  although  of  somewhat  different  structure,  no  doubt  corresponds 
with  the  axial  organ  of  other  Echinoderms.     This  homology  seems  to 
be  established  by  the  fact  that  the  dorsal  organ  of   the  Crinoids  shows 
relations  to  the  genital  system  similar  to  those  of  the  axial  organs  in 
the   Echinoidea,  the  Asteroidea,  and   the  Ophiuroidea  to   the    same 
system. 

5.  Holothurioidea. — In  this  class  there  is  no  axial  sinus  cut  off    ^ 
from  the  general  body  cavity. 


F.  The  Axial  Organ. 

(Dorsal  Organ.  Heart,  Pseudo-heart,  Kidney,  Plastidogenic  Organ,  Ovoid  Gland, 

Lymph  Gland. ) 

Xo  other  organ  of  the  Echinoderm  has  given  rise  to  so  many  contra- 
dictory statements  as  the  axial  organ.  The  names  given  in  the  above 
heading,  which  have  been  gathered  from  various  authors,  show  what 
different  functions  have  been  ascribed  to  it. 

According  to  the  most  recent  anatomical  and  ontogenetic  researches, 
the  following  points  may  be  stated  with  some  degree  of  certainty. 

(a)  The  axial  organ  lies  on  or  in  the  axial  sinus. 

(b)  It  is  developed  from  the  endothelium  of  the  body  cavity,  and, 
during  early  ontogenetic  stages,  it  grows   out   in   the  shape  of  pro- 
cesses, strands,  or  tubes,  which,  at  certain  definite  points  of  the  body, 
become  the  gonads  (ovaries  and  testes). 

(c)  Further,  in  the  adult  animal,  the  axial  organ  is  in  most  cases 
still  connected  with  the  genital  system,  functioning,  however  (at  least 
in  the  Asteroidea,  Ophiuroidea,  and  Echinoidea),  in  all  probability,  as  a 
lymph  gland. 

The  Holothurioidea  appear  to  possess  no  axial  organ. 

In  the  Asteroidea,  Ophiuroidea,  and  Echinoidea,  the  axial  organ 
consists  of  a  network  of  connective  tissue,  in  whose  meshes  (embedded 
in  blood  plasm)  round  cells  lie,  which,  by  continual  division,  yield 
lymph  corpuscles. 

1.  Asteroidea. — The  axial  organ  lies  in  the  axial  sinus,  to  the  wall  of  which  it 
is  attached  by  a  mesentery.  Below  the  madreporite  it  sends  off  a  process  into  a 
small  cavity,  which  is  completely  cut  off  from  the  axial  sinus.  Further,  it  pro- 


446  COMPARATIVE  ANATOMY  CHAP. 

trades  at  certain  points  through  the  wall  of  the  axial  sinus  into  the  general  body 
cavity. 

2.  Ophiuroidea. — The  axial  organ  is  developed  out  of  the  wall  of  the  sinus  which 
accompanies  the  stone  canal  on  the  side  which  is  turned  towards  the  periphery  of 
the  disc  ;  and,  further,  out  of  that  portion  of  the  wall  which  is  in  contact  with  the 
stone  canal.     It  projects  as  a  somewhat  massive  body  into  the  sinus,  occupying 
almost  its  entire  lumen  (cf.  Fig.  361,  8,  p.  422). 

3.  Echinoidea  (Fig.  358,  32,  p.  419). — The  axial  organ  lies  in  the  axial  sinus, 
which  it  almost  completely  fills,  and  to  the  wall  of  which  it  is  attached  by  means  of 
numerous  strands.     It  sends  off  a  process  into  the  sinus  which  lies  below  the  madre- 
porite  near  the  ampulla,  this  process  perforating  the  wall  which  divides  this  sinus 
from  the  axial  sinus. 

4.  Crinoidea  (Fig.  384  gp,  p.  482). — The  axial  organ,  which  is  in  this  class 
known  as  the  genital  stolon  (or  the  glandular  organ  or  the  dorsal  organ),  is  here 
differently  constructed.     It  originates  as  a  thin  strand  in  the  axis  of  the  chambered 
organ,  then  ascends  direct  through  the  axial  section  of  the  body  cavity  of  the  calyx 
towards  the  mouth,  widening  in  the  first  part  of  its  course,  and  then  again  narrowing. 
It  consists  of  a  complex  of  much-twisted  canals  with  narrow  lumina,  which  are  en- 
closed in  a  stroma  of  connective  tissue  ;  the  lumina  of  these  canals  may  disappear, 
and  they  may  become  strands.     They  are  lined  with  cylindrical  epithelium.     In  the 
axis  of  the  chambered  organ,  the  axial  organ  consists  merely  of  a  few  very  thin  strands 
or  canals,  but  when  it  leaves  the  chambered  organ  and  ascends  into  the  body  cavity, 
the  canals  swell  and  branch,  so  that  their  number  increases  till  the  middle  of  the  body 
cavity  is  reached.     Then  their  number  again  decreases,  one  canal  after  the  other 
ending  blindly.     Finally,  in  the  oral  region,  the  axial  organ  consists  of  only  a  few 
strands,  which  most  probably  are  continued  into  the  genital  tubes  or  strands  of 
the  arms.    Such  a  connection  has  at  least  been  demonstrated  in  the  young  Antedon, 
and  it  has  been  further  found  that,  ontogenetically,  the  genital  tubes  bud  out  of  the 
axial  organ. 

G.  The  Chambered  Sinus  of  the  Crinoidea  and  its  continuation  in 
the  Stalk  and  in  the  Cirri. 

Quite  in  the  apex  of  the  calyx,  and  in  Antedon  enclosed  in  the 
centrodorsal,  there  is  a  cavity  containing  the  apical  parts  of  the 
axial  organ.  This  cavity  is  of  enterocoelomic  origin.  It  is  divided, 
by  means  of  five  radially  arranged  partitions  of  connective  tissue,  into 
five  chambers,  which  are  covered  on  all  sides  by  epithelium.  This  is 
the  "chambered  organ"  (Fig.  384  cli,  p.  482). 

In  stalked  Crinoids  the  chambered  sinus  is  continued  into  the 
stalk,  forming  in  it  a  canal.  This  is  also  divided,  by  five  radially 
arranged  partitions,  into  five  sub-canals  (the  continuations  of  the  five 
chambers  of  the  sinus)  arranged  round  a  common  axis.  This  common 
axis  is  probably  formed  by  a  continuation  of  the  axial  organ. 

In  each  of  the  whorl  joints  of  the  stalk  in  those  Crinoids  which 
have  cirri,  the  five-fold  canal  widens  to  form  a  kind  of  repetition  of  the 
chambered  sinus,  giving  off  into  each  of  the  cirri  a  lateral  canal,  which 
runs  through  its  whole  length,  and  is  divided  into  an  upper  and  a  lower 
canal  by  a  horizontal  partition  reaching  inwards  as  far  as  to  the 
common  axis  of  the  stalk  canals. 


viii       ECHIXODERMATA—THE  PSEUDOHSEMAL  SYSTEM       447 

In  the  (.'oimitultilce,  in  consequence  of  the  absence  of  a  stalk,  the 
arrangement  is  somewhat  modified.  We  must  imagine,  however,  that 
only  the  internodes  of  the  stalk  are  wanting,  just  as  many  whorl 
joints  being  fused  with  one  another  and  with  the  centrodorsal  as  there 
are  whorls  of  cirri.  In  this  way  the  chambered  sinus  is  enlarged  by 
the  addition  of  the  widened  portions  of  the  canal  which  were  origin- 
ally present  in  the  whorl  joints.  The  cirrus  canals  now  rise  direct 
from  the  chambered  sinus. 

As  in  the  cirri  of  the  stalked  Crinoids,  so  also  in  those  of  the 
t'lu/i'ifuJidfe,  the  canal  is  divided  by  a  horizontal  partition,  and  in  these 
latter  animals  also  the  partition  of  each  cirrus  canal  is  continued  as  far 
as  to  the  axis  of  the  chambered  sinus  formed  by  the  axial  organ.  The 
chambered  sinus  of  the  Comatulidce,  in  sections  taken  in  the  direction 
of  the  principal  axis,  appears  therefore  to  be  divided  by  these  parti- 
tions into  as  many  spaces,  consecutively  superimposed,  as  there  are 
consecutive  whorls  of  cirri. 

The  five  chambers  of  the  chambered  sinus,  in  close  contact  with 
the  axial  organ,  are  produced  orally  for  a  short  distance  as  ever- 
mirrowing  canals  accompanying  the  axial  organ,  and  then  end  blindly. 

The  system  of  the  chambered  sinus  is,  therefore,  in  adult  Crinoids, 
completely  cut  off  from  the  rest  of  the  ccelom. 

For  the  relations  of  this  sinus  to  the  apical  nervous  system,  see  the  section  deal- 
ing with  this  latter,  p.  460. 


VIII.  The  Pseudohsemal  System. 
(Radial  Sinuses  and  Circular  Sinus  of  the  Schizoccel,  Subneural  Canals.) 

The  pseudohsemal  system  consists  of  canals  which  are  closely 
connected  in  the  same  manner  in  all  Echinoderms  with-  the  oral 
nervous  system.  As  radial  pseudohsemal  canals,  they  accompany 
the  radial  nerve  trunks  as  far  as  to  the  ends  of  the  radii ;  and  as 
a  pseudohsemal  ring  they  accompany  the  nerve  ring  in  its  course 
round  the  oesophagus.  They  always  lie  on  the  inner  side  of  the  nerve 
trunks  (that  turned  to  the  body  cavity),  between  these  trunks  and  the 
water  vascular  trunks.  The  radial  pseudohsemal  canals  give  off  side 
branches  which  accompany  the  nerves  of  the  tube-feet  to  their  bases. 

The  pseudoha3mal  canals  are  filled  with  a  fluid  which  resembles 
the  coelomic  fluid.  The  intimate  relation  subsisting  between  these 
canals  and  the  oral  nerve  ring  and  the  radial  nerve  trunks  makes  it 
probable  that  they  are  specialised  for  the  nourishment  of  these  nerves. 
It  has  also  been  conjectured  that  they,  together  with  the  epineural 
canals,  which  we  shall  describe  later,  are  of  essential  service  in  pro- 
tecting the  nerve  trunks  from  being  pressed  or  torn. 

In  the  Hdothiii'ioidea  and  Erhinoidea  the  pseudohsemal  system  is 
closed  on  all  sides  ;  in  the  Asteroidea  and  Ojjhiuroidea,  on  the  contrary, 


448  COMPARATIVE  ANATOMY  CHAP. 

it  communicates,  by  means  of  numerous  apertures,  with  the  general 
body  cavity,  and  at  one  point  of  the  pseudohremal  ring,  in  the  madre- 
poritic  interradius  with  the  axial  sinus. 

The  pseudohsemal  system,  in  the  Ophiuroidea  and  Asteroidea,  is 
said  to  arise,  ontogenetically,  as  a  cleft  in  the  connective  tissue 
(mesenchyme),  and  thus  to  be  a  sehizoecelomie  structure.  It  is, 
however  (as  has  been  proved  in  the  Holothurioidea),  lined  with  endo- 
thelium.  Such  a  lining,  in  the  case  of  a  schizocoelomic  cavity  is,  how- 
ever, so  incongruous,  that  we  are  justified  in  desiring  further  evidence. 
(For  the  position  of  these  canals,  see  Figs.  352-356,  pp.  409-413.) 

Special. — In  the  Holothurioidea  the  oral  pseudohyemal  ring  is,  in  the  Paractinopoda 
(Synaptidcc],  separated  from  each  of  the  radial  pseudohsemal  canals  by  a  septum. 
The  pseudohsemal  canals  stretch  only  a  short  way  backwards.  In  the  Aciinopoda, 
they  run  the  whole  length  of  the  body,  but  are  said  also  to  end  blindly  at  both  ends, 
and  the  pseudohremal  ring  is  said  to  be  wanting.  The  same  is  the  case  with  the 
well-developed  radial  pseudohsemal  canals  of  the  Echinoidea.  In  the  Crinoidea  the 
canals  are  certainly  very  much  reduced,  their  existence  is  altogether  denied  by  some 
authors.  The  pseudohsemal  canals  of  the  Ophiuroidea  give  off  lateral  branches  at 
regular  seginental  intervals  ;  these  branches  ascend  to  the  brachial  cavity  (dorsal 
canal),  and  open  into  it.  In  the  Asteroidea,  both  the  circular  and  the  radial  vessels  are 
divided  into  two  by  a  longitudinal  septum.  In  the  radial  canals  the  septum  is  vertical, 
in  the  pseudohsemal  ring  it  runs  slantingly,  dividing  it  into  an  outer  and  lower  and 
an  inner  and  upper  canal.  The  latter,  the  inner  and  upper  canal,  in  the  madreporitic 
interradius,  communicates  with  the  axial  sinus  ;  the  former,  the  outer  and  lower 
canal,  is  in  open  communication  with  the  body  cavity  of  the  disc  by  means  of  five 
ascending,  interradial,  lateral  canals.  At  regular  intervals,  between  every  two 
consecutive  tube-feet,  each  radial  pseudohsemal  canal  is  connected  with  two  marginal 
canals  which  run  longitudinally  at  the  edges  of  the  ambulacral  furrow.  Each  tube- 
foot  receives  two  canals  from  the  pseudohasmal  system,  which  run  to  its  tip ;  one  of 
these  comes  from  the  radial  canal,  and  the  other  from  the  lateral  canal.  The  lateral 
canal,  further,  sends  off  at  each  corner  between  two  consecutive  ambulacral  plates 
and  the  contiguous  adambulacral  plate,  a  lateral  branch,  which  runs  up  between 
these  plates.-  This  lateral  branch  opens  into  the  brachial  cavity. 

Two  specially  interesting  facts  deserve  notice :  (1)  the  mesentery  by  means  of  which 
the  axial  organ  is  attached  to  the  wall  of  the  axial  sinus  is  continued  into  the  septum 
of  the  pseudoheemal  ring,  and  through  this  into  the  septum  of  the  radial  pseudo- 
haemal  canals  ;  and  (2)  the  axial  organ,  although  in  a  reduced  condition,  may  even 
be  produced  along  a  greater  or  smaller  portion  of  these  septa.  These  facts  throw 
further  doubt  upon  the  schizocoalomic  nature  of  the  pseudohsemal  canals. 


IX.  The  Epineural  System. 

In  the  Holothurioidea,  the  Echinoidea,  and  the  Ophiuroidea  the  oral 
nervous  system  is  accompanied  by  canals  known  as  the  epineural 
canals,  which  run  between  it  and  the  adjacent  body  epthelium. 
This  epineural  system  thus  repeats  on  the  outer  side  of  the  oral 
nervous  system  the  pseudohsemal  system  which  accompanies  the 
nerves  on  their  inner  side,  and,  like  the  latter  system,  it  consists  of 
an  oral  circular  canal,  and  radial  canals.  In  the  Asteroidea  and 


viii          ECHINODERMATA—  BLOOD   VASCULAR  SYSTEM          449 


where  the  oral  nervous  system  still  lies  in  the  epithelium, 
the  epineural  canals  are  wanting.  This  fact  is  in  harmony  with 
what  we  know  of  the  formation  of  the  epineural  canals,  which,  as  has 
been  proved  in  the  Ophiuroidea,  arise  through  the  sinking  of  the  nerve 
trunks  below  the  surface  —  the  nerve  trunks  rising  ontogenetically 
in  the  epithelium.  These  originally  epithelial  trunks  become  covered 
by  two  lateral  integumental  folds,  which  finally  grow  together  over 
them  in  such  a  way  that  between  them  and  the  integument,  which 
now  forms  a  continuous  cover  over  them,  a  cavity,  the  epineural  canal, 
remains.  The  Synaptida  also  have  no  epineural  canals.  This  is 
probably  connected  with  the  special  way  in  which  their  (subepithelial) 
nerve  trunks  develop  ontogenetically. 

An  epineural  circular  canal  is  wanting  in  the  Holothurioidea,  and,  in  the 
Ecliiiioidca,  the  circular  canal  is  not  in  communication  with  the  radial  epineural 
canals.  A  small  epineural  (periambulacral)  space  occurs  in  connection  with  the 
development  of  a  circular  ganglion  at  the  base  of  each  tentacle  in  the  Ophiuroidea. 

X.  The  Blood  Vascular  or  Laeunar  System. 

Within  the  connective  tissue  of  various  parts  of  the  body  in  most 
classes  of  the  Echinodermata  there  occurs  a  strongly  developed  system 
of  very  small  spaces  OP  lacunae,  which  open  into  one  another,  and 
sometimes  form  at  the  surface  of  different  organs  a  fine,  close,  membrane- 
like  network  of  lacunae.  In  other  cases,  they  coalesce  to  form  bundles 
of  canals,  running  in  definite  directions,  and  anastomosing  with  one 
another.  This  lacunar  system  was  formerly  universally  called  a  blood 
vascular  system,  and  may  still  be  allowed  to  retain  this  name, 
although  a  regular  circulation  in  definite  directions  of  the  fluid 
it  contains  has  in  no  single  case  been  demonstrated. 

The  intercommunicating  laeunse,  of  which  the  blood  vascular 
system  consists,  have  no  walls  of  their  own,  and  no  endothelial 
lining,  and  their  arrangement  in  networks  or  plexuses,  which  are 
sometimes  spread  out  flatly,  and  at  others  thickened  into  "  vas- 
cular trunks,"  is  entirely  confined  to  the  Echinodermata. 

A  localised  propelling  apparatus  is  wanting.  What  was  for- 
merly called  the  heart  has  nothing  to  do  with  the  blood  vascular 
system,  but  is  the  axial  organ. 

The  fluid  (blood)  contained  in  the  blood  vascular  system  re- 
sembles that  in  the  body  cavity  and  in  the  water  vascular  system, 
but  contains  much  more  albumen  in  solution.  In  sections  of  fixed 
and  stained  animals,  a  vessel  can  easily  be  distinguished  from  the 
other  almost  empty  cavities  of  the  body  by  the  considerable  quantity 
of  coloured  coagulum  contained  in  its  spaces.  The  solid  constituents 
floating  in  the  blood  are  the  same  as  in  the  body  cavity  and  in  the 
ambulacra!  vascular  system. 

Leaving  altogether  on  one  side  the  Ophiuroidea  and  the  Asteroidea, 
in  which  the  existence  of  a  blood  vascular  system  is  still  doubtful, 
VOL.  II  2  G 


450  COMPARATIVE  ANATOMY  CHAP,  vm 

the  following  may  be  considered  as  the  chief  constituents  of  the  blood 
vascular  system  in  the  Echinodermata :  (1)  a  blood  vascular  network 
in  the  intestinal  wall,  whose  evident  function  is  to  absorb  the  digested 
and  dissolved  nourishment  as  albuminoid  out  of  the  intestinal  wall ; 
(2)  two  large  vascular  trunks,  which,  arranged  on  opposite  sides  of 
it,  accompany  the  intestine  in  its  course  ;  these  conduct  the  blood, 
which  has  become  enriched  with  albuminoid  in  the  vascular  network 
of  the  intestine,  to  other  parts  of  the  vascular  system  ;  (3)  a  blood 
vascular  ring1,  which  surrounds  the  mouth  or  oesophagus,  and  into 
which  the  two  intestinal  vascular  trunks  open  ;  (4)  five  radial  blood 
vessels,  which  run,  like  the  water  vascular  and  the  nerve  trunks,  in 
the  radii ;  (5)  a  vascular  network  on  the  surface  of  the  gonads 
(genital  glands) ;  (6)  a  vascular  network  on  the  surface  of  the  axial 
organ. 

The  connective  tissue  surrounding  the  vessels  may,  at  various 
points  of  the  vascular  system,  become  modified  for  yielding  corpuscles 
to  the  blood  (lymph  glands). 

Contractions,  very  irregular  and  indistinct,  have  been  observed 
only  in  the  vascular  trunks  of  the  intestine  of  the  Holothurioidea. 

1.  Holothurioidea  (Fig.  371). — The  simplest  arrangement  of  the 
blood  vascular  system  in  this  class  is  found  in  the  Paractinopoda 
(Synaptidce).  The  intestinal  lacuna?  pour  their  contents  first  into  two 
longitudinal  trunks,  one  of  wrhich  runs  along  the  dorsal,  and  the  other 
along  the  ventral  side  of  the  intestine.  The  ventral  trunk  opens 
anteriorly  into  the  dorsal,  which  then  runs  along  within  the  dorsal 
mesentery  direct  to  the  genital  gland.  It  is  here  continued  into  a 
spacious  lacunar  system,  which  develops  in  the  wall  of  the  gland  in 
such  a  way  as  to  split  into  two  diverging  lamellae,  an  outer  and  an 
inner,  the  latter  carrying  the  germinal  epithelium.  The  dorsal  vessel 
further  gives  off  a  small  lateral  branch  near  the  point  at  which  the 
stone  canal  enters  the  circular  canal.  According  to  recent  researches, 
neither  a  circular  canal  of  the  blood  vascular  system,  nor  radial 
vessels,  nor  tentacle  vessels  are  to  be  found.  In  the  Adinopoda  (Fig. 
371)  the  blood  vascular  system  is  more  completely  developed.  Here, 
again,  the  blood  from  the  lacunar  network  of  the  intestinal  wall  (which 
lies  on  the  inner  side  of  the  muscle  layer)  is  collected  by  two  vascular 
trunks  which  accompany  the  intestine  along  its  whole  length,  i.e.  to 
the  hind-gut ;  one  of  these  trunks  is  the  dorsal  or  mesenterial,  and 
the  other  the  ventral  or  anti-mesenterial.  They  both  open  anteriorly, 
immediately  behind  the  circular  canal  of  the  water  vascular  system, 
into  a  circular  vessel  which  surrounds  the  oesophagus,  from  which  five 
radial  vessels  run  in  the  radii.  Each  radial  blood  vessel  lies  between 
the  radial  nerve  trunk  on  the  outer  and  the  radial  water  vessel  on 
the  inner  side  (Fig.  352,  p.  409).  It  gives  off  lateral  branches  to 
the  oral  tentacles,  the  ambulacral  feet,  and  the  papilla?.  The  wall 
of  the  genital  glands  is  everywhere  richly  "  vascularised,"  either  by  a 
lacunar  network  or  by  a  more  simple  splitting  of  its  wall  of  connective 


, 


FIG.  371.— Organisa- 
tion of  Holothuria 
tubulosa.  The  blood 
vascular  system  is 
marked  black.  1,  Oral 
tentacles ;  2,  stone 
canals  ;  3,  water  vas- 
cular ring  ;  4,  Polian 
vesicle  ;  5,  gonads  ;  6, 
longitudinal  muscles ; 
7,  anterior  section  of 
intestine  ;  8,  ventral 
intestinal  vessel ;  9, 
radial  water  vessel ;  10, 
vascular  anastomoses  ; 
11,  dorsal  intestinal 
vessel ;  12,  filaments 
and  strands  (of  the 
nature  of  both  muscle 
and  connective  tissue) 
which  attach  the  cloaca 
to  the  body  wall ;  13, 
cloaca ;  14,  cloacal 
aperture  (anus) ;  15, 
middle  section  of  the 
intestine ;  16,  posterior 
section  of  the  same : 

17,  right  branchial  tree; 

18,  rete  mirabile ;  19, 
radial    canal    of    the 
water  vascular  system ; 

20,  left  branchial  tree  ; 

21,  tentacle    ampulla 
(after  Milne  Edwards 
and  Cams). 


452  COMPARATIVE  ANATOMY  CHAP. 

tissue.  The  blood  vascular  system  of  the  genital  gland  may  derive  its 
blood  in  three  mutually  exclusive  ways  :  (1)  by  means  of  a  special 
genital  vessel  from  the  blood  vascular  ring,  (2)  by  means  of  a  special 
genital  vessel  from  the  dorsal  intestinal  vessel,  (3)  direct  from  the 
latter,  with  which  the  genital  gland  is  in  contact. 

The  ventral  vessel  of  the  anterior  section  of  the  intestine  is  almost 
always  connected  with  that  of  the  middle  section  by  means  of  a 
usually  simple,  but  sometimes  complicated  anastomosis  (Figs.  371,  10, 
383,  27,  p.  477). 

In  the  Aspidochirotce  principally,  but  also  in  many  Dendrochirotce 
and  Molpadiidce,  the  dorsal  vessel  becomes  detached  from  the  intestine 
for  a  considerable  distance  ;  in  Holothuria  tubulosa  (Fig.  371)  this  occurs 
along  part  of  the  anterior,  the  whole  middle,  and  part  of  the  posterior 
section,  and  it  has  a  free  course  as  the  marginal  vessel  of  the  rete 
mirabile  through  the  body  cavity.  It,  however,  remains  in  connec- 
tion with  the  lacunar  network  which  is  developed  in  the  wall  of 
the  intestine  by  means  of  a  rich  plexus  of  blood  lacunas,  known  as  the 
rete  mirabile.  This  network  forms  a  much  perforated  membrane, 
one  edge  of  which  is  attached  to  the  intestine,  while  the  marginal 
vessel  runs  along  the  other.  The  blood  of  the  intestinal  lacunar 
system,  again,  may  collect  in  a  special  longitudinal  vessel  (collateral 
vessel,  pulmonary  vein)  before  passing  over  into  the  rete  mirabile 
(Fig.  371). 

The  rete  mirabile  is  often  particularly  richly  developed  in  the  loop 
formed  by  the  anterior  and  middle  intestinal  sections.  In  the  rest  of 
the  small  intestine,  the  vessels  which  enter  the  plexus  from  the  marginal 
vessel  first  break  up  into  a  bundle  of  very  fine  lacunas  (capillaries) ; 
these  finest  lacunae  then  collect  to  form  vessels  which  enter  the  collateral 
vessel.  It  may  be  said  that  here,  between  the  collateral  and  the  mar- 
ginal vessels,  numerous  small  retia  mirabilia  (Fig.  371,  18)  of  the 
second  order  are  developed,  and  that  they  form  webs  round  the 
terminal  ramifications  of  the  left  water  lung.  It  is  doubtful 
whether  they  serve  for  respiration,  since  they  are  not  developed  in 
the  wall  of  the  water  lung  itself,  but  merely  loosely  invest  the  latter. 

The  dorsal  vessel  (so  called  because  it  lies  on  the  anterior  intestinal  section 
close  to  the  dorsal  mesentery)  does  not  run  in  this  mesentery,  but  somewhat  to  the 
left  of  it.  In  the  middle  section  of  the  intestine  it  comes  to  lie  on  the  right,  and  in 
the  posterior  section  again  on  the  left  side  of  the  mesentery. 

At  certain  points  in  the  course  of  the  vessel,  blood  glands  may  be  developed. 
The  spongy,  alveolar  structure  of  the  vessel  then  becomes  more  marked,  and  many 
cells  are  found  embedded  in  the  strands,  filaments,  membranes,  etc.,  of  connective 
tissue  which  traverse  the  vessel  ;  these  are  the  formative  cells  of  the  blood  corpuscles. 
At  such  points  the  lumina  of  the  vessels  (i.e.  of  the  separate  lacunre,  which  together 
form  the  vessel)  are  much  reduced. 

2.  Eehinoidea. — The  arrangement  of  the  blood  vascular  system 
here  agrees  to  a  great  extent  with  that  in  the  Holothurioidea.  A  rich 
plexus  of  vacuoles  is  developed  in  the  connective  tissue  layer  of  the 


MII  ECHINODERMATA—  NERVOUS  SYSTEM  453 

intestinal  wall  (with  the  exception  of  a  larger  or  smaller  portion  of 
the  hind-gut) ;  from  this  plexus  the  blood  collects  into  two  longitudinal 
vessels,  one  outer  and  dorsal  and  the  other  inner  and  ventral.  These 
longitudinal  vessels  do  not  lie  in  but  on  the  wall  of  the  intestine,  in 
its  mesenteries.  The  ventral  or  inner  vessel,  along  the  part  where 
the  accessory  intestine  is  developed,  passes  on  to  this  latter.  Both 
vessels  open  into  a  blood  vascular  ring  which  encircles  the  oesophagus, 
in  close  contact  with  the  water  vascular  ring.  The  position  of  these 
two  rings  and  their  relation  to  one  another  has  already  been  sufficiently 
described  (p.  424).  From  the  blood  vascular  ring  five  radial  vessels 
run  to  the  radii,  within  the  bands  of  connective  tissue  which  separate 
the  pseudohaemal  canals  from  the  radial  trunks  of  the  water  vascular 
system  (Fig.  353,  p.  410).  In  Echinoids  provided  with  a  masticatory 
framework,  the  proximal  portions  of  these  radial  blood  vessels  descend 
in  the  axis  of  the  lantern  along  the  edges  of  the  five  single  pyramids 
forming  the  lantern,  which  are  turned  towards  the  oesophagus  and  to 
the  nerve  ring  (Fig.  358,  p.  419),  before  passing  through  the  auricles 
to  be  produced  as  mentioned  into  the  radii.  They  are  said  not  to  be 
in  open  communication  with  the  circular  vessel,  but  to  be  divided  from 
it  by  a  septum.  The  radial  blood  vessels  during  their  course  give  off 
lateral  branches  which  run  to  the  bases  of  the  tube-feet. 

A  lacunar  plexus  is  also  developed  in  the  axial  organ,  immediately 
below  the  surface ;  this  is  either  in  direct  communication  with  the 
blood  vascular  ring  which  encircles  the  oesophagus,  or  else  draws  its 
blood  from  the  dorsal  intestinal  vessel.  The  vacuolar  plexus  of  the 
axial  sinus  is  further  continued  into  the  wall  of  the  apical  circular  sinus 
of  the  body  cavity,  and  thence  on  to  the  wall  of  the  genital  glands. 

3  and  4.  Asteroidea  and  Ophiuroidea. — It  is  doubtful  whether  a 
blood  vascular  system  occurs  in  these  two  classes.  This  system  is 
certainly  wanting  in  the  intestinal  wall.  The  vessels  which  have 
hitherto  been  regarded  as  blood  vascular,  ring,  and  radial  blood  vessels 
(running  between  the  nerve  trunks  and  the  pseudohsemal  vessels)  agree 
in  structure  with  the  axial  organ,  and  are  said  to  be  direct  continua- 
tions of  this  organ.  As  such  they  fall  under  another  heading. 

5.  Crinoidea. — The  blood  vascular  system  is  here  well  developed, 
and  in  all  its  parts  shows  in  a  very  marked  manner  the  spongy 
structure  characteristic  of  Echinoderms  (i.e.  it  consists  of  lacunar  net- 
works or  plexuses).  One  lacunar  network  covers  the  axial  organ,  and 
another  spreads  out  over  the  intestinal  wall.  Both  are  in  open  com- 
munication with  a  lacunar  plexus  which  surrounds  the  oesophagus,  and 
which  may,  at  one  point,  become  differentiated  into  a  blood  gland 
(spongy  organ)  by  the  deposition  of  numerous  blood  formative  cells  in 
its  meshes. 

XL  The  Nervous  System. 

This  system  in  the  Echinodermata  is  developed  in  a  quite  peculiar 
manner,  unknown  in  other  animals.  It  is  composed  of  three  alto- 


454  COMPARATIVE  ANATOMY  CHAP. 

gether  independent  systems  :  (1)  the  superficial  oral,  (2)  the  deeper 
oral,  and  (3)  the  apical  system. 

(a)  The  superficial  oral  nervous  system  is  developed  on  the  oral 
side  of  the  body,   and  is  always  more  or  less  superficial.     Its  most 
important  and  constant  constituents  are  :  (1)  a  nerve  ring  encircling 
the  oesophagus,  and  (2)  radial  nerves  radiating  from  this  ring,  and 
corresponding  in  number  with  the  radii.     This  system  innervates  the 
integument,  the  ambulacral  appendages,  and  the  intestinal  canal.     It 
occurs  in  all  Echinodermata  without  exception. 

(b)  The  deeper  oral  nervous  system  accompanies  the  superficial 
system  on  its  inner  side  (i.e.  on  the  side  turned  towards  the  body 
cavity).      In  the  Ophiuroidea  and  Asteroidea  it  is  paired  in  each  radius, 
i.e.  its  trunks  or  ganglia  lie  on  the  two  sides  of  the  radial  nerves  of 
the  superficial  system.      In  the  Echinoidea  and  Holothurioidea,  on  the 
contrary,  it  is  unpaired  in  each  radius,  and  consists  of  a  trunk  or  ridge 
in  close  contact  with  the  radial  nerve  of  the  superficial  system  on  its 
inner  side.      In  the  Ophiuroidea  and  Asteroidea  a  more  or  less  complete 
ring  surrounding  the  oesophagus  seems  to  be  developed  in  this  system, 
but  this  is  wanting  in  the  Echinoidea  and  Holothurioidea.     The  Crinoids 
and   those  Echinoids  which  have  no   masticatory  apparatus   have  no 
deeper  oral  nervous  system. 

This  system  innervates  the  muscles  which  run  in  the  oral  side  of 
the  body  wall ;  in  the  Holothurioidea  it  perhaps  innervates  the  whole 
dermo- muscular  tube,  but  in  the  Echinoidea  probably  only  the  muscles 
of  the  masticatory  apparatus. 

(c)  The  apical  nervous  system  is  specially  strongly  developed  in 
the    Crinoidea.      It   consists    of  a    nerve    envelope,   surrounding   the 
chambered  organ,  and  forming  a  centre  from  which  five  nerves  run  out 
along  the  axial  canals  of  the  brachial  skeleton,  penetrating  as  far  as 
to  the  distal  joints  of  the  pinnulse  (cf.  Fig.  356,  8,  p.  413). 

The  apical  nervous  system  is  also  continued  into  the  stalk  and  the 
cirri.  It  innervates  the  whole  of  the  musculature  which  moves  the 
arms  and  the  cirri. 

In  the  Asteroidea  the  apical  nervous  system  consists  of  radially 
arranged  nerve  trunks  which  meet  at  the  centre  of  the  disc ;  there  is 
one  trunk  for  each  arm.  These  trunks  run  along  the  middle  line  of 
the  arms,  immediately  above  the  body  cavity,  and  innervate  the  dorsal 
muscles  of  the  arms. 

In  the  Ophiuroidea  and  Echinoidea  a  delicate  nerve  trunk  runs 
within  the  wall  of  the  aboral  circular  sinus  throughout  the  whole 
course  of  the  latter ;  this  is  the  genital  nerve  ring. 

The  Holothurioidea  have  no  aboral  system. 

A.  The  Superficial  Oral  Nervous  System. 

Until  comparatively  recently  this  system  was  the  only  nervous 
system  known  in  Echinodermata. 


viii  ECHINODERMATA— NERVOUS  SYSTEM  455 

In  the  Asteroidea  and  Crinoidea  it  occupies  throughout  life  an 
epithelial  position,  but  in  all  other  Echinoderms  it  sinks  and  becomes 
subepithelial,  except  at  the  ends  of  the  radii  (in  the  terminal  tentacles), 
and  in  the  intestine.  Here,  throughout  life,  it  is  epithelial. 

The  shifting  of  the  superficial  nervous  system  to  a  position  below 
the  body  epithelium,  in  Echinoderms,  goes  hand  in  hand  with  the 
formation  of  the  epineural  canals,  as  already  explained  (p.  449). 

1.  Asteroidea  (Fig.  354,  p.  411). 

The  radial  nerves  here  form  a  thickened  longitudinal  ridge  of  the 
epithelium  along  the  base  of  each  ambulacral  furrow,  and  the  circular 
nerve  a  thickened  ridge  round  the  mouth.  In  these  nerve  ridges  the 
(ciliated)  epithelial  cells  represent  the  nerve  cells.  At  their  bases  they 
are  continued  into  nerve  fibres,  which  run  in  the  longitudinal  direction 
of  the  nerve  ridges  (i.e.  of  the  radial  nerves),  and  together  form  the 
deeper  layer  of  the  ridge.  From  the  radial  nerves  a  close  plexus  of 
nerve  fibres  spreads  out  below  the  surface  of  the  outer  epithelium  over 
the  whole  body ;  this  is  especially  close  in  the  ambulacral  feet.  In 
the  same  way  a  layer  of  nerve  fibres  is  found  over  the  whole  intestinal 
epithelium ;  this  layer  increases  in  thickness  orally  until  it  enters  the 
circular  nerve. 

2.  Crinoidea  (Fig.  356,  p.  413). 

The  above  account  of  the  nervous  system  of  the  Asteroidea  is  also 
applicable  to  that  of  the  Crinoidea,  if  it  be  remembered  that  the  food 
grooves  of  the  arms  and  of  the  tegmen  calycis  of  the  Crinoids  correspond 
with  the  ambulacral  furrows  of  the  Asteroidea.  The  food  grooves 
branch  with  the  arms,  and  so  do  the  radial  nerve  ridges  of  the  super- 
ficial oral  system.  The  Crinoids,  however,  differ  from  the  Asteroids  in 
that  the  epithelial  nerve  plexus  is  limited  to  the  oral  side  of  the  calyx 
and  of  the  arms,  since  no  epithelium  can  be  made  out  on  the  apical 
side  of  the  calyx,  on  the  sides  and  backs  of  the  arms,  and  on  the  stalk 
and  the  cirri  of  adults. 


3.  Ophiuroidea  (Fig.  372). 

The  superficial  oral  nervous  system,  having  here  taken  up  a  sub- 
epithelial  position,  appears  in  the  form  of  distinct  nerve  trunks  and  of 
nerves  radiating  from  these.  The  central  portion  consists  of  the  nerve 
trunks  of  the  disc  and  of  the  five  radial  trunks  of  the  arms.  These 
latter  run  on  the  inner  side  of  the  row  of  ventral  shields,  between 
these  and  the  vertebral  ossicles.  The  circular  nerve  throughout  its 
whole  course  is  beset  with  nerve  cells  on  the  side  turned  towards  the 
oesophagus,  and  the  same  is  the  case  with  the  radial  nerves  on  the 
side  turned  towards  the  ventral  shields.  The  segmentation  of  the 
arms  appears  in  a  marked  manner  in  the  radial  nerve  trunks  which 


456 


COMPARATIVE  ANATOMY 


CHAP. 


run  through  it.  At  regular  segmental  intervals,  about  on  a  level  with 
the  consecutive  pairs  of  tentacles,  these  trunks  show  distinct  swellings, 
from  which  most  of  the  nerves  originate.  In  this  way  the  radial 
nerves  superficially  resemble  the  ventral  ganglionic  chain  of  many 
Annulata  and  Arthropoda. 

The  central  portion  of  the  deeper  oral  nervous  system  is  so  closely 
applied  to  the  corresponding  portion  of  the  superficial  nervous  sys- 
tem that  the  two  can  only  be  distinguished  from  one  another  by  means 


FIG.  372.— Nervous  system  of  an  Ophiurid  (Ophiothrix  fragilis)  (after  Cuenot).  Part  of  the 
disc  and  the  base  of  an  arm.  1,  Peripheral  brachial  nerve  ;  2,  tentacle  nerve  ;  3,  nerve  to  the 
muscles  between  the  vertebral  ossicles  ;  4,  radial  nerve  trunk  ;  5,  ganglion  at  the  base  of  a  spine  ; 
6,  marginal  nerve  of  the  disc  ;  7,  bursal  aperture  ;  8,  nervus  lateralis  ;  9,  gonad  projecting  into  the 
bursa ;  10,  Polian  vesicle  ;  11,  interradial  nerve  ;  12,  nerve  of  the  musculus  interradialis  aboralis 
(Simroth) ;  13,  nerve  ring ;  14,  dental  nerve  ;  15,  enterocoelic  nerve  ring ;  16,  spine  ;  I,  first  oral 
tentacle  ;  II,  second  oral  tentacle  ;  III-VIII,  tentacles  of  arms. 

of  careful  microscopic  examination,  e.g.  of  transverse  sections.  In  the 
following  description,  however,  we  shall  keep  the  two  systems  entire!}* 
distinct  from  one  another. 

Nerves  of  the  oesophageal  ring. — A  large  number  of  nerves  arise  from  the 
cesophageal  ring,  and  ramify  in  the  connective  tissue  layer  of  the  intestinal  wall. 
Further,  at  each  of  the  points  where  a  radial  nerve  trunk  joins  the  cesophageal  ring, 
the  latter  gives  off  two  nerves,  which  run  to  the  bases  of  the  first  pair  of  oral  tube- 
feet.  Each  of  them  there  forms  a  circular  ganglion,  almost  completely  surrounding 


vni  ECHINODERMATA—  NERVOUS  SYSTEM  457 

the  base  of  the  root,  and  then  sends  off  a  branch  which  runs  to  the  tip  of  the  tube- 
foot  (Fig.  372,  1). 

Nerves  of  the  radial  trunks.  — Two  pairs  of  nerves  arise  at  regular  intervals  from 
the  radial  nerve  trunks,  as  well  in  the  free  portion  of  the  arm  as  in  that  included  in 
the  disc.  These  are  the  tentacle  nerves  and  the  peripheral  nerves. 

The  tentacle  nerve  has  a  short  course  to  the  base  of  its  tentacle,  and  there  forms 
a  ganglion — the  tentacle  ganglion — which  embraces  the  base  of  the  tentacle.  The 
nerve  contains  along  its  course  nerve  cells,  which  are  still  more  plentiful  in  the  ten- 
tacle ganglion.  The  tentacle  nerve  and  the  ganglion  are,  like  the  radial  nerve  trunk, 
accompanied  by  an  epineural  canal.  From  this  basal  tentacle  ganglion,  the  tentacle 
nerve,  which  remains  in  the  subepithelial  layer,  ascends  to  the  tip  of  the  tentacle. 

The  peripheral  nerve  at  first  runs  in  close  contact  with  the  tentacle  nerve  of  the 
same  side  as  far  as  to  the  base  of  the  tentacle  ;  it  then  runs  further  laterally,  pene- 
trates into  the  lateral  wall  of  the  arm,  running  through  the  skeletal  mass  of  the 
latter,  and  breaks  up  into  branches  which  innervate  the  ventral,  lateral,  and  dorsal 
integument  of  its  own  side  of  the  arm. 

In  those  Ophiurids,  the  sides  of  whose  arms  are  provided  with  spines  (i.e.  in  the 
majority  of  cases),  peripheral  ganglia  are  formed  at  the  bases  of  these  spines. 

The  nervous  system  becomes  somewhat  more  complicated  in  the  portion  of  the 
arm  which  is  included  in  the  disc.  Branches  of  the  peripheral  nerve  ascend  apically 
at  the  side  of  the  bursa  which  is  turned  to  the  arm,  i.e.  on  the  radial  side  of  the 
genital  plate  described  on  p.  361,  when  it  meets  a  lateral  nerve  (8)  running  along  this 
plate.  Distally  this  lateral  nerve  is  continued  into  an  aboral  marginal  nerve  (6),  the 
branches  of  which  innervate  the  periphery  of  the  disc.  Proximally,  the  lateral 
nerve  passes  over  into  an  interradial  nerve  (11),  which  runs  along  the  outer  side  of 
the  interradial  muscle. 

In  some  cases  (Ophiothrix,  Ophiocoma,  Ophioglypha},  the  first  pair  of  tentacle 
nerves  and  their  accompanying  peripheral  nerves  are  followed  by  a  pair  of  nerves 
which  run  towards  the  bursal  aperture,  subsequently  uniting  with  the  lateral 
nerves. 

In  Ojihioglt/pha,  a  delicate  pair  of  nerves  which  innervate  the  integument  of  the 
ventral  (oral)  side  of  the  arm  runs  out  between  each  of  the  regularly  recurring  pairs 
of  tentacle  and  peripheral  nerves. 

4.  Eehinoidea  (Figs.  353  and  358,  pp.  410  and  419). 

In  all  Echinoids,  even  in  those  which  are  provided  with  a 
masticatory  apparatus,  the  oesophageal  ring  remains  in  close  proximity 
to  the  mouth,  on  the  inner  side  of  Aristotle's  lantern,  and  at  a  con- 
siderable distance  from  the  water  vascular  and  blood  vascular  rings, 
which  appear  to  be  in  a  manner  lifted  up  by  the  lantern.  From  the 
oesophageal  ring,  especially,  in  the  radii,  nerves  run  to  the  oesophagus, 
gradually  breaking  up  into  a  plexus,  which  is  still  traceable  even  in 
the  wall  of  the  first  coil  of  the  intestine.  From  the  radial  nerve 
trunks,  as  in  the  Ophiuroidea,  at  intervals  which  correspond  with  the 
ambulacral  feet,  nerves  for  these  feet  and  peripheral  nerves  are 
given  off.  In  the  regular  Eehinoidea  and  the  Clypeastroida,  the  tube- 
feet  nerves  and  peripheral  nerves  arise  together,  but  in  Spatangoidii 
they  arise  separately.  The  points  of  origin  of  these  two  sets  of 
nerves  from  the  radial  nerve  trunk  are  arranged  alternately,  in 
accordance  with  the  alternate  arrangement  of  the  two  rows  of 


458  COMPARATIVE  ANATOMY  CHAP. 

ambulacral  plates  of  an  ambulacrum,  and  with  the  alternate  arrangement 
of  the  ambulacral  feet  at  the  two  sides  of  the  water  vascular  trunk. 

Each  tube-foot  nerve,  with  its  peripheral  nerve,  passes  out  together 
with  the  tube-foot  canal  of  the  water  vascular  system,  through  the 
ambulacral  pore,  on  to  the  surface  of  the  test.  The  nerve  of  the 
tube-foot  is  then  continued  in  the  epithelial  layer  to  the  tip  of  the 
foot,  without  forming  a  ganglion.  The  peripheral  nerve,  however, 
enters  an  integumental  nerve  layer  which  covers  the  whole  body  and 
its  appendages. 

The  network  of  nerves  or  layer  of  nerve  fibres,  both  in  the  intes- 
tine and  in  the  external  integument,  lies,  in  regular  Echinoids  (Cidar- 
oida,  Diadematoida)  and  in  the  Clypeastroida,  deep  in  the  epithelium, 
but  is  subepithelial  in  the  Spatangoida. 

5.  Holothurioidea. 

The  superficial  oral  nervous  system  is  here  subepithelial  and 
agrees  in  all  respects  with  that  of  the  Echinoidea.  The  nerve  ring 
which  surrounds  the  mouth  gives  off  the  nerves  to  the  oral  tentacles 
and  the  intestinal  canal.  The  latter  innervate  also  the  skin  round  the 
mouth,  and  ramify  richly  in  the  connective  tissue  layer  of  the  intestine, 
especially  in  its  anterior  portion.  The  radial  nerve  trunks  give  off 
lateral  branches  to  the  tube-feet  or  ambulacral  papillae,  and  "peri- 
pheral nerves "  to  the  integument.  The  latter  break  up  into  a 
subepithelial  network  of  fibres. 

In  the  Synaptidce,  each  radial  nerve  trunk,  soon  after  it  rises  from 
the  oesophageal  ring,  gives  off  a  pair  of  nerves  to  the  auditory 
vesicles. 

B.  The  Deeper  Oral  Nervous  System. 

1.  Asteroidea  (Fig.  354,  p.  411). 

In  close  contact  with  the  inner  side  of  each  radial  nerve  ridge 
(which  is  here  epithelial)  a  subepithelial  longitudinal  band  of  nerve 
cells  and  nerve  fibres  run  along  each  side.  A  similar  band  accom- 
panies the  oesophageal  ring,  at  least  in  its  interradial  regions.  From 
the  radial  bands  of  the  deeper  nervous  system,  lateral  nerve  bands 
arise  at  regular  intervals  which  correspond  with  the  ambulacral  feet. 
These  lateral  bands  ascend  on  the  outer  side  of  the  radial  pseudo- 
haemal  canals,  and  soon  break  up  into  fibres,  which  probably  innervate 
the  muscles  of  the  ambulacral  skeleton.  Nerves  which  emerge  inter- 
radially  from  the  deeper  nerve  ring  seem  to  innervate  the  interradial 
muscles  of  the  oral  skeleton. 

2.  Ophiuroidea  (Fig.  355,  p.  412). 

Here  also,  two  lateral  bands,  consisting  of  nerve  cells  and 
longitudinal  fibres,  lie  in  close  contact  with  the  inner  side  of  each 


vin  ECHINODERMATA— NERVOUS  SYSTEM  459 

radial  nerve  trunk.  The  radial  nerve  trunk  of  the  superficial  system 
and  the  paired  nerve  bands  of  the  deeper  nervous  system  are  separated 
merely  by  a  thin  structureless  membrane.  The  superficial  circular 
nerve  is  similarly  accompanied  by  a  band  of  the  deeper  nervous 
system. 

The  paired  bands  become  thickened  at  regular  intervals,  simul- 
taneously with  the  radial  nerve  trunks  of  the  superficial  system. 
Between  the  consecutive  swellings  of  this  latter,  however,  they  are 
extremely  thin.  The  deeper  nerve  ring  is  much  thicker  in  its  inter- 
radial  than  in  its  radial  portions. 

In  each  interradius  two  nerves  are  given  off  by  the  deeper  nerve 
ring;  these  break  up  into  several  branches  and  innervate  the  inter- 
radial  muscles  of  the  oral  skeleton. 

The  paired  radial  bands  of  the  deeper  system  give  rise,  at  points 
which  regularly  alternate  with  the  tentacle  and  peripheral  nerves,  to 
nerves  which,  first  penetrating  the  cavity  of  the  pseudohaemal  canal, 
ascend  apically,  enter  the  vertebral  ossicles,  ramify  in  them,  and  inner- 
vate the  intervertebral  muscles.  The  vertebral  nerve  of  one  side 
always  innervates  the  dorsal  and  ventral  intervertebral  muscles  of  the 
same  side  of  the  arm,  which  counteract  the  homologous  muscles  of  the 
opposite  side. 

3.  Eehinoidea. 

Only  those  Echinoids  which  are  provided  with  a  masticatory 
apparatus  have  a  deeper  nervous  system  ;  this  bears  out  the  very 
probable  assumption  that  this  system  innervates  the  masticatory 
muscles.  It  consists  of  five  lamellae  consisting  of  nerve  cells  and 
fibres,  which  are  in  close  contact  with  the  radial  portions  of  the  super- 
ficial resophageal  ring  and  the  points  of  departure  of  the  radial  nerves. 
Each  lamella  gives  off  a  pair  of  large  nerves.  These  ascend  along  the 
edges  of  the  five  jaws,  then  ramify,  and  most  probably,  as  already 
stated,  innervate  the  jaw  muscles. 

4.  Holothurioidea  (Fig.  352,  p.  409). 

The  deeper  nervous  system  is  only  developed  on  the  radial  nerve 
trunks,  which  it  covers  on  their  inner  sides  in  the  form,  in  each  case, 
of  a  single  thin  band,  consisting  of  nerve  cells  and  longitudinal  fibres. 
The  nerves  which  proceed  from  these  radial  bands  seem  to  serve 
principally  for  the  innervation  of  the  dermo-muscular  tube. 


C.  The  Apical  or  Aboral  Nervous  System. 

The  apical  nervous  system  of  the  Asteroidea,  Eehinoidea,  and 
Ophiuroidea  has  already  been  sufficiently  described  at  the  beginning  of 
this  section,  p.  454.  That  of  the  Crinoidea,  however,  requires  some 
further  elucidation. 


460  COMPARATIVE  ANATOMY  CHAP. 

The  chambered  sinus  which  lies  in  the  centrodorsal  is  surrounded 
by  a  cup-shaped  envelope,  which  consists  of  ganglion  cells  and  nerve 
fibres.  The  latter  are  arranged  more  or  less  concentrically  around  the 
sinus.  The  nerve  envelope  of  the  chambered  sinus  is  continued  along 
its  prolongation,  the  stalk  canal,  and  along  the  canals  of  the  cirri.  All 
these  canals  are  in  fact  surrounded  by  a  nerve  sheath  whose  fibres  run 
longitudinally.  The  large  apical  nerve  trunks,  which  run  from  the 
nerve  envelope  of  the  chambered  sinus  into  the  radii  (enclosed  in 
the  nerve  canals),  consist  of  nerve  fibres  and  ganglion  cells.  A  kind 
of  metamerism  is  apparent  in  them,  since  they  become  somewhat 
thickened  in  the  consecutive  ossicles  of  the  arm,  and  give  off  nerves  at 
regular  intervals  corresponding  with  these  ossicles.  The  apical  nerve 
trunks  divide  with  the  arms,  in  whose  nerve  canals  they  are  enclosed, 
and  run  to  the  tips  of  the  pinnule. 

Kound  the  chambered  sinus,  commissures  are  formed  between  the 
nerve  trunks  arising  from  the  nerve-envelope  of  the  sinus.  The  courses 
of  these  commissures  are  explained  by  the  diagrams  of  the  nerve  canals 
in  which  they  are  enclosed  (Figs.  327-330,  p.  378). 

In  Antedon  and  other  forms,  in  the  second  costal,  where  the  five 
primary  nerve  trunks  fork  to  make  the  ten  brachial  nerves,  a 
peculiar  chiasma  nervorum  brachialium  is  formed  (cf.  Figs.  327 
and  329,  p.  378).  The  two  ramifications  of  the  nerve,  which  lyy 
their  crossing  form  the  chiasma,  run  one  above  the  other  without 
any  mingling  of  their  fibres.  Further,  the  two  brachial  nerves  form- 
ing one  pair  are  connected  immediately  beyond  the  chiasma  by  means 
of  a  transverse  commissure. 

In  each  ossicle  of  the  arm,  the  apical  brachial  nerve  gives  off  an 
upper  (oral)  and  a  lower  (apical)  pair  of  nerves.  These  nerves  seem  to 
be  principally  sensory.  They  ramify  richly  in  the  calcareous  substance 
of  the  joints.  The  increasingly  fine  ramifications,  which  radiate  to 
the  surface  of  the  arm  (Fig.  356,  p.  413),  finally  reach  special  groups 
of  epithelial  cells,  which  must  no  doubt  be  considered  as  sensory  cells. 
One  principal  branch  of  the  oral  (upper)  pair,  however,  is  said  to  run 
to  the  musculature,  uniting  the  joints  of  the  arm. 

Besides  these  two  pairs  of  nerves  proceeding  from  the  apical  nerve 
trunks,  nerves  are  said  to  run  out  at  the  level  of  the  joint,  from 
between  the  consecutive  brachial  joints,  these  having  for  their  special 
function  the  innervation  of  the  brachial  musculature. 

The  apical  pinnula  nerves,  which  are  given  off  in  alternating  order 
by  the  brachial  trunks  to  the  pinnulse,  arise  from  double  roots. 

According  to  the  present  state  of  our  knowledge  on  the  subject,  the 
apical  nervous  system  of  the  Crinoids  is  formed  by  the  ccelomic  endo- 
thelium.  Even  in  adult  animals,  those  portions  of  this  system  which 
envelop  the  chambered  sinus,  the  stalk  canal,  and  the  cirrus  canals 
show  a  close  connection  with  their  places  of  formation.  The  apical 
nervous  system  of  the  Asteroids  retains  its  endothelial  position  through- 
out life. 


viii  ECHINODEEMATA— NERVOUS  SYSTEM  461 


D.  The  third  Nervous  System  of  the  Crinoidea. 

Besides  the  superficial  oral  and  the  apical  systems,  the  Crinoids 
have  another  nervous  system.  This  is  developed  on  the  oral  side  of 
the  disc  and  of  the  arms,  and  is  subepithelial  in  position.  It  consists 
of  the  following  principal  parts  :  (1)  a  nerve  ring  encircling  the 
oesophagus  close  to  the  mouth ;  and  (2)  five  pairs  of  braehial 
nerves.  The  two  nerves  of  each  braehial  pair  run  down  the  arm 
longitudinally  at  the  sides  of  the  radial  water  vascular  canal  (Fig. 
356,  4,  p.  413).  They  are  found  on  the  branches  of  the  first,  second, 
and  other  orders.  Their  condition  at  the  points  where  the  arms 
divide  is  not  yet  known. 

In  addition  to  the  five  pairs  of  braehial  nerves,  the  cesophageal  ring 
gives  rise,  in  each  interradius,  to  two  nerves  which  branch  freely  and 
run  to  the  bands  and  mesenteries  which  traverse  the  body  cavity, 
giving  off  branches  also  to  the  tegmen  calycis. 

Lateral  branches  of  the  paired  braehial  nerves  innervate  the  mus- 
culature of  the  water  vascular  and  tentacle  canals  which  run  in  the 
arm  :  they  also  ascend  in  the  tentacles  to  innervate  their  sensory 
papillae. 

This  third  nervous  system  of  the  Crinoids  is  connected  with  the 
apical  system  by  means  of  branches  in  the  following  manner : — 

1.  The  two  braehial  nerves  of  each  arm  send  off  branches  alter- 
nately (first  from  the  right   nerve,  then  from  the  left)  towards  the 
apical  side  of  the  arm.     Each  of  these  unites  with  one  of  the  pair  of 
nerves  which  descend  orally  from  the  apical  nerve  trunk  within  the 
interior  of  the  braehial  joint  (Fig.  356,  10,  p.  413). 

2.  Certain  lateral  branches  of  the  pair  of  nerves  which  run  out 
from  the  cesophageal  ring  in  each  interradius  seem  to  run  along  the 
body  wall  apically,  and  to  unite  with  lateral  branches  of  the  apical 
nerve  trunks  which  come  from  the  nerve  envelope  of  the  chambered 
sinus. 

Although  recent  investigators  incline  to  the  opinion  that  this  third  nervous 
system  in  the  Crinoids  has  no  representative  in  the  other  Echinoderms,  the  question 
may  here  be  raised  whether  it  does  not  correspond  with  a  deeper  oral  nervous  system. 
If  we  imagine  the  deeper  oral  nervous  system  of  an  Ophiurid  or  an  Asteroid  to  have 
become  severed  from  the  superficial  system  and  to  have  shifted  further  below  the 
surface,  we  have  a  system  showing  considerable  resemblance  to  the  third  system  of 
the  Crinoids.  Because  no  connection  between  the  deeper  oral  system  and  the  apical 
system  has  yet  been  demonstrated  in  other  Echinoderms,  we  have  no  right,  considering 
the  difficulty  of  investigation  on  this  point,  to  conclude  that  no  such  connection 
exists.  It  is  difficult  to  believe  that  there  are  three  completely  independent  nervous 
systems  in  the  Echinoderm  body. 


462  COMPARATIVE  ANATOMY  CHAP. 


XII.  The  Sensory  Organs. 

The  sensory  organs  of  the  Echinodermata  show,  as  compared  with 
the  great  complexity  of  the  rest  of  their  organisation,  a  very  low 
degree  of  differentiation.  They  are  for  the  most  part  uridifferentiated 
integumental  (tactile)  sensory  organs.  Specific  sensory  organs  are 
only  rarely  developed ;  the  oral  feelers  are  considered  to  be  olfactory 
and  gustatory  organs ;  the  red  spots  at  the  tips  of  the  arms  of 
Echinoids  (on  the  terminal  tentacles),  and  also  the  shining  spots  on 
the  integument  of  the  Diadematidcc  and  related  forms,  are  thought  to 
be  eyes ;  the  sphseridia  of  the  Eehinoidea,  and  Baur's  vesicles 
(otolith  vesicles)  in  the  Synaptidce  and  Elasipoda  are  considered  to  be 
organs  of  hearing  or  of  orientation. 


A.  The  Ambulaeral  Appendages  as  Sensory  Organs. 
1.  The  Terminal  Tentacles. 

We  are  undoubtedly  justified  in  assuming  that  originally  the  radial 
canals  of  the  water  vascular  system  in  all  Echinoderms  ended  distally 
in  a  freely  projecting  tentacle  or  feeler,  which,  covered  with  a  highly 
developed  sensory  epithelium,  functioned  as  a  sensory  organ. 

Such  terminal  tentacles  are  well  developed  in  all  Asteroids  and 
Ophiuroids,  and  are  found  in  them  at  the  tips  of  the  arms,  where,  as 
we  have  seen,  the  radial  water  vascular  trunks  end.  They  are  sup- 
ported by  the  terminal  plates  of  the  skeleton,  and  are  surrounded  by 
small  spines  which,  when  the  slightest  stimulus  is  applied  to  the 
terminal  tentacle,  bend  towards  one  another  protectively  over  it. 

It  has  long  been  known  that  the  terminal  tentacle  of  the  Asteroids 
carries  a  pigment  spot,  which  has  been  regarded  as  an  eye. 

In  the  Eehinoidea^  the  terminal  tentacle  is  already  somewhat 
reduced.  The  five  tentacles  are  found  on  the  five  ocular  plates  of  the 
apical  system,  and  the  pore  which  perforates  each  ocular  plate  is  the 
aperture  of  the  distal  end  of  the  water  vascular  trunk,  which  runs  into 
the  tentacle. 

The  five  terminal  tentacles  are  still  further  reduced  in  the  Holo- 
thurioidea,  where  they  lie  round  the  anus. 

Adult  Crinoids  have  no  terminal  tentacles.  The  radial  canals  end 
blindly  before  reaching  the  ends  of  the  arms  and  pinnules. 

The  terminal  tentacles,  in  contradistinction  to  all  other  ambulacral 
appendages  (ambulacral  feet,  ambulacral  tentacles,  papilla),  may  be 
regarded  as  primary  appendages.  In  the  youngest  stages  of  the 
animal,  when  the  radial  water  vascular  trunks  have  only  just  arisen 
as  outgrowths  of  the  hydroccelomic  vesicle,  they  lie  close  to  the  mouth. 
They  attain  the  position  they  occupy  in  the  adult  animal  by  the 


VIII 


ECHINODERMATA— SENSORY  ORGANS 


463 


growth  and  prolongation  of  the  radial  canals,  during  which  process 
bulgings  arise  alternately  to  right  and  left,  which,  pushing  the  integu- 
ment before  them,  form  the  ambulacral  feet  or  tentacles.  Those  tube- 
feet  are  therefore  most  recent  in  origin  which  are  nearest  to  the 
terminal  tentacles  and  furthest  from  the  ring  canal. 

The  terminal  tentacle  is  the  oldest  ambulaeral  appendage  ;  it 
receives  the  distal  end  of  the  radial  canal,  and  is  the  only  unpaired 
ambulaeral  appendage  of  the  radius. 


Special — 1.   Asteroidea. — The   tentacle  which   rests   on   the  terminal   plate  is 
covered  by  a  very  deep  sensory  epithelium,  which  consists  of  long,  thin,  sensory 


FIG.  373.— Water  vascular  system  of  a  very  young  Asteroid.  1,  Terminal  tentacle ;  2,  eye 
spot  at  its  base  ;  3,  ambulaeral  feet ;  4,  radial  canal  of  the  water  vascular  system  ;  5,  circular  canal ; 
6,  mouth. 

cells.  It  carries  long  cilia,  and  contains  beneath  the  surface  a  thick  layer  of  nerve 
fibres,  which  represent  the  distal  end  of  the  radial  nerve  tract  of  the  arm.  At  its 
base,  on  the  side  turned  to  the  mouth,  it  carries  a  vivid  orange-red  pigment  spot. 

2.  Ophiuroidea. — The  tentacle  is  surrounded  by  the  terminal  plate  as  by  a  ring. 
There  is  no  eye.     The  subepithelial  radial  nerve  enters  the  terminal  tentacle  and 
ends   in   its   epithelium.      (These    facts   recall   an   analogous   arrangement   in  the 
Annelida,  where  the  ventral  cord  still  remains  epithelial  in  the  growing  caudal  end 
after  it  has  become  subepithelial  in  other  regions  of  the  body).     In  the  Euryalce, 
which  have  much-branched  arms,  no  terminal  tentacles  have  been  found. 

3.  Echinoidea. — The  terminal  tentacle  in  the  adult  is  usually  reduced  to  a  low 
papilla  which  rises  above  the  pore  in  the  radial  plate.     In  Ecliinocyamus  pusillus 


464 


COMPARATIVE  ANATOMY 


CHAP. 


alone  (Fig.  374)  does  this  papilla  project  someAvhat  further.  The  radial  canal  tra- 
verses the  pore  and  ends  blindly  under  the  epithelium  of  the  papilla.  The  radial  nerve 
trunks  also,  and  with  them  the  epineural  canal,  pass  through  the  pore.  After  reach- 
ing the  papilla,  the  fibres,  Avhich 
had  hitherto  been  subepithelial, 
enter  the  epithelium  of  the 
papilla  and  the  epineural  sinus 
ends.  The  pseudohsemal  sinus, 
on  the  contrary,  accompanies 
the  radial  canal  and  the  radial 
nerve  trunk  only  to  the  point 
where  these  latter  enter  the 
pore. 

In  a  few  rare  cases  the 
radials  are  perforated  by  two 
pores  for  the  passage  of  two 
terminal  tentacles  (Arbaciidce 
and  certain  Palccechinoidea : 
Melonitcs  multipora,  Palccc- 
chinus  elegans). 

4.  Holothurioidea. — In 
Cucumaria  cucumis  and  C. 
Lacazii,  the  radial  canals  end 
round  the  anus,  just  as  in  the 
Echinoidca,  but  the  last  ex- 
ternally visible  trace  of  the 
terminal  tentacle  has  dis- 
appeared. The  radial  canal  perforates  the  body  Avail,  accompanied  by  the  radial 
nerve  trunk  and  the  epineural  canal,  and  ends  blindly  close  below  the  surface.  The 
radial  nerve  unites  Avith  those  "nests"  of  cells  Avhich  represent  the  integumentary 
epithelium  and  Avhich  AA-ere  described  above,  p.  415.  The  pseudohtemal  canal  ends  at 
the  point  Avhere  the  radial  canal  enters  the  body  AA7all. 

In  other  Holothurioidea  (e.g.  Holothuria,  impaticiis)  eA'en  the  last  (intrategu- 
mentary)  trace  of  the  terminal  tentacle  is  Avanting.  The  Synaptidce,  haAre  no  terminal 
tentacle,  since  the  radial  canals  are  altogether  Avanting  in  them.  The  distal  ends  of 
the  radial  nerve  trunks,  however,  perforate  the  integument,  and  in  this  AATe  may 
perhaps  see  a  last  trace  of  the  terminal  tentacle. 


PIG.  374.— Section  through  the  terminal  tentacle  of 
Echinocyamus  pusillus  (after  Cue'not).  1,  Body  epithelium  ; 
2,  test ;  3,  epineural  canal ;  4,  radial  nerve  trunk  ;  5,  pseudo- 
haemal  canal ;  6,  radial  canal  of  the  water  vascular  system  ; 
7,  endothelium  of  the  body  cavity  ;  8,  genital  circular  sinus  ; 
9,  terminal  tentacle. 


2.  The  Ambulacral  Feet  and  Ambulacral  Tentacles. 

Experiments  on  the  living  animal  shoAV  that  all  the  ambulacral  appendages 
are  very  sensitive  to  external  stimuli,  especially  mechanical  and  tactile.  If  the 
extended  ambulacral  foot  of  an  Echinoid  be  irritated,  it  contracts,  and  the  neighbour- 
ing spines  bend  over  it  protectively.  The  same  is  the  case  with  all  the  ambulacral 
appendages  of  all  Echinoderms.  This  is  Avhat  we  should  expect  from  the  rich  inner- 
vation  of  these  appendages,  which,  in  addition  to  their  other  functions,  must  be 
regarded  as  organs  of  touch.  Tactile  functions  could  indeed  be  safely  attributed  to 
them  by  merely  observing  IIOAV  the  long,  thin,  suckerless  feet  at  the  end  of  the 
Asteroid  arm,  or  the  feet  on  the  anterior  side  of  the  Spatangoid  body,  stretch  out 
tentatively  in  various  directions  like  the  "  horns  "  of  a  snail. 

It  cannot  yet  be  demonstrated  that  all  or  any  of  the  ambulacral  appendages  of 
the  Echinodermata  have  any  other  special  sensory  functions.  It  has  been  con- 
jectured that  the  oral  tentacles  are  gustatory,  but  in  the  Synaptidce  alone  haA*e 


viii  ECHINODERMATA— SENSORY  ORGANS 

specific  sensory  organs  (the  so-called  sensory  buds)  been  found  on  the  inner  side  of 
the  oral  feelers  (i.e.  on  the  side  turned  to  the  mouth). 

It  has  further  been  conjectured  that  the  highly  developed  sense  of  smell  of  many 
Echinoderms  is  located  in  ambulacral  appendages,  and  in  the  Asteroids  it  has  been 
localised  especially  in  the  terminal  tentacles  of  the  arms,  above  described.  Decisive 
experiments,  however,  have  still  to  be  made  on  this  point. 

The  innervation  of  the  ambulacral  appendages  is  as  follows:  The  tube -feet 
or  tentacle  nerves  are  always  lateral  branches  of  the  radial  nerve  trunks  of 
the  superficial  system,  these  lateral  branches  being  accompanied  by  ganglion 
cells.  When  the  base  of  the  tube-foot  or  tentacle  is  reached  the  nerves  have, 
in  the  Echinoidea,  an  epithelial  course  ;  but  in  the  Ophiuroidea  and  Holo- 
thurioidea, they  are  subepithelial  even  within  the  foot  or  tentacle.  In  the  Crinoidea 
and  Asteroidea,  these  nerves,  like  the  superficial  nervous  system,  are,  as  we  should 
expect,  epithelial. 

The  condition  of  the  nerves  within  the  appendages  varies. 

In  the  Asteroidea  and  Crinoidea,  no  localised  distinct  tentacle  or  tube -foot 
nerve  can  be  distinguished,  a  layer  of  nerve  fibres  being  developed  within  the 
epithelium  of  the  entire  foot.  In  the  Ophiuroidea,  Holothurioidea,  and  Echinoidea, 
on  the  contrary,  these  nerves  are  distinct  even  in  the  feet  (in  the  first  two  groups 
they  are  subepithelial,  and  in  the  last  epithelial),  and  their  ramifications  can  be 
clearly  followed. 

When  the  nerve  reaches  the  base  of  a  tentacle  in  the  Ophiuroidea,  it  forms  a  large 
semilunar  or  semicircular  ganglion  encircling  the  base,  before  it  ascends  into  the 
tentacle  (Fig.  372,  p.  456). 

Round  the  terminal  disc,  in  the  tube-feet  of  the  Echinoidea  and  Asteroidea,  the 
(epithelial)  nerve  tissue  becomes  thickened  to  form  a  nerve  ring,  from  which  the 
nerve  fibres  (arranged  as  a  whole  radially)  run  inwards  close  to  the  epithelium  of  the 
terminal  disc. 

The  way  in  which  the  nerve  fibres  end  in  the  epithelium  of  the  ambulacral 
appendages  is  still  a  disputed  question.  According  to  one  view,  the  nerve  fibres  are 
connected  with  filiform  sensory  cells,  which  have  been  brought  to  light  by  means  of 
maceration.  According  to  another  view,  no  such  cells  are  present,  the  nerve  fibres 
and  epithelial  cells  being  merely  in  contact. 

Special. — Any  special  terminal  apparatus  of  the  sensory  nervous  system  has 
rarely  been  observed  in  the  ambulacral  appendages.  We  have,  however : — 

(a)  The  sensory  buds  on  the  oral  tentacles  of  the  Synaptidce,  already  mentioned 
above.     These  are  arranged  in  two  longitudinal  rows  along  the  inner  side  of  each 
tentacle.     They  are  conical  or  papilla-like  projections  of  the  tentacle  wall,  with  a 
pit-like  invagination  at  the  tip.     A  nerve  from  below  the  surface  of  the  cutis  enters 
the  base  of  the  pit,  which  consists  of  strongly  ciliated  sensory  cells.     It  has  already 
been  pointed  out  that  olfactory  or  gustatory  functions  have  been  attributed  to  these 
sensory  buds. 

(b)  The  tentacles  of  the  Crinoidea  carry  scattered  sensory  papillae  in  the  form  of 
distinct  slender  projections.     Each  papilla  is  composed  of  the  fine  processes  of  the 
circle  of  sensory  cells,  which  form  its  base.    It  is  traversed  by  a  shiny  axial  (muscle) 
fibre,  and  carries  at  its  freely  projecting  end  three  delicate,  thin,  immobile  sensoiy 
hairs  (Fig.  356,  14,  p.  413). 

(c)  Similar  sensory  papillae   are   found  on  the  tentacles  of  the  Ophiuroid  form 
Ophiact-is  vircns. 

(d)  In   certain  species  of  the   Ophiuroid  genus   OfMatkrix,   the  tentacles  are 
covered  with  circular  rows  of  conical   sensoiy  papillae.      Each   papilla  seems  to 
consist  of  a  bundle  of  long  sensory  cells,  and  carries  freely  projecting  sensory  hairs 
(Fig.  375). 

VOL.  II  2  H 


466 


COMPARATIVE  ANATOMY 


CHAP. 


(e)  Sensory  papillae,  sensory  hairs,  etc.,  have  also  been  observed  on  the  various 
sensory  feet  of  the  Echinoidea. 

[On  the  polymorphism  of  the  ambulacral  appendages,  see  the  section  on  the 
Water  Vascular  System,  D,  p.  431. 


B.  Nerve  Endings  in  the  Integument. 

A  close  plexus  of  nerve  fibres  is  developed  within  the  body  epithelium  of  the 
Echinoidea  and  Asteroidea.  This  plexus  is  more  strongly  developed,  i.e.  closer  and 
thicker  at  points  which  are  specially  sensitive  to  external  stimuli,  such  as  the 

fascioles  of  the  Echinoidea,  round 
the  bases  of  the  pedicellarite,  and  on 
the  gills  (the  so-called  papulte)  of 
the  Asteroidea,  and  at  the  bases  of 
the  spines  of  the  Echinoidea. 

[For  the  sensory  prominences  on 
the  pedicellariae,  see  p.  399.] 

In  the  Crinoidca  and  Ophi- 
uroidea,  sensory  nerves  continually 

-  : -;s^^**iii^       ^  ?       ramifying   more   and   more   finely, 

*  <  ''?;i0>S5^6  nm  through  the  (calcined)  cutis  to 
the  surface  of  the  body.  The 
manner  in  which  these  nerve  fibres 
terminate  is  unknown.  Investiga- 
tion on  this  point  is  the  more 
difficult  as  the  epithelium  appears 
to  be  hardly  distinguishable  from 
the  cutis. 

At  the  edges  of  the  food  grooves 
(on  the  arms  and  the  oral  disc)  of 
the  Crinoidca,  alternating  with  the 
trilobed  tentacles,  groups  consisting 
of  five  to  six  sensory  cells  with 
delicate  immobile  hairs,  occur. 

Among    the    Holothurioidea,    a 
system  of  nerve  fibres  ramifying  in 
I)  •.    \  the    cutis    has    been    described    in 

'X,^  ,.-.  Cucumaria.     From  these  branches 

run  to  the  nests  of  epithelial  cells 

FIG.  375.— Half  of  a  transverse  section  through  ,    * 

an  ambulacral  tentacle  of  Ophiothrix  fragilis  (com-  sunk  below  the  surface,  which  were 
bined  from  figures  by  Hamann).  1,  Body  epithelium  ;  mentioned  in  connection  with  the 
2,  sensory  papillse ;  3,  cuticular  connective  tissue 


4,  nerves  to  the  sensory  papillae;  5,  longitudinal  mus- 
culature ;  6,  epithelium  of  the  tentacle  canal  (7) ; 
8,  tentacle  nerve. 


integument,  p.  415.  A  similar 
arrangement  has  been  found  in 
other  Actinopoda. 

In  the  Paractinopoda  (Synapta, 
Anapta]  numerous  scattered  sensory  or  tactile  papillae  are  found  on  the  integument, 
which,  at  such  points,  bulges  out  to  form  prominences.  At  the  centre  of  such  a 
prominence  a  group  of  sensory  cells  forms  the  tactile  papilla.  A  distinct  nerve  runs 
from  each  papilla  to  a  large  tactile  ganglion  lying  in  the  cutis  beneath  it.  The 
epithelial  cells  surrounding  the  papilla  are  differentiated  into  glandular  cells 
inhaerens). 


ECHINODERMATA— SENSORY  ORGANS 


467 


C.  Auditory  Organs,  Organs  for  Orientation. 

Two  types  of  organs  for  hearing  or  for  orientation  have  been 
observed  in  Echinoderms  :  (1)  the  auditory  vesicles  (Baur's  vesicles, 
otoeysts)  of  certain  Holothurioidea ;  and  (2)  the  sphseridia  of  the 
Eclrinoidea,  which  have  already  been  described. 

1.  Auditory  vesicles  are  found  only  in  the  Holothurioidea,  and 
among  these  in  the  Paradinopoda  (Synaptidce),  and  among  the  Actino- 
poda  in  the  Elasipoda. 

They  have  been  best  observed  in  the  Synaptidce  (Fig.  376).  In 
this  family,  five  pairs  of  these  vesicles  occur  in  the  cutis  of  the  body 
wall,  near  the  tentacles,  at  the  points  where  the  five  radial  nerve 
trunks  emerge  from  the  calcareous  ring.  On  the  outer  side  of  each 


7  ff 

FIG.  376.— Section  tlirough  the  two  auditory  vesicles  of  a  radius  of  Synapta  ''after  Cuenot). 
1,  Epineural  sinus  ;  2,  epithelial  wall  of  the  auditor}*  vesicle;  3,  otoliths;  4,  nervus  acusticus  ; 
5,  longitudinal  muscles  ;  6,  pseudohaemal  canal ;  7,  radial  nerve  trunk. 

nerve  trunk  lies  a  pair  of  otoeysts.  Each  otocyst  consists  of  a  vesicle 
filled  with  fluid,  with  a  wall  of  (ciliated)  plate  epithelium.  Numerous 
otoliths  are  found  vibrating  in  the  fluid.  These  otoliths  are  vesicular 
cells,  the  cavity  of  each  being  filled  by  a  hard  mass  (phosphate  of 
lime).  The  nerves  to  the  otoeysts  (nervi  acustici)  come  from  the 
radial  nerve  trunk. 

The  auditory  vesicles  of  the  Elasipoda  occur  in  great  numbers ; 
there  may  be  fourteen  to  one  hundred  or  even  more.  Xot  infrequently 
their  manner  of  distribution  is  bilaterally  symmetrical.  For  instance, 
in  Elpidia  glacialis,  six  of  the  fourteen  auditory  vesicles  occur  on  the 
two  lateral  radii  of  the  trivium,  and  one  on  each  of  the  two  radii  of 
the  bivium.  In  this  case  no  vesicles  occur  in  the  ventral  median 
radius, 

2.  The    sphaeridia    of    the    EchinoiJea,   which    are   regarded    as 


468 


COMPARATIVE  ANATOMY 


CHAP. 


transformed  spines,  have  already  been  described  in  the  section  on  the 
skeletal  system  (pp.  392-3).  According  to  recent  researches,  there  is 
only  a  loose  connection  between  the  sphaeridium  and  the  shell  tubercle, 
fibres  of  connective  tissue,  and  not  muscle  fibres,  uniting  them.  When 
an  Echinoid  is  in  the  natural  position,  the  sphseridia,  which  are 
developed  only  on  the  oral  side,  hang  down  perpendicularly  in  their 
niches  or  chambers,  owing  to  the  weight  of  the  dense  calcareous  mass 
which  forms  their  rounded  ends.  They  are  thus  able,  by  pressing  on 
the  nerve  cushion  at  their  base,  to  orientate  an  animal  as  to  its  position 
in  space.  Echinoids  which  are  laid  on  the  back,  quickly  turn  them- 
selves over  again. 

D.  Eyes. 

1.  The   eye-spots   of  Asteroids    have   already   been   mentioned 
(p.  463)  in  connection  with  the  terminal  tentacles.     A  vivid  red  eye- 


FIG.  377.— Section  through  the  optic  cushion  at  the  base  of  the  terminal  tentacle  of  an 
Asteroid.  1,  Cuticle  of  the  optic  cup  ;  2,  pigment  cells  ;  3,  cuticle  of  the  tentacle  epithelium  (4) ; 
5,  nerve  layer  below  the  surface  of  the  same  ;  6,  cutis  of  connective  tissue  ;  7,  epithelium  of  the 
tentacle  canal. 

spot  is  found  at  the  base  of  each  of  these  tentacles,  on  the  side  turned 
to  the  mouth.  On  closer  examination  each  eye-spot  is  found  to  break 
up  into  a  large  number  of  single  eyes,  shaped  like  cups  or  hollow 
cones.  The  tips  of  these  conical  cups  are  directed  inward  towards 
the  highly  developed  layer  of  nerve  fibres  below  the  surface  of  the 
tentacle  epithelium,  while  their  cavities  open  outward  (Fig.  377).  The 
wall  of  each  optic  cup  is  formed  of  pigment  cells  (with  interspersed 
urpigmented  retinal  cells).  The  cuticle  of  the  tentacle  epithelium 


VIII 


ECHINODERMATA— SENSORY  ORGANS 


469 


also  is  continued  into  each  cup.  The  portions  of  the  cuticle  which 
belong  to  the  different  cells  forming  the  wall  of  the  cup  are  distinct 
from  one  another,  and  have  been  described  as  rods. 

Living  Asteroids  carry  the  arm  with  its  tip  directed  upwards,  so 
as  to  enable  the  eye-spot  to  function. 

2.  In  Diadema  setosum  (Euechinoidea  diadematoida),  an  animal 
highly  sensitive  to  light,  the  skin,  which  is  as  black  as  velvet,  is 


FIG.  37S.— Part  of  a  compound  eye  of  Diadema  setosum  (after  P.  and  F.  Sarasin). 
p,  Pigment  cups. 

ornamented  with  numberless  shiny  blue  spots ;    these  are  gradually 
lost  on  the  oral  side. 

Each  of  these  blue  spots,  when  its  surface  is  examined  with  the 
microscope,  breaks  up  into  a  number  of  pentagonal  or  hexagonal 
portions.  The  number  of  these  varies,  according  to  the  size  of  the 
spot,  sometimes  being  many  hundreds.  Each  is  a  refractive  body, 
which  stands  in  a  cup  of  black  pigment  (Fig.  378).  The  blue  colour 


FIG.  379.— Section  through  the  eye  of  Diadema  setosum,  diagrammatic  (after  P.  and  F.  Sara- 
sin).  1,  Layer  of  ganglion  cells;  2,  "cornea"  ;  3,  refractive  body ;  4,  pigment  cups;  5,  nerve 
layer,  or  plexus  ;  6,  fibres  of  connective  tissue  ;  7,  collection  of  pigment  below  the  nerve  layer. 

of  the  spots,  which  are  regarded  as  compound  eyes,  is  due  to  inter- 
ference. 

A  section  made  through  such  an  eye  (Fig.  379)  reveals  :  (1)  that 
the  body  epithelium,  which  is  covered  by  a  ciliated  cuticle,  spreads, 
much  thinned,  over  the  whole  eye  (cornea)  •  (2)  that  each  "  refractive 
body "  consists  of  a  number  of  vesicular  cells  (modified  epithelial 
cells) ;  (3)  that  a  pigment  cup  (consisting  of  cells  which  are  often 
branched  and  star-like)  surrounds  the  basal  portion  of  each  refractive 
body ;  (4)  that  the  whole  eye,  with  its  numerous  pigment  cups,  rests 


470  COMPARATIVE  ANATOMY  CHAP. 

directly  upon  a  nerve  layer  provided  with  ganglion  cells,  which,  at 
the  edge  of  the  eye,  passes  into  the  usual  layer  of  nerve  fibres,  found 
in  all  Echinoids  below  the  surface  of  the  body  epithelium. 

Similar  spots  are  found  in  other  Diadematidce,  and  species  of  the  related  genus 
Astropyga. 

In  the  Synaptidce  (S.  vittata),  at  the  base  of  each  tentacle,  two  pigment  spots 
occur.  It  has  now  been  proved  that  these  "  eyes  "  are  sensory  organs,  but  a  detailed 
account  of  them  has  not  yet  been  given. 


XIII.  The  Body  Musculature. 

The  special  development  of  the  body  musculature  of  the  Echino- 
derms  is  directly  connected  with  the  peculiar  development  of  the 
skeleton. 

Regarding  the  musculature  and  the  skeleton  alone,  the  Echino- 
derms  may  be  divided  into  three  groups. 

Holothurioidea. — The  skeleton  here  consists  merely  of  isolated, 
and  usually  microscopically  small,  calcareous  bodies.  The  presence  of 
these  in  the  leathery  integument  does  not  prevent  changes  of  form  in 
the  tubular  body.  The  body  musculature  is  developed  as  a  dermo- 
museular  tube.  By  means  of  co-ordinated  contractions  of  the 
circular  and  longitudinal  musculature  of  which  this  tube  is  com- 
posed, the  animal  is  able  to  make  slow,  worm-like  movements. 

Asteroidea,  Ophiuroidea,  and  Crinoidea. — The  body  in  these 
classes  is  drawn  out  in  the  direction  of  the  radii  in  the  form  of 
arms,  which  are  supported  by  a  jointed  skeleton.  The  dermo- 
museular  tube  breaks  up  into  separate  muscles,  which  connect  the 
joints  of  the  skeleton  together.  In  the  Asteroidea  alone,  a  dermo- 
muscular  tube  persists  side  by  side  with  these  isolated  muscles. 

Eehinoidea.  —  The  skeleton  of  the  Echinoidea  (with  but  few 
exceptions)  is  a  rigid  test  or  capsule.  A  body  musculature  would 
here  be  useless,  and  is  therefore  wanting. 

An  exception  to  this  rule  is  found  among  the  Euechinoidea  in  the 
Streptosomata,  in  which  the  plates  of  the  more  or  less  flexible  test 
imbricate.  It  has  now  been  proved  that  in  the  fichinothuridce,  five 
pairs  of  muscle  lamellae  run  in  meridians  from  the  oral  to  the  apical 
side  on  the  inner  surface  of  the  test.  The  contraction  of  these 
muscle  lamellae  causes  a  depression  of  the  test. 

The  musculature  of  the  Echinodermata  consists,  as  a  rule,  of  smooth  muscle  fibres. 
A  longitudinally  striated  fibril  of  contractile  substance  lies  on  one  side  of  the  undiffer- 
entiated  protoplasm  of  the  formative  cell  which  contains  the  nucleus.  Transversely 
striated  muscle  fibres  are  of  less  frequent  occurrence,  but  are  found  in  the  adductor 
muscles  of  the  seizing  pedicellarise  in  the  Echinoidea  (p.  398),  and  in  the  muscles  of 
the  rotating  anal  spines  of  Centrostephamis  longispinus. 

The  musculature  of  the  various  organs  or  systems  of  organs  will  be  described  in 
the  sections  dealing  with  those  organs. 


ECHIXODESMA  TA—BOD Y  MUSCULA  TURE  4 7 1 


A.  Holothurioidea. 

The  dermomuscular  tube  consists  everywhere  of  an  outer  circular 
muscle  layer,  and  of  five  radial  longitudinal  muscles  (Figs.  371  and 
3 S3,  pp.  451  and  477). 

The  circular  musculature  lies  immediately  within  the  cuticle.  It 
is  usually  interrupted  in  the  five  radii,  and  thus  consists  of  five  longi- 
tudinal interraclial  strips  or  bands  of  muscle,  the  fibres  of  which  run 
transversely.  In  the  Paradinopoda  (Synaptidce)  alone,  where  there  are 
no  radial  canals  of  the  water  vascular  system,  the  fibres  run  uninter- 
ruptedly round  the  body. 

The  longitudinal  musculature  consists  of  five  strong  muscles  or 
pairs  of  muscles  traversing  the  body  longitudinally  along  the  radii. 
These  muscles  thus  cover,  on  the  side  of  the  body  cavity,  the  radial 
organs  enumerated  on  p.  409.  Anteriorly  (at  the  oral  pole)  they  are 
inserted  into  the  radial  pieces  of  the  calcareous  ring,  posteriorly  (at 
the  apical  pole)  they  end  near  the  anus. 

In  the  DendrochirotcRj  the  longitudinal  musculature  is  differentiated 
in  a  peculiar  manner.  At  the  middle  of  the  body,  or  somewhat  in 
front  of  it,  the  fibres  of  each  of  the  five  longitudinal  muscles  divide 
into  two  bundles.  One  of  these  bundles  is  continued  simply  as  a 
longitudinal  muscle  along  the  body  wall,  while  the  other  freely  tra- 
verses the  bod}'  cavity,  and  is  attached  anteriorly  to  a  radial  piece  of 
the  calcareous  ring  (Fig.  349,  p.  404).  The  retractor  muscles  of  the 
oral  region  have  in  this  way  been  derived  by  the  splitting  up  of  the 
originally  simple  longitudinal  muscles,  and  this  specialisation  became 
more  marked  as  the  oral  tentacles  of  the  Dendrochirotcf,  became  more 
and  more  highly  developed,  and  required  increasing  protection. 
Species  are  to  be  found  in  which  the  separation  and  branching  off  of 
retractors  from  the  longitudinal  muscles  has  not  yet  been  perfected. 

Apart  from  the  Demt rock  i rot ce,  retractors  occur  only  in  the  genus 
Molptttlia,  and  in  species  of  the  genera  Clrirodota  and  Synapta. 


B.  Echinoidea. 

The  longitudinal  muscles  of  the  Eclrinothuridce  (Asthenosoma)  have 
the  shape  of  semilunar  leaves,  the  convex  sides  of  which  are  directed 
outwards,  and  are  attached  to  the  inner  surface  of  the  test;  the 
concave  edges,  on  the  other  hand,  face  the  axis  of  the  test  (Fig.  380). 
They  are  inserted  into  the  test  at  the  boundaries  between  the 
ambulacra  and  the  interambulacra,  at  the  lateral  edges  of  the  ambu- 
lacral  plates. 

In  each  muscle  lamella,  the  fibrous  bands  radiate  fan-like  (Fig.  380)  from  a 
"centrum  tendineum  "  on  the  inner  edge  of  the  lamella.  The  uppermost  fibres  are 
attached  to  the  radials,  the  lowermost  to  the  outer  side  of  the  auriculae.  Anasto- 
moses between  the  fibres  are  not  infrequent. 


472 


COMPARATIVE  ANATOMY 


CHAP. 


The  five  pairs  of  longitudinal  muscles  or  pairs  of  muscle  lamellae  in  the  Echino- 
thuridce  invite  comparison  with  the  five  longitudinal  muscles  or  pairs  of  muscles  in 
the  Holothurioidea.  No  true  homology  can,  however,  be  proved  with  certainty, 


Fig.  380.— Test  of  Asthenosoma,  broken  open  so  as  to  show  the  longitudinal  muscles.  1,  In- 
terambulacral  plates  ;  2,  ambulacral  plates ;  3,  radial  canals ;  4,  centrum  tendineum ;  5,  muscle 
bands ;  6,  ambulacral  apophysis  (auricula)  ;  7,  opening  muscle  of  the  teeth  ;  8,  retractors  of  the 
masticatory  apparatus. 

since  no  direct  relation  between  the  calcareous  ring  of  the  Holothurian,  and  any 
definite  portions  of  the  Echinoid  skeleton  (such  as  the  auricules,  or  the  pieces  of 
the  masticatory  apparatus)  can  be  established. 


C.  Asteroidea. 

On  the  apical  side  of  the  arms  and  of  the  disc,  a  dermomuscular 
layer  has  been  observed,  which  consists  of  external  transverse,  and 
internal  radial  fibres,  and  runs  lengthwise  in  the  arm.  This  does  not 
appear  to  spread  (as  a  muscle  layer)  to  the  oral  side  of  the  body,  where 
the  ambulacral  skeleton  is  developed.  It  may  perhaps,  however,  have 
become  differentiated  here  into  the  special  musculature  of  the  ambu- 
lacral skeleton. 

This  latter  is  developed  as  follows  : — 

Ten  muscles  occur  in  each  skeletal  segment. 

1.  On  each  side   two  vertical  muscles  (or  bands),    one  distal  and  the  other 
proximal,  connect  the  adambulacral  with  the  ambulacral  plate  (cf.  Fig.  309,  p.  351). 

2.  On  each  side  an  upper  longitudinal  muscle  connects  every  two  consecutive 
ambulacral  plates  on  their  apical  side  (that  turned  to  the  body  cavity).     The  func- 
tion of  these  muscles  is  to  bend  the  arm  upward  (Fig.  381,  2  and  7). 

3.  On  each  side  a  lower  longitudinal  muscle  connects  every  two  consecutive 
adambulacral  pieces  ;  this  muscle  counteracts  the  upper  longitudinal  muscle. 


ECHINODERMATA—BODY  MUSCULATURE 


473 


4.  An  upper  transverse  muscle  connects  the  two  opposite  ambulacral  plates  of 
one  and  the  same  skeletal  segment,  on  their  apical  side  (that  turned  to  the  body 
cavity).     These  muscles,  by  their  contraction,  widen  the  ambulacral  furrow  (Fig. 
382,  3  and  6). 

5.  A  lower  transverse  muscle  connects  the  two  ambulacral  plates  of  a  segment 
on  the  lower  side,  which  is  turned  to  the  furrow.     These  muscles,  by  their  contrac- 
tion, narrow  the  furrow. 

The  musculature  of  the  oral  skeleton  (Fig.  381)  is  arranged  as  follows  : — 


FIG.  381.— Oral  skeleton  and  basal  part  of  the  brachial  skeleton  of  Pentaceros  turritus, 
with  the  musculature  of  these  parts  (after  Viguier).  From  within.  1,  First  adambulacral  plates  ; 
I-IV,  first  to  fourth  ambulacral  plates  ;  or,  orals ;  2,  dorsal  longitudinal  muscles  ;  3,  dorsal  trans- 
verse muscles  (for  opening  the  ainbulacral  furrow) ;  4,  interbrachial  pillars  ;  5,  muscle  apophyses 
of  the  first  adambulacral  plates  ;  6,  facets  of  the  ambulacral  plates  for  the  attachment  of  the  dorsal 
transverse  muscles  ;  7,  ditto  for  the  attachment  of  the  dorsal  longitudinal  muscles  ;  8,  transverse 
muscles  between  the  first  adambulacral  plates  (teeth) ;  9,  dorsal  transverse  muscles  between  the  first 
pair  of  ambulacral  plates  ;  10,  dorsoventral  muscle ;  11,  stone  canal ;  12,  crossed  ligament ;  13,  ab- 
ductor dentium  ;  14,  adductor  dentium  ;  o,  aperture  for  the  first  ambulacral  feet. 

1.  A  single  or  double  radial  muscle  connects  the  distal  ends  of  the  first  adam- 
bulacral plates  (teeth,  1  in  Fig.  310,  p.  352)  of  one  and  the  same  radius  (Fig.  381, 13), 
and  opens  these  plates. 

2.  A  muscle,  which  connects  the  distal  ends  of  the  first  two  adambulacral  plates 
of  two  neighbouring  radii,  and  is  therefore  interradial.     These  interradial  muscles, 
by  contracting,    close   the  pairs   of   teeth   (Fig.    381,    14).      This    closing  action 
is  assisted  by  a  transverse  muscle,  which  joins  the  opposing  edges  of  each  pair  of 
teeth  (8). 

3.  The  first  pair  of  ambulacral  plates  of  a  radius,  like  all  succeeding  pairs,  are 
connected  together  by  means  of  a  dorsal  transverse  muscle  (Fig.  381,  9). 


474  COMPARATIVE  ANATOMY  CHAP,  vm 

4.  Five  pairs  of  dorsoventral  muscles  connect  the  first  five  pairs  of  ambulacral 
plates  with  the  dorsal  wall  of  the  disc.  By  the  contraction  of  these,  the  apical  wall 
of  the  disc  is  approximated  to  its  oral  wall  (Fig.  381,  10). 

D.  Ophiuroidea. 

A  dermomuscular  tube  is  altogether  wanting.  The  muscles  which 
move  the  brachial  skeleton  (the  intervertebral  muscles)  have  already 
been  described,  p.  357. 

The  musculature  of  the  oral  skeleton  (cf.  Figs.  314  and  386,  pp.  359  and  486). 

1.  Amusculus  interradialis  externus  connects  transversely  the  opposite  distal 
surfaces  of  the  oral-angle  plates  of  neighbouring  arms.  This  is  the  most  powerful 
of  the  muscles. 

2  and  3.  The  two  oral-angle  plates  of  one  and  the  same  arm  are  connected 
by  an  upper  and  a  lower  transverse  muscle  (musculus  radialis  superior  et  inferior), 
and  approximated  by  means  of  their  contraction. 

The  three  muscles  just  described  form  an  outer  circle,  which  is  followed,  orally, 
by  a  second  inner  circle,  consisting  of  the  following  muscles  : — 

4.  A  musculus  interradialis  internus  inferior  connects  transversely  the  oral 
ends  of  the  oral-angle  plates  of  the  different  arms. 

5.  The  innermost  muscles  of  the  oral  skeleton  consist  of  fibres  which  radiate  out- 
wards.    They  run  as  five  interradially  placed  pairs  of  muscles,  from  the  oral-angle 
plates  to  the  teeth  (in  Ophiactis  only  to  the  upper  teeth),  for  whose  movement  they 
serve.     These  are  the  musculi  interradiales  interni  superiores. 

E.  Crinoidea. 

A  dermomuscular  tube  is  wanting.  The  musculature  which  moves 
the  jointed  skeleton  has  already  been  described,  p.  376. 


XIV.  The  Alimentary  Canal. 

A.  General  Review. 

The  alimentary  canal,  which  runs  through  the  body  cavity,  being 
attached  or  suspended  to  the  body  wall  in  various  ways  by  means  of 
mesenteries  or  mesenterial  filaments,  commences  with  the  mouth  and 
ends  with  the  anus. 

The  absence  of  the  anus  in  the  Ophiuroidea  and  in  the  Asteroid 
family  Astropedinidce  cannot  be  regarded  as  an  original  condition. 

In  no  ease  does  the  alimentary  canal  run  as  a  straight  tube  from 
mouth  to  anus,  although,  in  many  Synaptidce,  its  condition -is  almost 
as  simple.  As  a  rule,  the  secreting  and  resorbing  surface  of  the  canal 
is  increased  in  one  of  two  ways  : — 

1.  The  alimentary  canal,  between  the  mouth  and  anus,  becomes 
increasingly  lengthened,  and  thus  necessarily  forms  loops,  and  has  a 
winding  course  (Holothurioidea,  Echinoidea,  Crinoidea). 


R 
Hblofhuricr 


JTntedon,  Pentacrmus. 


anas 


x«  anus 

FIG.  382.— Diagram  of  the  course  of  the  alimentary  canal  in  various  Echinoderms.  The 
body  is  viewed  from  the  oral  side,  rnd,  ms,  Mesenteries  ;  as,  food-grooves  or  ambulacral  furrows 
of  the  tegmen  calycis  ;  m,  madreporite  ;  x,  commencement  of  the  backward  coil  of  the  intestine  ; 
ii  first,  L2  second  (backward)  coil  of  the  intestine  ;  s,  siphon,  accessory  intestine  ;  1  and  2  (in  F) 
the  two  points  at  which  the  accessory  intestine  enters  the  principal  intestine  ;  «i,  free  portion  of 
the  accessory  intestine  ;  so,  portion  of  the  same  in  contact  with  the  principal  intestine  ;  coec,  in- 
testinal ccecum.  [Combined  from  several  sources.] 


476  COMPARATIVE  ANATOMY  CHAP,  vm 

2.  The  alimentary  canal  runs  direct,  without  coiling,  from  mouth 
to  anus,  but  has  sae-like  widening^  (Ophiuroidea),  and  further,  in  the 
Asteroids,  sends  off  branched  outgrowths  into  the  arms. 

The  wall  of  the  intestine,  as  a  rule,  in  the  Echinoderm,  consists  of 
the  following  layers :  (1)  a  deep  inner  epithelium,  with  numerous 
glandular  cells  ;  (2)  a  layer  of  connective  tissue  ;  (3)  a  muscle  layer  ; 
(4)  an  outer  epithelium,  the  endothelium  of  the  body  cavity.  A 
system  of  blood  lacunae  (absorbing  canals)  is  developed  in  the  layer 
of  connective  tissue  in  the  Holothurioidea,  Ecliinoidea,  and  Crinoidea. 


B.  Holothurioidea  (Figs.  371,  p.  451,  and  383). 

Course  of  the  alimentary  canal. —  The  mouth  lies  at  the  oral 
pole  (i.e.  at  the  anterior  end  of  the  body),  the  anus  at  the  apical  pole. 
For  the  exceptions  to  this  rule,  especially  ffhopalodina,  in  which  the 
mouth  lies  close  to  the  anus,  cf.  section  III.,  p.  408. 

The  alimentary  canal  is,  as  a  rule,  considerably  longer  than  the 
body  (on  an  average  three  times  as  long),  and  therefore  has  a  looped 
or  winding  course.  From  the  mouth,  it  first  runs  backward  towards 
the  anus  (first  or  anterior  section),  it  then  bends  for  the  first  time, 
and  runs  forward  again  (second  or  middle  section) ;  lastly,  it  bends 
again  near  the  anterior  end  of  the  body,  and  runs  backward  once 
more,  this  time  to  the  anus  (third  or  posterior  section). 

In  making  these  bends,  the  alimentary  canal  forms  a  spiral  round 
the  principal  (longitudinal)  axis  of  the  body  ;  this  is  very  clearly 
seen  by  following  the  line  of  attachment  of  its  mesenteries  to  the 
body  wall. 

The  anterior  section  is  attached  to  the  median  dorsal  line  interradially.  From 
this,  at  the  first  bend,  the  mesentery  passes  across  the  left  dorsal  radius  into  the  left 
dorsal  interradius.  The  whole  of  the  central  section  is  attached  in  this  interradius. 
At  the  second  bend,  the  mesentery  passes  over  the  left  ventral  radius  and  interradius, 
and  over  the  middle  ventral  radius,  into  the  right  ventral  interradius.  The  third 
or  posterior  section  is  attached  in  this  latter  interradius  (Figs.  350,  p.  407,  and 
383). 

If  a  Holothurian  is  placed  upright,  with  the  oral  pole  upward  and 
the  apical  downward,  and  if  we  project  the  loops  of  the  alimentary 
canal  on  to  a  horizontal  plane,  or,  if  we  simply  view  the  intestinal 
loops  of  a  Holothurian  from  the  oral  pole,  we  see  that  the  digestive 
tract  runs  from  left  to  right,  i.e.  in  the  direction  of  the  hands  of  a 
clock.  In  other  Echinoderms  with  coiled  intestine,  the  coils  also  run 
in  this  direction. 

It  was  mentioned  above  that  the  alimentary  canal  of  many  Parac- 
tinopoda  (Synaptidce)  is  almost  straight.  This  is,  however,  not  an 
original  condition,  as  is  seen  from  the  following  facts  :  (1)  the  older 
larva  and  the  quite  young  Synapta  have  a  bent  alimentary  canal ;  (2) 
the  intestinal  mesentery  is  inserted  in  the  body  wall  exactly  in  the 


Fig.  383.— Organisation  of  an  Aspidochirotan  Holothurian.  In  the  dorsal  interradius,  the 
body  wall  is  cut  through  on  the  left,  near  the  dorsal  mesentery,  and  is  spread  out  (after  Ludwig,  in 
Leuckart's  Tafelwerk).  1,  Genital  aperture  ;  2,  radial  plates  ;  3,  interradial  plates  of  the  calcareous 
ring ;  4,  genital  duct ;  5,  dorsal  or  anterior  mesentery  of  the  intestine  ;  6,  stone  canals  with  their 
inner  madreporites  ;  7,  dorsal  intestinal  vessel  ;  8,  gonads  ;  9,  anterior  section  of  the  alimentary 
canal ;  10,  ventral  intestinal  vessel ;  11,  posterior  section  of  intestine  ;  12,  longitudinal  muscles  ;  13, 
posterior  edge  of  the  dorsal  mesentery  ;  14,  right  branchial  tree  (water  lung)  ;  15,  circular  muscula- 
ture of  the  body  wall ;  16,  longitudinal  muscles ;  17,  cut  edge  of  the  body  wall ;  18,  longitudinal 
muscles ;  19,  partly  muscular  filaments  running  from  the  wall  of  the  cloaca  to  the  body  wall ;  20,  cloa- 
cal  aperture ;  21,  cloaca ;  22,  Cuvierian  organs ;  23,  left  branchial  tree ;  24  and  25,  longitudinal  muscles ; 
26,  middle  section  of  the  alimentary  canal  ;  27,  vascular  anastomosis  ;  28,  fore-gut ;  29,  Polian 
vesicle  ;  30,  blood  vascular  ring  ;  31,  water  vascular  ring  ;  32,  commencement  of  the  radial  vessel. 


UBS*; 

OF    VHK 

UNIVERSITY 


478  COMPARATIVE  ANATOMY  CHAP. 

manner  above  described,  and  thus  runs  in  a  spiral ;  (3)  in  most  cases, 
close  examination  reveals  that  even  in  the  adult  the  canal  is  coiled  in 
a  spiral,  although  drawn  out  to  a  great  length,  and  that  the  first  and 
second  bendings  can  still  be  distinguished  as  slight  curves. 

If  the  typical  alimentary  canal  of  an  Adinopod  were  to  be 
shortened  until  it  was  of  almost  the  same  length  as  the  body,  the  con- 
dition found  in  the  Synaptidce  would  arise. 

The  divisions  OP  sections  of  the  alimentary  canal. — In  the 
intestine  of  the  Holothurioidea,  consecutive  sections  have  been 
distinguished,  but  these  are  never  very  marked  microscopically. 
Throughout  its  whole  course,  the  canal  retains  its  tubular  shape. 
The  different  sections  are  distinguished  by  their  sizes  and  by  the 
thickness  of  their  walls,  by  their  colour,  their  vascularisation,  and 
especially  by  their  histological  structure.  The  boundaries  of  the  con- 
secutive sections  are  usually  externally  indicated  by  circular  constric- 
tions of  varying  distinctness;  these  constrictions  not  infrequently 
correspond  with  circular  folds  projecting  into  the  canal. 

The  mouth. — Around  the  mouth,  the  circular  musculature  becomes  thickened  into 
a  small  sphincter  muscle. 

The  more  strongly  the  oral  tentacles  are  developed,  the  more  marked  is  the 
capacity  for  invaginating  the  mouth  with  its  tentacles,  and  with  a  larger  or  smaller 
portion  of  the  anterior  end  of  the  body,  into  the  body  cavity.  In  the  Dendrochirotcc, 
in  which  the  tentacles  are  strongly  developed,  the  invaginable  portion  of  the  anterior 
end  of  the  body  is  called  the  proboscis.  It  is  not  infrequently  distinguished  by  the 
different  colouring  and  constitution  of  its  integument.  In  all  cases,  in  invagination, 
the  chief  part  is  played  by  the  retractor  muscles  (cf.  p.  471).  At  the  posterior 
boundary  of  the  proboscidal  region  five  (interradial  or  radial)  calcareous  valves  are 
occasionally  developed  ;  these,  when  the  proboscis  is  invaginated,  close  the  aper- 
ture (e.g.  Psolus,  Figs.  227  and  228,  p.  287). 

The  oesophagus  reaches  from  the  mouth  to  the  circular  canal  of  the  water 
vascular  system,  or  even  further.  It  is  attached  to  the  water  vascular  ring,  the  cal- 
careous ring,  the  radial  canals  of  the  water  vascular  system,  etc. ,  by  means  of  bands 
which  run  out  radially,  traversing  the  pericesophageal  sinus  (see  Fig.  365,  p.  428). 
These  bands  are  chiefly  of  the  nature  of  connective  tissue,  but  also  contain  muscle 
fibres.  The  oesophagus,  with  the  complex  of  surrounding  organs,  is  sometimes 
called  the  pharyngeal  bulb. 

The  oesophagus  is  followed  by  a  shorter  portion  known  as  the  stomach  intestine, 
and  this  again  by  the  longest  part  of  the  digestive  tract,  the  small  intestine.  This 
last  forms  the  larger  posterior  portion  of  the  first  section  of  the  intestine,  the  whole 
of  the  second  section,  and  by  far  the  greater  portion  of  the  third  and  last  section. 

The  last  part  of  the  alimentary  canal,  the  cloaca  or  rectum,  is  distinguished  by 
special  thickness,  and  is  attached  by  radially  arranged  strands  and  filaments  to 
the  neighbouring  body  wall.  These  strands  consist  of  connective  tissue  and  muscle 
fibres. 

Into  the  cloaca  or  rectum  open  the  water  lungs  and  the  Cuvierian  organs, 
where  these  are  present.  These  will  be  described,  pp.  487,  488.  In  some  Elpidiidce, 
the  anterior  part  of  the  cloaca  bulges  out  to  form  a  large  caecum,  which  projects  more  or 
less  far  into  the  body  cavity,  sometimes  reaching  almost  to  the  middle  of  the  body. 
Since  the  Elpidiidce  possesses  no  water  lungs,  there  is  some  justification  for  the 


viii  ECHINODERMATA— ALIMENTARY  CANAL  479 

suggestion  recently  made  that  their  cloacal  caecum  may  function  as  a  rudimentary 
organ  of  this  kind. 

The  inner  surface  of  the  alimentary  canal  often  shows  a  longitudinal  fold. 
Transverse  intestinal  folds,  arranged  in  longitudinal  rows,  have  been  observed  in 
the  small  intestine  of  the  Aspicl.odiirotaz. 

Finer  structure  of  the  alimentary  canal. — The  wall  of  the  digestive  tract 
consists  of  the  following  layers,  which  may  vary  greatly  in  thickness  and  special 
structure  in  the  different  sections  :  (1)  An  inner  intestinal  epithelium  ;  (2)  an  inner 
layer  of  connective  tissue  with  the  blood  lacunae  ;  (3)  a  muscle  layer  (generally  con- 
sisting of  an  inner  layer  of  longitudinal  and  an  outer  layer  of  circular  fibres,  but  in 
some  Synaptidce  and  Aspidochirotce  this  order  is  reversed)  ;  (4)  an  outer  layer  of  con- 
nective tissue  (often  very  thin)  ;  (5)  the  ciliated  endothelium  of  the  body  cavity. 

The  inner  intestinal  epithelium  is  ciliated,  especially  in  the^  small  intestine. 
Over  most  of  the  digestive  tract  it  is  found  as  a  very  deep  epithelium  covered  by  a 
fine  cuticle,  its  cells  being  pallisade  or  thread  cells.  Glandular  cells  of  various 
sorts  are  specially  numerous  in  the  epithelium  of  the  stomach.  The  epithelium  of 
the  cloaca  resembles  the  outer  body  epithelium.  Into  it  open  the  processes  of  long 
subepithelial  glands,  which  are  unicellular  and  tubular. 

The  anus  can  be  closed  by  means  of  a  sphincter  muscle.  Calcareous  plates, 
papilla?,  etc.,  may  occur  round  it. 


C.  Eehinoidea. 

For  the  position  of  the  mouth  and  the  anus,  cf.  p.  338. 

In  all  adult  Eehinoidea,  the  length  of  the  tubular  intestine  is 
greater  than  that  of  a  straight  line  from  the  mouth  to  the  anus,  so 
that  the  course  of  the  alimentary  canal  is  necessarily  coiled. 

The  simplest  arrangement  is  found  in  the  Clypeastroida  (Fig.  382, 
E,  p.  475).  The  direction  of  the  intestinal  coils  will  here  be  described 
in  the  same  way  as  in  the  Holothurioidea,  the  viscera  being  viewed 
from  the  oral  side.  After  traversing  the  masticatory  apparatus,  the 
alimentary  canal  turns  to  the  right  (following  the  direction  of  the 
hands  of  a  clock),  and  makes  rather  more  than  a  complete  coil  round 
the  principal  axis  of  the  body.  So  far,  the  course  exactly  resembles 
that  of  the  intestine  in  the  Holothurioidea.  In  the  Clypeastroida, 
however,  the  canal  now  bends  back  upon  itself,  and  runs  direct  back 
to  the  anus,  which  in  this  division  of  the  Eehinoidea  lies  orally  in  the 
posterior  unpaired  interradius. 

In  the  endoeyelie  or  regular  Eehinoidea,  the  arrangement  is  not 
simpler,  but  still  more  complicated.  After  leaving  the  masticatory 
apparatus,  the  alimentary  canal  ascends  towards  the  apical  system, 
then  bends  round  and  follows  the  direction  of  the  hands  of  a  clock 
(attached  to  the  inner  surface  of  the  test  by  the  mesentery)  till  it  has 
run  about  once  round  the  principal  axis.  It  then  bends  back  upon 
itself,  coiling  in  the  reverse  direction  backwards  to  the  apical  anus 
(Fig.  382,  D,  p.  475). 

The  intestine  of  the  exocyclic  Spatangoldea  resembles  in  its  course 
that  of  the  endoeyelie  Eehinoidea  with  one  difference,  caused  by  the 
facts  that  the  mouth  has  shifted  anteriorly  along  the  oral  surface,  and 


480  COMPARATIVE  ANATOMY  CHAP. 

the  anus,  out  of  the  apical  system  into  the  posterior  interradius.  The 
mouth  therefore  draws  the  commencement  of  the  first  intestinal  coil 
(which  runs  in  the  direction  of  the  hands  of  a  clock)  forward,  while 
the  anus  draws  back  (i.e.  posteriorly)  the  end  of  the  spiral  which  runs, 
as  above  described,  backwards  (Fig.  382,  F,  p.  475). 

It  is  worthy  of  note  that,  in  quite  young  Spatangoidea  (Hemiaster 
cavernosus,  2  mm.  long),  the  intestine,  which  appears  to  end  blindly, 
ascends  direct  from  the  oral  to  the  apical  pole.  At  a  rather  later 
stage  the  mouth  is  still  central,  while  the  apical  end  of  the  alimentary 
canal  has  already  somewhat  shifted,  and  opens  through  the  anus  out- 
side of  the  apical  system.  At  this  stage  (when  the  length  of  the 
animal  is  from  two  to  three  mm.)  the  intestine  runs  up  from  mouth 
to  anus  in  one  single  coil,  as  a  spiral  in  the  direction  of  the  hands 
of  a  clock.  The  complicated  arrangement  in  the  adult  is  thus 
secondary,  and  is  no  doubt  due  to  the  fact  that  the  canal  increases  in 
length  more  than  does  the  interval  between  mouth  and  anus. 

Finer  structure  of  the  intestinal  wall. — This  agrees  essentially  with  the  struc- 
ture described  in  connection  with  the  Holothurioidea.  No  distinct  sections  can  be 
made  out  in  the  alimentary  canal.  That  part  of  it  which  runs  through  the  mastica- 
tory apparatus  is  often  called  the  pharynx.  Its  lumen  in  section  is  five-rayed,  the 
layer  of  connective  tissue  forming  five  longitudinal  ridges  which  bulge  in  the 
epithelium.  It  is  connected,  in  a  manner  which  cannot  here  be  further  described, 
bv  means  of  five  pairs  of  longitudinal  bands  of  connective  tissue,  with  the  surround- 
ing masticatory  apparatus. 

The  name  ossophagus  is  generally  given  to  the  portion  of  the  digestive  tract 
which  follows  the  pharynx  (and,  in  the  Spatangoidea,  to  the  whole  of  the  first  por- 
tion of  the  intestine)  as  far  as  the  point  where,  in  regular  Echinoids,  there  is  a  sac- 
like  widening,  and  in  the  Spatangoidea  a  large  csecum.  In  regular  Echinoidea,  it 
includes  that  part  of  the  intestine  which  ascends  from  the  lantern  towards  the 
apical  system,  together  with  the  first  portion  of  the  first  spiral.  In  the  Spatangoidea 
it  runs  back  from  the  mouth  and  then  bends  forward,  forming  the  commencement 
of  the  first  intestinal  spiral. 

The  oesophagus  is  followed  by  the  first  intestinal  spiral,  which  runs  in  the 
direction  of  the  hands  of  a  clock.  It  commences  with  a  slight  sac-like  swelling 
(regular  Echinoids)  or  a  large  caecum  (Spatangoidea).  In  this  part  of  the  alimen- 
tary canal  a  rich  system  of  blood  lacunse  is  developed  in  the  connective  tissue  layer 
on  the  inner  side  of  the  otherwise  weak  musculature. 

In  the  second  or  reverse  spiral  this  lacunar  network  is  wanting.  This  spiral  is 
distinguished,  more  especially  in  the  regular  Echinoidea,  by  its  peculiar  colouring, 
being  yellow,  whereas  the  first  spiral  appears  brown. 

In  regular  Echinoidea,  the  two  intestinal  spirals  have  an  elegantly  undulating 
course,  regularly  ascending  and  descending. 

The  second  spiral  passes  without  any  sharp  boundary  into  the  rectum,  which,  in 
the  Spatangoidea,  runs  back  from  the  middle  of  the  body.  At  its  commencement,  in 
Echinocardium  (flavesccns)  and  Sehizaster,  it  has  a  small  diverticulum. 

The  alimentary  canal  of  the  Spatangoidea,  which  is  distended  with  sand  and 
mud,  is  thicker  and  its  walls  are  firmer  than  in  the  regular  Echinoidea,  whose  in- 
testine usually  contains,  besides  mud,  a  large  number  of  unicellular  algse.  There  is  a 
corresponding  difference  in  the  mesenteries.  In  the  regular  Echinoidea  the  intes- 
tine is  attached  by  means  of  mesenteries  practically  only  to  the  test,  and  these 


vin  EGHINODERMATA—  ALIMENTARY  CANAL  481 

mesenteries  are  broken  through  in  such  a  manner  as  to  form  elegant  arcades.  In  the 
Xpatanyoidea,  hoAvever,  the  different  coils  of  the  canal  are  further  connected  together 
inter  se  by  mesenteries  which  are  not  perforated. 

Unicellular  glands  of  various  kinds  are  found,  chiefly  in  the  epithelium  of  the 
first  section  of  the  intestine.  In  the  Spatangoidea,  in  the  commencement  of  the  first 
spiral,  there  are  multicellular  flask-shaped  glands  ;  these  lie  in  the  connective  tissue 
layer,  the  neck-like  duct  alone  opening  into  the  lumen  of  the  alimentary  canal. 

The  accessory  intestine  (siphon),  which  occurs  in  nearly  all 
Echinoidea,  deserves  special  attention.  Near  the  commencement  of 
the  first  spiral,  the  siphon  branches  off  from  the  main  intestine  as  a 
narrow  tube,  which  again  enters  the  intestine  at  the  end  of  that  spiral, 
to  which  it  thus  belongs.  The  siphon  always  runs  along  the  inner 
side  of  the  main  intestine  (that  turned  to  the  principal  axis  of  the 
body).  In  regular  Echinoids,  it  follows  the  main  intestine  in  its 
course ;  in  the  Clypeastroida,  on  the  contrary,  its  course  is  somewhat 
shortened.  In  the  Spatangoida,  the  first  part  of  its  course  is  shorter 
than  that  of  the  main  intestine,  while  the  rest  follows  the  coils  of  the 
latter. 

The  Cidaroida  (Doroddaris  papillata)  have  no  distinct  siphon,  but 
it  is  very  probable  that  this  organ  is  here  represented  by  a  longitu- 
dinal furrow  bordered  by  two  folds,  which  furrow  is  either  not  yet,  or 
no  longer,  shut  off  from  the  lumen  of  the  intestine.  This  furrow 
occurs  in  the  same  region  of  the  intestine  as  the  siphon,  and  also  on 
the  axial  side  of  the  canal. 

In  the  Spatangoid  genera  Brissus,  Briss&psis,  and  Schizaster  a 
second  siphon  has  been  discovered,  running  parallel  to  the  intestine. 

The  structure  of  the  siphon  resembles,  in  essential  points,  that  of 
the  main  intestine.  It  has  been  conjectured  that  it,  like  the  accessory 
intestine  of  certain  worms,  subserves  intestinal  respiration. 


D.  Cpinoidea. 

In  this  class  the  alimentary  canal  is  tubular.  It  descends  from 
the  mouth  into  the  calyx,  coiling  in  the  direction  of  the  hands  of  a 
clock  (when  the  body  is  viewed  from  the  oral  side).  From  the  base 
of  the  calyx  it  again  ascends,  continuing  the  same  curve,  towards  the 
tegmen  calycis,  and  here  enters  the  anal  cone  in  the  anal  interradius  ; 
it  then  runs  through  the  anal  cone,  opening  outward  at  its  tip  through 
the  anus. 

During  its  course  through  the  calyx,  the  intestine  makes  one 
complete  coil  round  the  principal  axis  (Fig.  382,  B,  p.  475).  The 
alimentary  canal  of  Adinometra  affords  a  striking  exception  to  this 
rule,  forming,  in  the  same  direction  as  in  other  Crinoids,  as  many  as 
four  coils  (Fig.  382,  C).  It  may  be  remembered  that  Adinwnetra  is 
also  distinguished  from  all  other  Crinoids  by  the  eccentric  position  of 
the  mouth  in  the  tegmen  calycis. 

The  section  of  the  intestine  which  lies  at  the  bottom  of  the  calyx 

VOL.  II  2  I 


482 


COMPARATIVE  ANATOMY 


CHAP. 


FIG.  384.— Radial-interradial  section  in  the  direction  of  the  principal  axis  through  the 
calyx  of  Pentacrinus  decorus  (after  P.  H.  Carpenter),  left  half  radial,  right  interradial,  dia- 
grammatic. In  many  respects  the  figure  is  obsolete  and  incorrect.  The  most  noteworthy  points 
are  emphasised  by  special  type  in  the  following  lettering,  sa,  Subambulacral  plates  ;  rn,  radial 
nerve  trunk  (hatched) ;  rv,  radial  pseudoluemal  canal  (black) ;  ra,  radial  canal  of  the  water  vascular 
system  (dotted);  d\,  brachial  coelom  ;  pi,  spongy  organ  of  the  blood  vascular  system  ;  art,  circular 
canal  of  the  water  vascular  system  ;  r>,  mouth  ;  mi  (to  the  right),  nerve  ring ;  ar,  blood  vascular 
ring?  pseudohsemal  ring?;  an  (to  the  left),  anus;  re,  rectum;  in,  plates  of  the  interambulacral 
areas ;  pa,  calyx  pores ;  d,  the  coelom,  traversed  by  a  spongy  network ;  it,  intestinal  coils 
(cut  through);  gp,  axial  organ;  mu,  muscles;  <vo,  nerve  commissure  between  the  axial  trunks: 
bu,  basal ;  cv,  cirrus  cMial ;  m,  cirrus  nerve  ;  ch,  chambered  sinus,  with  the  centre  of  the  apical 
nervous  system;  •/-,  radial  plate;  c1;  <•.>,  first  and  second  costal;  di,  distichal;  si/,  syzygial 
suture  ;  gr,  genital  canal ;  uni,  apical  nerve  trunk  of  the  arm. 


viu  ECHINODERMATA— ALIMENTARY  CANAL  483 


is  occasionally  somewhat  widened,  and  is  then  called  the  stomach. 
In  Rhizocrinus  and  Bathycrinus  there  are,  on  the  external  side  of  the 
digestive  tract,  interradially  placed  outgrowths.  Similar  outgrowths 
occur  in  great  numbers  on  the  inner  side  of  the  tract  in  Antedon  (the 
side  facing  the  axis  of  the  calyx).  Such  a  diverticulum,  when  espe- 
cially large  and  branched,  has  been  called  a  hepatic  caecum,  but  this 
name  must  not  be  accepted  in  any  strict  sense. 

The  finer  structure  of  the  intestine  agrees  in  essentials  with  that 
in  other  Echinoderms.  The  intestinal  epithelium  is  everywhere  ciliated 
except  in  part  of  the  rectum.  The  musculature  is  weakly  developed 
or  altogether  wanting,  except  near  the  mouth  and  in  the  rectum, 
where  sphincters  are  formed.  The  anal  tube  or  cone  consists  of  the 
body  wall  externally  and  of  the  wall  of  the  rectum  internally. 
Between  these  two  the  reduced  body  cavity  is  traversed  by  radially 
placed  strands  of  connective  tissue. 


E.  Asteroidea  (Fig.  385). 

That  part  of  the  oral  area  which  is  left  free  by  the  skeleton  is 
covered  by  a  soft  oral  integument,  in  the  middle  of  which  lies  the 
mouth.  This  organ  can  be  opened  by  muscles  which  run  out  from  it 
radially  in  the  oral  integument ;  it  can  be  closed  by  circular  muscle 
fibres,  which  run  round  it  in  the  oral  integument  and  in  the 
oesophagus. 

The  mouth  leads  into  an  oesophagus,  which  ascends  perpendicu- 
larly, widens  rapidly,  and  passes  over  without  any  sharp  boundary 
into  the  stomachal  sac. 

In  Echinaster  scpositus,  the  oesophagus  has  around  it  ten  outgrowths,  whose 
walls  are  very  much  folded,  and  whose  (inner)  epithelium  is  richly  supplied  with 
glands. 

The  membranous  stomachal  sac  of  the  Asteroids  is  very  spacious, 
filling  the  whole  disc.  Its  wall  is  irregularly  folded,  and  provided 
with  outgrowths ;  it  is  connected  with  the  wall  of  the  disc  by  means 
of  mesenterial  strands,  partly  of  connective  tissue,  partly  of  muscle. 

In  the  upper  (apical)  portion  of  the  stomachal  sac,  five  pairs  of 
braehial  divertieula  open ;  these  are  the  radial  caeca,  or  hepatic 
appendages,  which  stretch  more  or  less  far  into  the  arms,  according 
to  family,  genus,  or  species.  There  is  one  pair  in  each  arm.  These 
divertieula  of  the  stomachal  sac  (which,  ontogenetically,  develop  com- 
paratively late)  have  the  following  general  structure.  Each  diver- 
ticulum consists  of  a  median  common  tube,  which  runs  in  the  longi- 
tudinal direction  of  the  arm,  giving  off  lateral  tubes  alternately  to 
right  and  left.  Each  lateral  tube  receives  from  each  side  the  openings 
of  closely  crowded  glandular  lobes,  so  that  the  secreting  surface  is 
very  large. 


484 


COMPARATIVE  ANATOMY 


CHAP. 


In  the  Echimisteridce  and  Asterinidce,  the  common  tube  swells  into 
a  large  sac. 

At  the  point  where  the  stomachal  sac  narrows  to  form  the  short 


FIG.  385.— Alimentary  canal  and  genital  organs  of  an  Asteroid,  diagrammatic.  1,  Bradrial 
diverticula  of  the  stomach  ;  2,  gonads  ;  3,  base  of  the  gonad,  which  corresponds  with  its  aperture  ; 
4,  stomachal  sac ;  5,  anus  ;  6,  rectal  diverticula  ;  7,  apical  circular  sinus  and  trunk  ;  8,  one  of  the 
ten  radial  sinuses  and  trunks  running  from  this  latter  to  the  gonads ;  9,  stone  canal  in  the  axial 
sinus ;  10,  madreporite. 

rectum,  i.e.  high  up  in  the  apex  of  the  disc,  it  is  once  more  provided 
with  diverticula.     These  rectal  diverticula,  whose  number,  arrange- 


EGHINODERMATA— RESPIRATORY  ORGANS  485 

ment,  and  size  are  subject  not  only  to  specific  and  generic,  but  often 
also  to  individual  variations,  are,  as  a  rule,  much  smaller  than  the 
brachial  diverticula  of  the  stomach,  with  which  they  otherwise  agree 
in  structure. 

The  anus  is  wanting  only  in  the  Astropectinidce.  Elsewhere  it 
lies  somewhat  eccentrical^  (never  exactly  at  the  apical  pole)  in  the 
interradius  which  follows  the  madreporitic  interradius  (in  the  direction 
of  the  hands  of  the  clock),  when  the  disc  is  seen  from  the  apical 
side. 

Finer  structure  of  the  intestine. — The  intestinal  epithelium  is  ciliated.  Glandular 
cells,  as  goblet  cells,  mucus  cells,  and  granular  cells,  are  everywhere  found  together 
with  epithelial  cells.  The  last  named  appear  to  secrete  especially  the  digestive 
ferment ;  they  preponderate  at  the  commencement  and  terminal  part  of  the  canal, 
and  are  particularly  numerous  in  the  brachial  and  rectal  diverticula.  The  muscle 
layer  is  well  developed  in  the  oesophagus,  the  rectum  and  the  rectal  diverticula,  less 
strong  in  the  stomach,  and  wanting  in  the  brachial  diverticula. 

The  manner  in  which  the  brachial  diverticula  are  suspended  to  the  apical  brachial 
wall  has  already  been  described  (p.  440). 

The  Asteroidea  are  carnivorous,  feeding  on  other  marine  animals,  especially 
Bivalves  and  Gastropods.  When  feeding,  they  evaginate  the  greater  part  of  the 
stomach  out  of  the  oral  aperture,  enveloping  their  prey  with  it.  The  secretion  of 
the  mucus  cells  yielded  during  the  process  appears  to  be  poisonous  and  to  have  a 
decomposing  action.  The  animals  attacked  quickly  die,  and  are  passed  on  to  the 
part  of  the  stomach  still  remaining  within  the  disc,  where  they  undergo  the  digesting 
action  of  the  secretion  yielded  by  the  granular  glands  (Kornerdrusen). 

The  evagination  of  the  stomach  is  brought  about  by  the  musculature  of  the  disc,  \ 
and  its  withdrawal  by  the  (partially)  muscular  mesenterial  bands  which  attach  it  to  \ 
the  body  wall. 

The  anal  aperture  certainly  does  not  serve  for  the  ejection  of  all  the  fecal  masses. 
It  is  impossible  that  large  masses,  such  as  the  shells  of  Bivalves  and  Gastropods, 
which  are  found  in  the  stomachs  of  Asteroids,  can  be  ejected  through  such  a  narrow 
aperture  ;  they  are  no  doubt  passed  out  again  through  the  mouth. 

F.  Ophiuroidea  (Figs.  386  and  388,  p.  494). 

The  condition  of  the  alimentary  canal  in  this  class  is  simpler  than 
in  any  of  the  others.  The  somewhat  spacious  buccal  cavity  which  is 
surrounded  by  the  oral  skeleton  leads  into  the  digestive  sac  which 
fills  the  body  cavity  of  the  disc,  in  so  far  as  it  is  not  occupied  by  the 
bursae.  An  anus  is  wanting.  Special  intestinal  appendages  in  any 
way  corresponding  with  the  brachial  or  rectal  diverticula  of  the 
Asteroids  are  wanting. 


XV.  Respiratory  Organs. 

There  are  no  respiratory  organs  which  are  homologous  throughout 
the  whole  phylum  of  the  Echinodermata.  Portions  of  the  body 
belonging  to  very  different  organs  and  systems  of  organs  are  function- 


486 


COMPARATIVE  ANATOMY 


CHAP. 


ally  modified  for  the  purpose  of  respiration.     All  these  respiratory 
organs  (except  the  respiratory  trees  of  the  Holothurioidea)  are  described 


FIG.  380.  —  Radial  -  interradial  section 
through  the  disc  and  the  base  of  the  arm 
of  an  Ophiurid,  in  the  direction  of  the  prin- 
cipal axis.  Left  half  interradial,  right  half 
radial  (after  Ludwig).  mu,  mu\,  mil*,  Muscles 
of  the  oral  skeleton  ;  an  (black),  nerve  ring  ; 
am-2+adi,  oral-angle  plates;  or,  oral  =  ventral 
wall  of  the  disc;  aav,  apical  circular  sinus 
with  ring-like  strand  (c/.  Fig.  390).  vP,  Polian 
vesicle  ;  In,  mesenterial  filaments  between  the 
stomachal  sac  and  the  wall  of  the  disc ; 
cl,  coelom ;  alw,  apical  wall  of  the  disc ;  ra, 
radial  canal  of  the  water  vascular  system ; 
rph,  radial  pseudohsemal  canal ;  n;  continua- 
tion of  the  axial  organ  in  the  arm(?),  radial 
blood  vessel  (?);  rn,  radial  nerve  trunk  of  the 
oral  system:  bsi-bs^,  first  to  sixth  ventral 
shield  ;  teo\  and  teo->,  first  and  second  oral 
tentacles ;  tn,  torus  angularis ;  D,  teeth ; 
o,  buccal  cavity  ;  la,  entrance  to  the  stomachal 
sac;  iph,  peripharyngeal  sinus;  o.nij,  peri-" 
stomal  plates ;  aa-,  water  vascular  ring ; 
it,  stomachal  sac  ;  am.?  -  am^,  second  to  sixtli 
brachial  vertebral  ossicles ;  spt,  septum  be- 
tween the  body  cavity  and  the  peripharyngeal 


in  connection  with  the  systems  of  organs  to  which  they  belong, 
review  of  these  organs  will  be  found  below. 


viii  ECHINODERMATA— RESPIRATORY  ORGANS  487 


A.  The  (inner)  Respiratory  Trees  of  the  Holothurioidea  (Figs.  371 
and  383,  pp.  451  and  477). 

These  organs,  which  are  known  as  water  lungs  or  respiratory 
trees,  occur  as  two  tree-like  delicate- walled  branched  canals  or  tubes, 
which  lie  to  the  right  and  left  in  the  body  cavity,  their  principal 
trunks  opening  posteriorly  into  the  anterior  part  of  the  cloaca.  They 
open  either  separately  or  through  a  common  terminal  portion.  The 
last  branches  of  the  respiratory  trees  end  in  vesicular  widenings, 
similar  "  ampullae  "  being  found  also  along  the  branches  themselves. 
When  well  developed,  the  respiratory  trees  reach  far  forward  into  the 
body  cavity,  being  attached  at  various  points  by  muscle  fibres  or 
filaments  of  connective  tissue  to  adjacent  organs,  i.e.  to  the  body  wall, 
the  alimentary  canal,  the  pharynx,  and  the  mesenteries.  In  many 
Aspidochiroto  the  left  respiratory  tree  is  associated  with  the  rete  mira- 
bile  of  the  blood  vascular  system  in  the  way  described  on  p.  452. 
The  delicate  wall  of  the  organ  consists  of  an  inner  ciliated  epithelium, 
a  thin  layer  of  connective  tissue,  a  muscle  layer  (in  which  an  inner 
layer  of  longitudinal  fibres  and  an  outer  layer  of  circular  fibres  can 
be  more  or  less  distinctly  made  out),  and,  finally,  of  the  ciliated 
endothelium  of  the  body  cavity. 

There  can  be  no  doubt  that  the  respiratory  trees  actually  function 
as  respiratory  organs.  At  regular  intervals  water  flows  into  them 
from  the  cloaca,  and  is  from  time  to  time  expelled  through  the  anus, 
discoloured  by  the  admixture  of  faecal  masses. 

Respiratory  trees  are  wanting  in  all  Paradinopoda  (Symptidce), 
the  Pelar/olhuriidce,  and  the  Elasipoda,  unless,  in  the  last-named  family, 
the  diverticulum  of  the  rectum  described  in  the  section  on  the 
alimentary  canal,  p.  478,  represents  a  rudimentary  lung. 


B.  Review  of  the  Respiratory  Organs  of  the  Eehinodermata. 

(a)  Holothurioidea  Aetinopoda  (excluding  Elasipoda  and  Pelago- 
fhuiiidce). 

1.  The  respiratory  trees,  which  open  into  the  cloaca. 

2.  The   oral  tentacles,  and  to  some  extent   the  delicate-walled 

ambulacral  tentacles  as  well. 

(!>)  Holothurioidea,  Paraetinopoda  and  Pelagothuriidse. 

The  whole  of  the  body  wall  and  the  oral  tentacles.  Respiration 
is  promoted  by  the  circulation  of  the  body  fluid,  kept  up  by  means  of 
the  ciliated  urns. 

('.•)  Eehinoidea. 

1.  The  external  gills,  as  outgrowths  of  the  peripharyngeal  sinus, 

p.  442. 

2.  The  ambulaeral  feet,  especially  those  on  the  apical  surface  of 


488  COMPARATIVE  ANATOMY  CHAP. 

the  body,  and  more  particularly  the  branchial  tentacles  on 
the  petaloids,  cf.  p.  433. 

3.  The  accessory  intestine,  in  which,  at  least  in  regular  Echinoids, 
a  streaming  of  water  takes  place  which  does  not  interfere 
with  the  digestive  processes  going  on  in  the  principal 
intestine,  cf.  p.  481. 

(d)  Asteroidea. 

1.  The  papulae,  cf.  p.  439. 

2.  The  ambulaeral  feet. 

(e)  Ophiuroidea. 

1.  The  bursse  (respiratory  and  genital  chambers). 

2.  The  ambulaeral  tentacles. 
(/)  Crinoidea. 

1.  The  ambulaeral  tentacles. 

2.  The  anal  tube  (anal  cone),  which  alternately  takes  in  and 

gives  out  water. 

XVI.  The  Cuvierian  Organs  of  the  Holothurioidea  (Fig.  383,  22, 

p.  477). 

In  certain  Holothurioidea,  peculiar  accessory  structures  are  found 
.connected  with  the  terminal  portion  of  the  respiratory  trees ;  these 
are  known  as  the  Cuvierian  organs.  They  occur  chiefly  in  the  Aspi- 
dochirotce  (especially  in  the  genera  Holothuria  and  Mnlleria) ;  in  other 
Holothurioidea  they  only  occur  in  isolated  cases  (Molpadia  chilensis, 
Cucumaria  frondosa).  The  Cuvierian  organs  are  usually  very  numerous, 
even  as  many  as  a  hundred  occurring  in  some  of  the  species  provided 
with  them.  Although,  as  already  mentioned,  they  are  usually  found 
in  the  terminal  portion  of  the  respiratory  trees,  they  may  shift  higher 
up  the  principal  trunk,  and  may  even  pass  over  on  to  the  branches. 

It  is  not  improbable  that  they  represent  morphologically  trans- 
formed branches  of  these  trees,  the  structure  of  their  walls  agreeing 
in  general  plan  with  that  of  these  latter. 

Two  kinds  of  Cuvierian  organs  are  distinguished — (1)  glandular, 
and  (2)  non-glandular. 

The  Cuvierian  organs  of  the  glandular  kind  are  long  tubes,  the 
very  narrow  axial  canals  of  which  open  into  the  terminal  section  of 
the  respiratory  tree.  Each  of  these  axial  canals  has  a  spiral  course, 
and  is  lined  by  a  unilaminar  epithelium.  This  epithelium  is  followed 
externally  by  a  very  thick  layer  of  connective  tissue,  which  projects 
into  the  axial  canal  in  the  form  of  a  spiral  fold.  The  next  layer 
consists  (1)  of  isolated  circular  muscle  fibres,  and  (2)  of  external 
longitudinal  muscle  fibres  gathered  into  small  bundles.  Outside  of 
the  muscle  layer  there  is  another  layer  of  connective  tissue,  which, 
on  the  side  of  the  body  cavity,  is  covered  by  a  peculiarly  developed 
glandular  layer ;  this  no  doubt  represents  the  modified  endothelium 
of  the  body  cavity. 


ECHINODERMATA—SACCULI  OF  CRINOIDS  489 

In  this  glandular  layer  the  cells  can  no  longer  be  recognised  except  by  their 
nuclei,  no  boundaries  being  distinguishable.  The  layer  is  closely  packed  with 
secreted  granules.  "Wandering  cells  and  calcareous  corpuscles  are  found  in  the  con- 
nective tissue  wall. 

The  animal,  when  irritated,  vehemently  ejects  its  Cuvierian  organs  through  the 
cloaca.  (The  susceptibility  to  irritation  which  leads  to  such  ejection  varies  greatly 
in  different  forms. )  In  this  process  the  tubes  are  not  turned  inside  out,  but  are 
thrown  out  complete,  just  as  they  are  in  the  body  cavity,  probably  through  a  rent 
in  the  cloacal  wall.  When  these  tubes  are  thus  thrown  out,  water  is  almost  certainly 
pressed  out  of  the  respiratory  trees  into  their  axial  canals.  The  discharged  Cuvierian 
organs  are  remarkable  (1)  for  their  extreme  viscidity;  (2)  for  their  extraordinary 
extensibility.  They  can  be  drawn  out  to  more  than  thirty  times  their  ordinary 
length.  Their  viscidity  is  no  doubt  produced  by  the  secreted  granules  of  the 
glandular  layer.  In  consequence  of  these  peculiarities,  the  discharged  Cuvierian 
organs  are  weapons  of  defence  ;  they  remain  attached  to  the  body  of  an  enemy,  and 
impede  its  movements.  They  may  also  be  weapons  of  attack,  the  prey  being 
caught  and  held  fast  till  it  dies,  when  its  decomposing  remains  serve  for  food. 

The  non-glandular  Cuvierian  organs  are  either  tubular,  like  the  glandular,  or 
branched.  They  are  mostly  beset  with  stalked  vesicles.  The  smooth  endothelium 
of  the  body  cavity  which  covers  them  shows  no  glandular  development  of  any  sort. 
The  part  played  by  these  non-glandular  and  consequently  non-viscid  organs  is 
entirely  problematical. 

XVII.  Excretion. 

Special  excretory  organs  are  altogether  wanting  throughout  the 
Echinodermata.  It  is  probable  that  fluid  excrement  is  osmotically 
given  off,  together  with  the  carbonic  acid,  at  the  respiratory  surfaces 
of  the  body.  Further,  coloured  and  occasionally  crystalline  corpuscles, 
which  are  met  with  in  very  different  parts  of  the  body,  chiefly  in 
the  connective  tissue  layers  in  most  Echinoderms,  have  been  regarded 
as  products  of  excretion.  They  appear  to  remain  in  the  places  of 
their  formation,  this  conclusion  being  arrived  at  from  the  fact  that 
they  are  present  in  far  greater  quantities  in  old  than  in  young  animals. 
They  are  also  found  within  the  wandering  cells,  and  it  might  be 
worth  investigation  whether  these  wandering  cells,  which  force  their 
way  into  the  body-  and  the  intestinal-epithelium,  do  not  play  some 
part  in  excretion. 


XVIII.  The  Saeeuli  of  the  Crinoidea. 

These  are  peculiar  organs  which,  in  certain  Crinoids,  occur  in  great 
numbers  below  the  integument,  principally  at  the  edge  of  the  food 
grooves  of  the  pinnulae,  the  arms,  and  the  disc,  less  frequently  else- 
where (intestinal  wall,  mesenteries).  They  are  globular  sacs  lying 
close  below  the  surface,  but  having  no  outer  aperture,  and  are  closely 
packed  with  strongly  refractive  spherules,  which,  during  life,  are 
colourless,  but  turn  red  after  death.  Close  examination  shows  that 


490  COMPARATIVE  ANATOMY  CHAP. 

these  spherules  are  enclosed,  at  least  at  first,  in  cells,  each  of  which 
has  a  nucleus  lying  in  the  base,  which  is  turned  away  from  the 

surface.       These    are    regarded    as 
connective  tissue  cells. 

According  to  recent  ontogenetic  research, 
each  individual  spherule  is  the  metamor- 
phosed product  of  a  mesen  chyme  cell,  and 
the  sacculi  on  their  first  appearance  are  said 
to  be  nests  of  such  cells. 

Sacculi   are   specially   numerous   in   all 

f^^^/^t^^Wv^~^^-^_         species  of  Antedon,  but  are  also  found  in 
\~^&^^^^  >"4     Eudiocrinm,  Promachocrinus,  Pentacrinus, 

\/ -.,.,-,      „..._,._•>          Rhizocrinus,  Bathycrinus.     They  are  want- 

j,  ing  in  Adinometra,   Thaumatocrinus,   and 

FIG.  387. -Diagram  of  a  sacculus.  1,  Super-  Holopus.  Their  significance  has  not  been 
licial  layer  of  integument  passing  over  the  discovered.  They  have,  been  regarded  by 
sacculus ;  2,  granular  masses  within  special  various  authors  as  calcareous  glands,  ex- 
cells ;  5,  the  nuclei  of  these  cells;  4,  nuclei  cret  of  unicelllliar  aig8e 
of  the  surrounding  cutis  (3).  , . 

and   slime   glands,   but,   according   to   the 

most  recent  opinion,  they  are  proteid  corpuscles,  deposited  in  the  connective  tissue 
cells  as  reserve  stuff',  to  be  used  as  occasion  requires,  for  the  regeneration  of  broken - 
off  arms  or  of  the  viscera. 

In  other  Echinoderms  the  contents  of  wandering  cells  (especially  of  those  cells 
which  are  massed  together  below  the  surface  of  the  Holothurian  integument)  have 
also  been  claimed  as  reserves  of  nutrition. 


XIX.  Genital  Organs. 

A.  General  Morphology. 

With  rare  exceptions,  which  will  be  dealt  with  separately,  the 
sexes  are  separate  in  Echinoderms. 

The  genital  organs  are  throughout  distinguished  by  great  sim- 
plicity, as  evidenced  by:— 

1.  The   entire   absence   of   every   kind  of  eopulatory   organ. 
The  sexual  products  are  ejected  from  the  body,  and  fertilisation  takes 
place  in  the  water  (except  in  cases  of  care  of  the  brood  to  be  men- 
tioned later). 

2.  The   entire   absence    of  accessory   glands,  of  widenings   or 
outgrowths   of  the   ducts,    and    of    complicated    adaptations    for    the 
nourishment  of  the  ripening  sexual  products. 

The  genital  organs  consist  of  variously  shaped  tubes,  within  which 
the  spermatozoa  or  eggs  ripen,  and  from  which  they  are  discharged 
through  simple  efferent  ducts. 

These  gonadial  tubes  lie  in  any  part  of  the  body  cavity  ;  in  the 
most  complicated  cases  their  wall  consists,  from  without  inward,  of 
(1)  the  endothelium  of  the  body  cavity;  (2)  a  thin  muscle  layer; 


vin  E(  'HIXODERMA  TA —GENITAL  ORGANS 


(3)  a  layer  of  connective  tissue  ;  and  (4)  the  inner  germinal  epithelium. 
The  muscle  layer  is  often  wanting. 

According  to  the  morphology  of  the  genital  organs  the  Echino- 
derms  fall  into  two  groups. 

In  the  larger  principal  group,  containing  the  Echinoidea,  Aste- 
1-n'nl-n,  Ophnrroidea,  and  Crinoidw,  there  are  several  gonads,  each  with 
a  duct  and  an  aperture  ;  they  follow,  in  their  arrangement,  the  radial 
structure  of  the  body,  showing  at  the  same  time  close  relations  to  the 
axial  organ  (or  to  the  wall  of  the  axial  sinus).  The  axial  organ  has 
been  compared  to  a  trunk,  of  which  the  gonads,  as  direct  prolonga- 
tions, are  the  fruitful  branches,  on  which  the  sexual  products  ripen. 
as  fruit. 

The  direct  organic  connection  between  the  axial  organ  and  the 
gonads  persists  throughout  life  in  the  Asteroidea,  Ophiuroidea,  and 
perhaps  also  in  the  Crinoidea ;  in  the  Echinoidea  it  is  only  ontogenetic, 
and  ceases  in  the  adult. 

The  second  group  is  formed  by  the  Holothurioidea,  in  which  there 
is  neither  axial  organ  nor  axial  sinus.  The  genital  organs  consist  of 
a  single  tuft  of  gonadial  tubes.  This  tuft  lies  in  the  body  cavity  in 
the  median  (dorsal)  interradius,  and  sends  off  a  duct  which  runs 
forward  in  the  dorsal  mesentery,  and  opens  outward  in  the  anterior 
region  of  the  body,  often  very  near  the  mouth. 

There  is,  as  a  rule,  no  difference  in  the  microscopic  structure 
and  external  appearance  of  the  male  and  female  genital  organs  in 
Echinoderms.  In  some  cases,  however,  at  the  time  of  maturity,  they 
may  be  distinguished  by  their  different  colouring. 

Secondary  sexual  characters  .have  been  noticed  only  in  very  rare 
cases. 

B.  Holothurioidea  (Figs.  371  and  383,  pp.  451  and  477). 

In  all  Holothurioidea,  the  gonads  are  developed  as  a  single  tuft  of 
genital  tubes,  lying  in  the  dorsal  interradius.  All  the  tubes  of  the 
tuft  converge  towards  one  point,  and  open  into  the  base  of  the  gonad. 
which  lies  in  the  dorsal  mesentery,  and  is  often  somewhat  widened 
for  the  reception  of  the  sexual  products. 

The  gonadial  tubes  are  either  simple  or  branched  ;  in  number  and  size  they  vary 
within  pretty  wide  limits  according  to  the  species  and  the  stage  of  maturity  attained. 
They  may  exceed  the  body  in  length.  They  usually  hang  from  the  base  of  the 
gonad,  on  the  two  sides  of  the  mesentery,  into  the  body  cavity,  but  there  are  cases 
in  which  they  are  developed  only  on  one  side — the  left  —  of  the  mesentery  (in 
Hnlnthurifi.  Miillri'ia,  Labldodcinas  among  the  Aspidochirotce,  and  in  many 
£•///// /<'///••». ').  The  base  of  the  gonad  lies  in  the  anterior  half  of  the  body,  often  very 
near  its  anterior  end  (especially  in  Synaptidas  and  Molpadiidce,  but  also  in  many 
Aspidochirotce  and  Dcnclrochirotce). 

From  the  base  of  the  gonad  the  genital  duet  runs  more  or  less 


492  COMPARATIVE  ANATOMY  CHAP. 

far  forward  in  the  dorsal  mesentery,  to  pass  through  the  body  wall 
at  some  point  of  the  anterior  half  of  the  body  in  the  dorsal  median 
line,  and  to  open  outward  through  the  usually  single  genital  aperture. 

The  distance  of  this  aperture  from  the  extreme  anterior  end  of  the  body,  how- 
ever, varies  very  greatly.  It  is  greatest  in  the  Elasipoda,  and  becomes  smaller  in 
the  Aspidochirotce.  In  the  Molpadiidce  and  Synaptidce  the  genital  aperture  lies 
immediately  behind  the  circle  of  tentacles,  and  in  the  Dciidrochirotce  it  shifts  into 
that  circle,  even  reaching  its  inner  side.  It  is  found  behind  the  middle  of  the  body 
only  in  Psychropotes  longicauda. 

The  genital  aperture  is  usually  inconspicuous.  Occasionally  it  is  found  on  the 
tip  of  a  genital  papilla  ;  in  species  of  the  genera  Thyone  and  Cucumaria,  this  is  the 
case  only  in  males,  a  slight  sexual  dimorphism  thus  arising. 

The  occurrence  of  several  genital  apertures  (2,  4,  8,  16  in  certain  Elasipoda)  is 
quite  exceptional.  They  always  belong  to  one  and  the  same  gonad,  and  to  one 
genital  duct.  This  latter  in  such  cases,  before  emerging,  divides  dichotomously  into 
as  many  branches  as  there  are  apertures. 


C.  Asteroidea  (Fig.  385,  p.  484). 

The  genital  organs  are  here  developed  as  five  pairs  of  bundles  of 
gonadial  tubes,  or  as  five  pairs  of  rows  of  such  bundles.  These  pro- 
ject freely  into  the  body  cavity ;  their  bases  are  attached  to  the  apical 
(dorsal)  body  wall,  generally  at  the  apical  edge  of  the  supramarginal 
plates,  or  on  a  level  with  this  edge.  Exactly  over  the  point  of 
attachment,  i.e.  over  the  base  of  each  gonadial  tuft,  the  efferent  duct 
traverses  the  body  wall  (between  two  neighbouring  skeletal  plates)  to 
open  outward  at  the  surface  through  one,  less  frequently  through 
several,  genital  apertures.  These  apertures  are  quite  small,  and  are 
often  only  clearly  visible  at  the  season  of  sexual  maturity,  when  the 
genital  products  are  ejected. 

The  bases  of  all  the  gonadial  bundles  are  still  connected  with  the 
axial  organ  even  in  the  adult  (cf.  p.  445  on  the  axial  organ  and  the 
axial  sinus).  The  axial  organ  is  continued  along  the  inner  apical 
(dorsal)  body  wall  (that  turned  to  the  ccelom)  as  a  pentagonal  strand 
running  round  the  apical  pole  and  the  anus,  which  agrees  in 
structure  with  the  organ  itself.  At  each  of  the  five  interradially 
placed  corners  of  the  ring  it  sends  off  a  pair  of  strands  which  run 
peripherally.  There  are  thus  in  all  five  pairs  of  strands  radiating 
from  the  ring ;  these  run  to  the  bases  of  the  five  pairs  of  gonadial 
tufts,  and  where  these  are  in  rows,  from  tuft  to  tuft  of  each  row, 
connecting  their  bases. 

Just  as  the  axial  organ  is  surrounded  by  the  axial  sinus,  so  are 
all  its  derivatives  surrounded  by  a  coelomic  sinus,  a  direct  prolonga- 
tion of  the  former. 

The  aboral  ring-like  strand  lies  in  a  ring-sinus,  attached  to  its 
wall  by  a  suspensory  band.  This  sinus  is  also  continued  along  the 
five  pairs  of  strands  which  radiate  from  the  ring-like  strand ;  when  it 


viii  ECHINODERMATA— GENITAL  ORGANS  493 


reaches  the  bases  of  the  gonads  it  is  further  continued  along  all  the 
individual  tubes  to  their  tips.  The  gonadial  tubes  thus  have  a  double 
wall — first,  their  own  wall,  which  is  a  continuation  of  that  of  the  out- 
growths from  the  axial  organ ;  and  secondly,  an  outer  wall,  which  is 
a  continuation  of  the  wall  of  the  axial  sinus.  Between  these  two 
walls  lies  the  narrow  ccelomic  sinus,  which  is  in  open  communication 
by  means  of  the  sinuses  of  the  genital  strands,  with  the  ring  sinus, 
and  through  this  latter  with  tlie  axial  sinus. 

The  relations  existing  between  the  gonads,  the  axial  organ,  and 
the  system  of  sinuses,  is  clearly  elucidated  by  the  ontogeny  of  the 
Asteroidea,  which  shows  that  in  quite  young  animals  the  axial  organ 
grows  out  apically,  and  first  forms  the  ring  strand.  Out  of  this  the 
genital  strands  bud,  and  from  these  latter  again  the  bundles  of 
gonadial  tubes  arise,  which  are  at  first  solid  outgrowths,  and  only 
become  hollow  secondarily.  During  this  whole  process  the  growing 
axial  strand,  which  finally  produces  the  rudiments  of  the  gonads, 
continually  carries  the  axial  sinus  along  with  it,  so  that  the  ring-like 
strand,  the  genital  strand,  and  the  genital  tubes  are  entirely  sur- 
rounded by  a  sinus,  which  constantly  remains  in  open  communication 
with  the  axial  sinus. 

At  those  points  of  the  genital  strands  from  which  the  gonadial 
bundles  bud,  i.e.  at  the  future  bases  of  the  gonads,  the  duct  which 
perforates  the  body  wall  is  formed  from  within  at  a  later  stage. 

The  form  of  the  individual  gonadial  bundles  requires  little  notice.  The  genital 
tubes  of  which  each  bundle  is  composed  are  usually  not  long,  sometimes  they 
resemble  short  sacs  and  are  vesicular,  they  are  rarely  branched. 

Of  much  greater  interest  are  the  number  and  arrangement  of  these  bundles. 

In  the  simplest  cases,  five  pairs  of  gonadial  bundles  are  present ;  this  is  the  case, 
as  far  as  examination  of  the  various  species  on  this  point  has  taken  place,  in  the 
following  families  :  the  Aster  inidcc,  Solastcridce,  Echinasteridce,  Linckiidw,  Asteri- 
ii.lr.  In  these,  the  five  pairs  either  lie  in  the  disc,  one  bundle  at  each  side  of  each 
interradius,  or  have  shifted  into  the  bases  of  the  arms  (Echinasteridce,  Linckiidaz). 
More  than  five  pairs  of  gonads  are  found  in  the  families  of  the  Astropedinidai, 
Pcntaccrotida',  and  Gymnasteriidcc.  They  either  lie  in  the  disc  in  rows  at  the  sides 
of  the  interradii,  or  the  five  pairs  of  rows  stretch  into  the  arms.  This  last  arrange- 
ment is  found  in  the  most  extreme  form  in  Luidia,  where,  on  each  side  of  each  arm, 
a  row  of  nine  runs  to  near  its  tip,  one  or  two  pairs  occurring  on  each  skeletal 
segment. 

In  all  cases,  each  bundle  has  its  separate  genital  aperture. 

As  a  rule,  each  bundle  has  only  one  aperture,  but  it  sometimes  happens  (Asterias, 
Solaster]  that  the  duct  which  traverses  the  apical  body  wall  branches,  and  opens 
through  several  genital  pores  lying  near  one  another. 

Asterina  gibbosa  is  an  exception  to  the  rule  that  the  genital  apertures  lie  on  the 
apical  side  of  the  disc  or  arms.  The  apertures  here  lie  on  the  oral  side,  a  peculiarity 
connected  with  the  fact  that  these  Asteroids  do  not  simply  eject  their  eggs  into  the 
water,  but  attach  them  in  combs  or  plates  to  stones,  etc. 

It  must,  finally,  be  noted  that  the  aboral  ring  sinus  is  not  always  simple,  but 
may  break  up  into  a  circular  network  of  sinuses  (e.g.  Echinaster  sepositus). 


494 


COMPARATIVE  ANATOMY 


CHAP. 


D.  Ophiuroidea. 

In  structure  and  development  the  genital  organs  in  this  class 
strongly  resemble  those  of  the  Asteroidea.  The  gonad  is  connected 
with  the  axial  organ  by  means  of  an  aboral  ring-like  strand,  and  both 
the  gonads  and  this  strand  are  surrounded  by  coelomic  sinuses,  which 
communicate  with  the  axial  sinus. 

The  only  important  difference  in  the  genital  organs  of  the  two 
classes  is  caused  by  the  fact  that,  in  the  Ophiuroidea,  the  gonads  do  not 
open  outward  directly,  but  by  means  of  five  pairs  of  large  sac-like 


FIG.  388.— Stomach  and  bursse  of  a  young  Ophioglypha  albida,  in  its  natural  position  in 
the  disc,  the  dorsal  wall  of  which  is  removed.  1,  Bursse  ;  2,  cavity  of  the  disc  ;  3,  interradial ;  4, 
radial  bulgings  of  the  digestive  sac  (after  Ludwig). 

invaginations  of  the  body  wall  into  the  coelom  of  the  disc,  these 
sacs  themselves  communicating  with  the  exterior  through  five  pairs 
of  slit-like  apertures  lying  at  the  sides  of  the  bases  of  the  arms  on  the 
lower  (oral)  side  of  the  disc.  These  sac-like  invaginations  of  the 
body  wall  are  the  bursae  or  bursal  pockets,  their  outer  slit-like 
apertures  being  known  as  the  bursal  (genital)  apertures,  which  have 
already  been  mentioned  (Figs.  245,  246,  and  314,  pp.  300,  301, 
and  359). 

1.  The  Bursse  (Figs.  388  and  389). 

These  are  large  sacs  within  walls,  which  fill  up  the  body  cavity  of 
thf1  disc  round  the  digestive  sac.     Their  walls  are  attached  to  that  of 


ECHIXODERMATA— GENITAL   ORGANS 


495 


the  digestive  sac  and  the  apical  body  wall  by  means  of  strands  of 
connective  tissue,  and  consist  of  the  same  layers  as  the  body  Aval), 
though  in  the  bursae  these  layers  are  much  thinner.  Calcareous 
corpuscles  may  be  either  present  or  wanting  in  the  connective  tissue. 
The  inner  epithelium  of  the  bursae  is  in  some  parts  strongly  ciliated. 

The  outer  apertures  of  the  bursae  lie  at  the  sides  of  the  proximal 
portions  of  the  arms,  which  are  included  in  the  disc.  Each  bursa  has, 
as  a  rule,  one  aperture,  but  in 
the  genus  Ophinn.i.  (formerly 
Ophioderma)  there  are  two  aper- 
tures on  each  side  of  the  base  of 
an  arm,  one  distal  and  the  other 
proximal.  Both  these  apertures, 
however,  lead  into  one  and  the 
same  bursa,  and  the  double  aper- 
ture (in  Ophiura)  can  be  deduced 
from  the  ordinary  single  aperture 
by  assuming  that  the  margins  of 
the  latter  fuse  at  about  'the 
middle  of  their  length. 

The  gonads  are  attached  to 
the  wall  of  the  bursa,  on  the 
side  turned  to  the  body  cavity 
(Figs.  391  and  392).  The 
spinal  nrorhirts  nass  into  the  of  the  stomach  and  the  gonads  (after  Ludwig). 
56XUal  prc  a  Of  the  two  burs*,  that  on  the  left  has  been  removed. 

blirsa,     and     are     ejected     thence    j,  Dorsal  shields  of  the  ami;  2,  dorsal  wall  of  -the 


FIG.  389.— Part  of  a  preparation  of  Ophio- 
glypha similar  to  that  in  Fig.  388,  after  removal 


disc;  3>  bursa  with  its  tJP  (4>; 


through  the  aperture.     This   is, 

i  F    ,1        F  6,  vertebral  ossicle  in  the  base  of  the  arm  ;  7,  genital 

however,    Only    Olie    Of    the    func-    ptate  :  8>  row  of  tawal  plates  or  scales. 

tions  of  the  bursa,  and,  in  most 

Ophiuroidea,  as  it  appears,  not  the  principal  function. 

The  bursse  have  a  more  important  function  as  respiratory  organs. 
Sea-water  can  enter  them,  and  through  their  thin  walls  exchange  of 
gases  can  take  place  between  it  and  the  body  fluid.  It  would  be  in- 
teresting if  it  could  be  proved  that,  as  in  the  mouth  and  oesophagus 
of  the  Corals,  the  sea-water  enters  through  one  (more  or  less  proximal) 
part  of  the  bursal  aperture,  and  flows  out  again  through  another  (more 
distal)  part.  The  proximal  aperture  of  each  bursa  in  Ophiura  is  per- 
haps an  inhalent,  and  the  distal  an  exhalent  aperture. 

In  certain  Ophiuroidea  (e.g.  Amphiura  squainata,  magellanica,  Ophia- 
ntntha  ricijuii'ii.  marsupialis,  Ophioglypha  he.radis,  Ophiomym  rivipara,  etc.), 
the  bursae  serve  as  brood  chambers.  The  eggs  pass  through  their 
Avhole  development  in  them,  until  all  the  organs  of  the  young  Ophiurid 
are  formed. 

2.  The  Genital  Apparatus  (Figs.  390-393). 

The  most  interesting  point  in  connection  with  the  genital  apparatus 
is  the  peculiar  course  of  the  apical  ring  sinus  with  the  ring-like  strand 


496 


COMPARATIVE  ANATOMY 


CHAP. 


it  contains.     Fig.  390,  which  represents  the  ring  sinus  in  horizontal  pro- 
jection, illustrates  its  course  in  five  outwardly  directed  radial  and  five 


FIG.  390.— Course  of  the  aboral  circular  sinus,  with  the  ring-like  strand  contained  in  it  in 
the  Ophiuroidea  (diagram  after  Ludwig).  1,  Gonads  ;  2,  axial  sinus  with  axial  organ  ;  3,  mouth  ; 
4,  circular  sinus  with  ring-like  strand,  on  the  side  of  the  bursal  wall  turned  to  the  interradius  ;  5, 
interradial  portion  of  the  ring  sinus  and  strand,  bent  downwards  orally  (Fig.  386,  left  aav)  ;  6,  bursal 
aperture  ;  7,  radial  (apical)  region  of  the  ring  sinus  (Fig.  386,  right  anv) ;  8,  lateral  branches  of 
the  same  on  the  bursal  wall  turned  to  the  radius. 

inwardly,  ie.  orally  directed  interradial  curves.     In  this  undulating 
course  the  ring  sinus  descends  on  the  inner  wall  of  the  disc  alternately 

from  the  apical  to  the  oral  side, 
and  then  again  ascends  to  the 
apical  side,  the  radial  curves  lying 
apically  and  the  interradial  (those 
near  the  bursse)  orally. 

This  peculiar  course  is  no  doubt  con- 
nected (1)  with  the  orally  directed  course 
of  the  axial  sinus,  the  axial  organ,  and 
the  stone  canal  which  opens  outward 
orally  through  its  madreporite  (Fig. 
361,  6,  p.  422).  For  the  ring  sinus 
is  the  continuation  of  the  axial  sinus, 
and  the  ring-like  strand  is  the  continua- 
tion of  the  axial  strand.  It  is  now  im- 
possible to  determine  whether  the  axial 
sinus  and  the  axial  organ,  in  bending 
oralty,  drew  the  ring  sinus  interradially 

in  the  oral  direction  (in  the  first  place  this  could  of  course  only  apply  to  the  madre- 
poritic  interradius),  or  whether,  on  the  contrary,  the  ring  sinus,  shifting  orally,  drew 


FIG.  391.— Bursa  of  Ophioglypha,  seen  from 
the  side  turned  towards  the  interradius  (diagram 
after  Ludwig).  1,  The  tip  of  the  bursa,  lying  on 
the  dorsal  side  of  the  digestive  sac  ;  2,  the  gonads 
sessile  on  thr  bursal  wall ;  3,  distal  portion  of  a 
bursa  (that  turned  to  the  periphery  of  the  disc)  ; 
5,  proximal  portion  of  the  same  (that  turned  to 
the  centre  of  the  disc) ;  4,  the  rows  of  bursal 
scales  along  the  aperture. 


VIII 


EGHINODERMATA— GENITAL  ORGANS 


497 


the  axial  sinus,  etc.  with  it  downwards  ;  i.e.,  it  is  impossible  to  decide  which  organ 
took  the  lead  in  shifting.  (2)  As  the  gonads  which  bud  from  the  ring-like  strand 
open  into  the  bursae,  which  latter,  however,  open  outward  orally,  it  is  to  some 
extent  explicable  why  the  ring- 
like  strand  descends  interradi- 
ally  to  the  bursre. 

The  whole  problem  is  still 
further  complicated  by  the  ques- 
tions :  (1)  What  was  the  original 
function  of  the  bursse  ?  (2)  Is 
the  ventral  position  of  the  bursse 
the  primitive  position  ?  (3)  Is 
the  opening  of  the  gonads  into 
the  bursse  a  recent  specialisation 


in  the  Ophiuroidea  ? 


The  curved-in  portion 
of  the  ring -like  strand 
(with  the  sinus  enclosing 


FIG.  39-2.—  Transverse  section  through  the  disc  of  an 
Ophiurid  (Ophioglypha)  at  the  base  of  an  arm  (after 
Ludwig).     1,  Dorsal  wall  of  the  disc  ;   2,  bulging  of  the 
it)  rUllS  along   that    side  Of    digestive  sac  ;  3,  bursa  ;  4,  gonad  on  the  bursal  wall  ;  5,  base 

each  bursa  which  is  turned   of  the  arm  ;  6-  ventral  waU  of  the  disc  ;  7)  bursal  ap61^6  : 

,  ..  :  .  ,  .  T 

to    the    interradms.       It, 

however,  gives   off  a  branch   to  the  wall 

the  radius  (of  the  arm),  this   branch  running  along  this  wall   from 

its  periphery  to  its  proximal  part.     Both  wralls  of  the  bursa,  therefore, 


8,  genital  plate  ;  9,  bursal  scale. 

which  is   turned   towards 


FIG.  393.— Section  through  an  ovary  of  an  Ophiurid  (Ophioglypha  lacertosa)  (after  Cudnot). 
1,  Muscle  trunk,  cut  through  transversely  ;  2,  nerve  ring  ;  3,  bursal  wall ;  4,  aperture  of  the  ovary 
into  the  bursa  ;  5,  wall  of  the  genital  sinus  ;  6,  genital  sinus ;  7,  the  endothelium  of  the  genital 
sinus,  which  covers  the  gonadial  wall ;  8,  cavity  of  the  gonad  ;  9,  eggs  in  a  more  mature  condition 
than  the  rest ;  10,  ring-like  strand  in  the  aboral  ring  sinus  (11). 

the  abradial  wall,  i.e.  that  turned  to  the  interradius,  and  the  adradial 
wall,  i.e.  that  turned  to  the  arm,  have  a  genital  strand.  The  abradial 
genital  strand  of  each  bursa  is  merely  a  part  of  the  apical  ring 
strand,  while  the  adradial  is  a  lateral  branch  of  that  strand.  These 
VOL.  II  2  K 


498  COMPARATIVE  ANATOMY  CHAP. 

five  pairs  of  adradial  genital  strands  recall  the  five  pairs  of  genital 
strands  of  the  Asteroidea. 

The  gonadial  tubes  are  sessile  upon  the  genital  strands  of  the 
bursae,  and  project  freely  into  the  body  cavity  of  the  disc.  These 
gonadial  tubes  are  either  single  pear-shaped  tubes,  great  numbers  of 
which  are  arranged  in  rows  along  the  genital  strands,  or  they  are 
collected  into  tufts,  and  then  there  is  one  tuft  on  the  adradial  and  one 
on  the  abradial  wall  of  the  bursa. 

In  the  former  case  (e.g.  Ophioglyplia,  Ophiomyxa,  Ophiocoma)  the  two  rows  of 
genital  tubes  (the  adradial  and  the  abradial)  stand  rather  low  down  on  the  wall  of 
the  bursa,  near  its  aperture,  almost  parallel  with  the  edges  of  the  latter.  Each 
genital  tube  has  its  special  aperture  into  the  bursa. 

In  the  second  case,  the  points  of  insertion  of  the  two  tufts  of  gonads  within  the 
ventral  region  of  the  bursal  wall,  i.e.  the  position  of  the  bases  of  the  gonads, 
seems  to  vary  greatly,  and  each  tuft  appears  to  have  only  one  aperture  into  the 
bursa  (Ophiopholis,  Ophiothrix). 

It  is  still  an  open  question  whether  the  genital  apertures  are  constant  in  adult 
Ophiurids,  or  whether  they  only  break  through  into  the  bursal  cavity  at  the  time  of 
maturity. 

The  gonadial  tufts  arise  as  originally  solid  buds  from  the  genital  strands,  and, 
while  forming,  bulge  out  the  wall  of  the  sinus  which  contains  the  strand  ;  the  tubes 
are  thus  here  also  surrounded  by  a  genital  sinus,  which  communicates  with  the  ring 
sinus,  and  through  it  with  the  axial  sinus  (Fig.  393). 

The  ring-like  strand  is  attached  by  a  thick  band  to  the  wall  of  the  ring  sinus. 
It  is  solid,  and  consists  of  two  kinds  of  cells  :  (1)  cells  which  entirely  resemble 
those  of  the  axial  organ,  of  which  the  ring-like  strand  is  a  prolongation  ;  (2) 
enclosed  in  these  cells,  there  is  a  strand  of  cells  proved  to  be  genital  germ  cells 
(rachis  genitalis).  The  cells  of  the  former  kind  progressively  decrease  in  number, 
and  those  of  the  second  kind  increase  in  number  the  nearer  the  ring-like  strand 
approaches  the  gonads.  The  former  are  not  even  continued  into  the  gonadial 
tubes,  while  the  latter  kind  yield  the  germinal  cell  material  of  the  gonads.  It  is 
very  probable  that,  after  the  sexual  products  have  been  ejected,  a  new  formation 
of  germinal  cell  material  takes  place,  by  some  kind  of  forward  movement,  from  the 
rachis  genitalis. 

The  development  of  the  genital  system  from  the  axial  organ  and  the  axial  sinus 
proceeds  in  the  same  manner  as  in  the  Asteroidea. 

Ophiactis  virens,  a  form  distinguished  by  reproduction  by  means  of 
fission,  and  by  the  peculiar  arrangement  of  the  appendages  of  the 
water  vascular  system,  is  the  only  Ophiurid  in  which  the  bursse  are 
altogether  wanting.  The  gonads  open  direct  outward  on  the  oral  side. 


E.  Eehinoidea  (Figs.  358  and  370,  pp.  419  and  443). 

Although  the  genital  system  of  the  Eehinoidea  appears  to  resemble 
in  its  development  that  of  the  Asteroidea  and  the  Ophiuroidea  (a  point 
on  which,  however,  further  research  is  desirable),  marked  deviation 
takes  place  in  the  adults. 

The  gonads,  at  least  in  regular  Echinoids,  are  five  in  number,  and 


VIII 


ECHINODERMATA— GENITAL  ORGANS 


499 


lie  in  the  apical  region  of  the  body  cavity,  in  the  interambulacra.  The 
five  genital  ducts  ascend  towards  the  apex,  there  perforate  a  ccelomic 
circular  sinus  which  surrounds  the  rectum,  pass  through  the  genital 
pores  of  the  basals,  and  then  open  outward,  sometimes  at  the  tips  of 
projecting  papillae. 

The  gonads. — These,  in  a  mature  condition,  are  large  acinose 
organs,  which  are  suspended  to  the  inner  wall  of  the  test  by  an 
exactly  interradial  principal 
suspensor,  and  by  various 
other  bands  of  connective 
tissue.  They  are  not  sur- 
rounded by  a  genital  sinus. 

The  number  of  gonads 
was  originally  five.  Five 
are  found  in  all  the  regular 
Echinoids  (Cidaroida  and 
Diademaioidd),  and  also  in 
many  Clypeastroida.  In  the 
Spatangoida,  the  Holectypoida, 
and  many  Clypeastroida,  the 
number  is  reduced,  the  pos- 
terior unpaired  gonad  with 


FIG.  394.— Cystechinus  vesica  A.  Ag.   Apical  portion 
of  the  test  from  within,  with  the  three  gonads.     1,  An- 
belonging    terior  ambulacrum  ;   2,  left  anterior ;  3,  left  posterior ; 
3    4,  right   posterior    gonad  ;    5,   circular  sinus  (after  A. 
Agassiz). 

appear.      In  the  Spatangoida, 

the  reduction  may  go  still  further,  the  right  anterior,  and  in  a  few 

cases  the  left  anterior  as  well,  disappearing  (Fig.  394). 


the    genital 

to  it  beinsr  the  first  to  dis- 


Further  details  on  this  point  are  to  be  found  in  the  section  on  the  skeletal  system 
(cf.  pp.  321-324  on  the  genital  pores).  It  is  there  shown  that  these  pores  are  by  no 
means  necessarily  limited  to  the  basals. 

The  genital  apertures. — The  genital  papillae,  on  the  tips  of  which  the  genital 
apertures  lie.  are  specially  well  developed  in  the  Spatangoida. 

The  ring  sinus  encircles  the  anus  with  the  periproctal  sinuses,  the  stone  canal, 
and  the  axial  sinus.  Its  wall  is  formed  on  the  one  side  by  the  test,  and  on  the 
other  by  a  circular  lamella  of  connective  tissue  which  is  covered  on  both  surfaces  by 
endotheliuni,  on  the  apical  surface  by  that  of  the  ring  sinus,  and  on  the  oral  by  that 
of  the  general  body  cavity. 

The  lower  wall  of  the  apical  ring  sinus  is  broken  through  in  Dorocidaris,  so 
that  the  circular  sinus  is  here  in  open  communication  with  the  general  body 
cavity. 

In  all  other  cases,  the  ring  sinus  is  entirely  closed  on  all  sides  in  adult 
Echinoids. 


In  adults,  there  is  no  trace  of  a  ring-like  strand  enclosed  in  the 
ring  sinus.  The  connection  between  the  axial  organ  and  the  gonads 
is  thus  lost. 


500 


COMPARATIVE  ANATOMY 


CHAP. 


F.  Crinoidea  (Fig.  395). 

In  the  Crinoids,  a  genital  strand  runs  through  the  arms,  branching 
with  them,  and  entering  into  their  last  ramifications — the  pinnulse. 
While  this  genital  strand,  which  is  to  be  found  even  below  the  food 
grooves  of  the  tegmen  calycis,  remains  as  a  rule  infertile  in  the  calyx 

and  in  the  arms,  in  the 
pinnules  the  germinal 
cells  which  it  contains 
give  rise  to  the  genital 
cells.  The  genital  strand 
in  the  pinnulse  becomes 
a  gonadial  tube. 

On  the  position  of  the 
genital  strand  cf.  p.  414 
and  Fig.  356.  It  runs 
between  the  three  brachial 
sinuses  of  the  body  cavity 
(between  the  dorsal  canal 
and  the  two  ventral 
canals),  below  the  food 
grooves  of  the  arms. 

It  is  contained  in  a 
special  coelomie  sinus 
ovariai  (like  the  ring-like  strand 
and  the  genital  strands 
of  the  Asteroidea  and  the 
Opliiuroidea),  to  the  wall 

5,  deeper  longitudinal  nerves  of  the  pinnula  ;  6,  tentacle  canal ;  of  which  it  is  attached  by 
7,  radial  canal  of  the  water  vascular  system  ;  8,  nerve  ridge  of  fi]ampnt<,  nf  rnrmPPtivp 
the  superficial  oral  system  ;  10,  sacculus,  see  p.  490. 

tissue. 

The  coelomie  sinus  is  continued  on  to  the  gonadial  tubes  of  the 
pinnulae  and  there  becomes  the  genital  sinus. 

The  genital  strand  is  at  first  solid,  but  at  a  later  stage  becomes  a 
hollow  genital  tube.  This  genital  tube  widens  in  the  pinnules  into 
the  gonadial  tube,  which,  in  mature  pinnules,  is  filled  either  with  eggs 
or  spermatozoa,  these  having  their  origin  in  the  cells  of  the  wall  of  the 
gonadial  tubes  (the  germinal  epithelium). 

In  a  transverse  section  of  the  genital  tubes  of  the  arms,  the  wall 
appears  thickened  at  one  part.  This  thickening  is  the  section  of  a 
ridge  whose  cells  seem  to  correspond  with  those  of  the  germinal  epi- 
thelium of  the  gonadial  tubes. 

It  is  very  probable  also,  that  after  the  ejection  of  the  sexual  pro- 
ducts from  the  pinnules  in  Crinoids,  the  new  formations  of  these  pro- 
ducts proceed  from  the  germinal  cells,  which  are  pushed  forward  out 
of  the  ridge  of  the  genital  tube  into  the  pinnules. 


FIG.  395. —Transverse  section  through  an 
pinnule  of  a  Crinoid,  diagrammatic.  1,  nerve  trunk  of  the 
apical  nervous  system  in  the  joint  of  the  pinnula  ;  2,  genital 
sinus  ;  3,  germinal  epithelium  of  the  gonadial  rachis  (genital 
strand  or  tube) ;  4  and  9,  sinuses  of  the  brachial  coelom  ; 


ECHINODERMATA— GENITAL  ORGANS  501 

That  the  cells  of  the  genital  ridge  (and,  indeed,  originally  all  the  cells  of  the 
genital  strand)  are  germinal  cells  is  further  proved  by  the  fact  that,  in  exceptional 
cases,  gonads  may  develop  in  the  arms  also,  and  even  under  the  ambulacral  furrows 
of  the  calyx  (e.g.  in  individuals  of  the  species  Antedon  and  Actinometra,  and  in  one 
species  not  specified). 

The  gonadial  tubes  are  sometimes  long,  sometimes  egg-shaped.  They  run 
through  a  larger  or  smaller  number  of  joints  of  the  pinnule.  At  the  time  of 
maturity  they  swell  and  often  bulge  out  the  wall  of  the  pinnule  in  such  a  way  as 
to  show  at  a  glance  which  pinnules  contain  ripe  sexual  products. 

The  manner  in  which  the  ripe  products  are  ejected  from  the  pinnules  is  not  yet 
satisfactorily  explained.  There  seem  to  be  no  constant  genital  apertures  in  the 
adult.  It  appears  that  the  ejection  takes  places  through  two  merely  temporary 
apertures  (one  on  each  lateral  wall  of  the  pinnule). 

Round  the  mouth,  finally,  there  are  five  genital  strands  with  the 
sinuses  in  which  they  lie,  running  from  the  periphery,  i.e.  from  the 
bases  of  the  arms  below  the  food  grooves  of  the  tegmen  calycis.  It  is 
not  certainly  known  what  becomes  of  these  genital  strands  ;  according 
to  some  accounts,  they  are  continued  round  the  mouth  into  the  strands 
of  the  axial  organ.  They  are  said :  also  to  develop  ontogenetically  as 
outgrowths  of  that  organ  (cf.  p.  446). 

If  the  axial  organ  of  the  Crinoids  is  homologous  with  that  of  the  Ophiuroids, 
Asteroids,  and  Echinoids  (which  homology  cannot  be  considered  as  certainly  estab- 
lished), then  we  should  have  the  same  relations  subsisting  between  the  axial  organ 
and  the  genital  organ  in  the  Crinoids  as  in  the  other  groups  above  mentioned.  But 
in  the  Crinoids  the  genital  strands,  which  only  become  fruitful  as  gonadial  tubes  in  the 
pinnulae,  are  oral  outgrowths  of  the  axial  organ,  whereas  in  other  Echinoderms  (apart 
from  the  Holotliurioidca,  which  are  quite  isolated)  they  are  apical  outgrowths. 

G.  Origin  of  the  Sexual  Products. 

The  first  origin  of  the  sexual  products  has  been  accurately  described 
for  the  Ophiurid  Amphium  squamata.  They,  and  the  cells  of  the  axial 
organ,  arise  out  of  one  and  the  same  rudiment,  which  consists  of  the 
endothelial  cells  of  the  eoelom.  Tie  Echinoderms  would  thus,  as 
far  as  the  origin  of  the  sexual  products  is  concerned,  agree  with  the 
Annulata,  the  Mollusca,  and  the  Vertebrata. 

The  specific  cells  of  the  axial  organ  seem  incapable  of  becoming 
germinal  cells. 

H.  Hermaphroditism  in  Eehinoderms. 

Hermaphroditism  is  an  altogether  exceptional  phenomenon  in 
Echinoderms,  and  is  only  of  frequent  occurrence  in  one  order  of  the 
Holothurians,  the  Paractinopoda  (Synaptidce).  Apart  from  this  order, 
it  is  only  certainly  established  in  one  Asteroid  (Asterina  gibbosa)  and 
one  Ophiurid  (Amphium  squamata). 

(a)  Paractinopoda. — All  species  of  the  genera  Synapta  and  Anapta,  examined 
with  regard  to  this  point,  and  a  few  species  of  the  genus  Chirodota,  are  herma- 
phrodite. 


502  COMPARATIVE  ANATOMY  CHAP. 


as  well  as  spermatozoa  are  produced  in  the  gonadial  tubes,  but  the  two 
products  do  not  ripen  simultaneously  (Synapta  inhcerens}.  The  spermatozoa  only 
ripen  after  the  ejection  of  the  eggs. 

(£>)  Asterina  gibbosa. — Here  also  the  eggs  and  the  spermatozoa  are  formed  in  the 
same  genital  organs,  again  not  being  simultaneously  produced.  The  young  animals 
are  males,  the  adults  females. 

(c]  Amphiura  squamata. — The  simple  pear-shaped  gonads  are  very  few  in  number. 
On  the  average,  the  adradial  and  the  abradial  walls  of  a  bursa  have  only  one  gonad 
each  sessile  on  it.  The  adradial  gonads  are  testes,  the  abradial  ovaries.  Only  a  few 
eggs  in  the  ovary  and  a  small  number  of  spermatozoa  in  the  testes  ripen  at  one  time. 
These  two  kinds  of  sexual  products  here  also,  as  it  appears,  do  not  ripen  simul- 
taneously in  one  and  the  same  animal.  The  eggs  are  developed  in  the  bursse. 

I.  Care  of  the  Brood  and  Sexual  Dimorphism. 

Little  by  little,  somewhat  numerous  cases  of  care  of  the  brood  have  become  known 
among  the  Holothurioidea,  Echinoidea,  Asteroidea,  and  Ophiuroidea.  These  are  not 
infrequently  connected  with  a  more  or  less  pronounced  sexual  dimorphism,  the 
adaptations  for  care  of  the  brood  being  found  only  in  the  female. 

The  eggs  of  an  Echinoderm  in  which  the  brood  is  cared  for  are,  as  far  as  investiga- 
tion on  this  subject  has  gone,  distinguished  by  remarkable  size,  and  by  a  rich  pro- 
vision of  nutritive  yolk,  from  those  which  are  ejected  into  the  water,  and  are  destined 
to  develop  into  free-swimming  larvae. 

The  following  review  makes  no  claim  to  be  exhaustive. 

(a)  Holothurioidea. — In  Psolus  ej)hippifer  (cf.  Fig.  228,  p.  287)  large  plates  are 
found  on  the  back  of  the  female,  raised  from  the  dorsal  integument  by  means  of 
stalks.  Between  the  stalks  a  brood  chamber,  roofed  over  by  the  contiguous  plates, 
arises  ;  in  this  the  fertilised  eggs  which  emerge  through  the  dorsal  genital  aperture 
pass  through  their  development. 

In  Cucumaria  crocea,  the  developing  eggs  are  retained  in  a  dorsal  trough,  which 
arises  by  the  swelling  up  and  bulging  outward  of  the  body  wall  in  the  two  dorsal 
radii. 

Another  kind  of  care  of  the  brood  is  found  in  Cucumaria,  Icevigata  and  C.  minuta. 
Two  sacs  here  project  from  the  body  wall  into  the  body  cavity  ;  these  are  brood 
pouches,  which  shelter  the  developing  brood.  The  sacs  are  probably  mere  invagina- 
tions  of  the  body  wall ;  their  outer  apertures,  however,  have  been  discovered  only  in 
C.  minuta.  The  two  sacs  belong  to  the  two  ventral  interradial  areas  ;  in  C.  Icevigata 
they  lie  near  the  middle  of  the  body,  in  C.  minuta  anteriorly. 

In  Phyllophorus  urna  and  Chirodota  rotifera,  the  body  cavity  serves  as  a  brood 
chamber.  It  is,  however,  unknown  how  the  fertilised  eggs  pass  in  arid  the  young 
Holothurioidea  out  of  it. 

In  other  Echinoderms,  as  might  be  anticipated,  we  find  the  spines  occasionally 
acting  as  protections  for  the  brood. 

(6)  Echinoidea. — In  a  few  Cidaroida  (e.g.  C.  canaliculaia,  C.  nutrix,  C.  mem- 
branipora)  the  eggs  are  retained  on  the  apical  area  of  the  test,  and  here  develop, 
protected  by  the  spines,  which  bend  together  over  them.  The  same  is  the  case  in 
many  Spatangoida,  but  the  members  of  this  order  have  become  still  more  specialised 
for  this  function.  In  certain  forms  either  some  or  all  of  the  petaloids  (cf.  p.  347) 
sink  in  deeply,  and  thus  give  rise  to  brood  chambers  (marsupia)  into  which  the  eggs 
pass  from  the  genital  aperture.  The  brood  developing  in  such  a  marsupium  is  pro- 
tected by  the  bending  together  of  the  larger  spines  which  border  it.  In  the  Schizaster 
figured  on  p.  294,  the  anterior  impaired  petaloid  ;  in  Hemiaster  cavernosus,  in  which 
this  arrangement  is  best  known,  the  paired  petaloids  are  the  most  deeply  sunk.  As 


VIII 


ECHINODERMATA—CARE  OF  BROOD 


503 


this  is  only  the  case  in  the  female,  we  here  have  a  striking  sexual  dimorphism. 
Similar  adaptations  for  the  care  of  the  brood  seem  to  occur  in  Moira,  Anochanus, 
etc. 

(c)  Asteroidea. — Among  the  Asteroidea,  the  Pterasterince  (Pteraster,  Hymenaster) 
are  very  remarkable  for  the  care  of  the  brood.  The  whole  of  the  apical  body  wall 
carries  large  peculiarly  shaped  paxillae  or  calcareous  pillars,  from  the  free  ends  of 
which  radiate,  like  the  spokes  of  a  wheel,  a  varying  number  of  calcareous  rods  (cf. 
p.  391).  All  these  calcareous  stars  of  the  paxillee  are  connected  by  an  integument 
in  such  a  way  that,  between  this  in- 
tegument (supradorsal  membrane)  and 
the  dorsal  wall  of  the  body  beneath  it, 
a  large  brood  chamber  is  formed.  This 
chamber  communicates  with  the  ex- 
terior at  many  points  :  (1)  through  a 
large  aperture  at  the  apical  pole 
(osculum)  usually  surrounded  by  five 
valves  of  considerable  size  (Fig.  396)  ; 
(2)  through  numerous  contractile  pores 
or  spiracles  in  the  membrane  which 
covers  the  brood  cavity  ;  (3)  through 
regular  segmeutally  recurring  apertures 
at  the  sides  of  the  arms.  These  aper- 
tures can  be  closed  by  means  of  small 
spines  or  scales.  These  "segmental" 
apertures  are  regarded  by  the  present 
writer  as  ventilating  apertures,  as  they 
appear  to  serve  the  purpose  of  keeping 
up  an  active  circulation  of  water  in  the 

brood  cavitv. 

FIG.  39t5.—  Hymenaster  pellucidus,  Wyv.  Thorn- 
The   sexual    arrangements    in    the    son>  from  the  apical  side>     The  osculum  is  seeDj 

Pterasterince  are  unfortunately  still  un-    surrounded  by  five  valves  (after  Sladen). 

known.    All  specimens  as  yet  described 

show  the  brood  membrane.     Possibly  they  are  all  females,  and  the  males  are  still 

unknown,  or  the  Pterasterince  may  be  hermaphrodite.     Or,  again,  there  may  be  a 

far-reaching  dimorphism,  which  has  led  to  the  males  being  described  as  a  separate 

species. 

Leptoptychaster  kerguelenensis,  an  Astropcctinid,  shows  us  the  care  of  the  brood, 
seen  in  the  Ptcrasteriiuc,  to  a  certain  extent  in  statu  nascendi.  The  eggs  which 
emerge  from  the  genital  aperture  pass  into  the  interstices  between  the  stalks  of  the 
still  separate  paxillre,  and  there  pass  through  the  first  stages  of  their  development. 
At  a  later  stage,  also,  as  young  Asteroids,  they  remain  for  some  time  on  the  body  of 
the  mother. 

In  Asterias  spirabilis,  similar  arrangements  are  found,  but  the  embryo  is  con- 
nected by  means  of  a  stalk  to  the  body  wall  of  the  mother. 

Other  Asteroids  (e.g.  species  of  Echinaster  and  Asterias)  protect  the  brood  which 
collects  on  the  oral  side  ;  it  develops  under  the  shelter  of  the  arms,  which  simply 
bend  round  over  it,  so  forming  a  temporary  brood  chamber. 

Ophiuroidea. — In  the  description  of  the  bursse,  p.  495,  it  was  mentioned  that,  in 
many  Ophiurids,  these  function  as  brood  chambers,  and  the  best-known  cases  were 
given. 


504  COMPARATIVE  ANATOMY  CHAP. 


XX.  Capacity  for  Regeneration  and  Asexual  Reproduction  by  means  of 
Fission  and  Gemmation. 

The  capacity  for  regeneration  is,  as  a  rule,  highly  developed  in  Echinoderms. 
Defects  in  the  body  wall  are  in  this  way  easily  and  quickly  repaired  in  all  Echino- 
derms. The  Echinoidea,  even,  in  which  this  capacity  is  in  other  ways  slightly 
developed,  easily  repair  smaller  or  greater  defects  in  the  body  epithelium  which  covers 
the  test.  For  example,  in  Dorocidaris  papillata,  portions  of  the  test  over  which 
the  epithelium  has  been  damaged  or  destroyed  are  cast  off,  and  as  soon  as  a  fresh 
integument  has  formed  a  new  test  can  undoubtedly  be  produced  below  it. 

The  capacity  for  regeneration  may  increase  to  an  extraordinary  degree  within  the 
different  divisions,  and  the  sensitiveness  to  external  stimulus  increases  in  proportion, 
till  a  stage  is  reached  when  voluntary  amputation  by  means  of  muscular  contraction 
takes  place  in  response  to  external  stimuli. 

The  Crinoids  completely  regenerate  lost  viscera,  and  it  even  appears  as  if  such 
loss  is  not  altogether  involuntary,  in  certain  species  and  under  certain  conditions 
voluntary  amputation  taking  place.  This,  however,  is  not  certain. 

Crinoids  easily  regenerate  broken-off  portions  of  arms  or  whole  arms  ;  several  or 
indeed  all  the  arms  may,  under  favourable  circumstances,  be  regenerated.  The  arms 
break  off  easily  at  their  bases  ;  it  even  appears  as  if  Antedon,  in  response  to  injurious 
stimuli,  voluntarily  throws  off  its  arms. 

The  regeneration  of  the  portions  of  arms  (bitten  off,  possibly  by  enemies)  or  of 
whole  arms  takes  place  very  easily  in  many  Asteroidea  and  Ophiuroidea.  The 
frequency  with  which  Asteroids  and  Ophiurids  with  regenerated  arms  or  arm  tips  are 
met  with  demonstrates  both  the  frequency  of  mutilation  and  the  great  utility  of 
regeneration. 

Species  of  Asteroids  in  which  the  disc  with  the  other  arms  are  regenerated  at  the 
base  of  broken-off  arms  are  less  common.  Such  regenerations  give  rise  to  the  well- 
known  "comet"  form  of  Asteroids  (Fig.  397,  B).  Regeneration  of  the  whole  body 
from  one  arm  never  occurs  in  Ophiurids.  It  has  been  suggested  that  the  difference 
between  Asteroids  and  Ophiurids  in  this  respect  is  connected  with  the  fact  that,  in 
Asteroids,  intestinal  diverticula  project  into  the  arms,  and  that  the  genital  products 
are  often  developed  in  them,  which  is  never  the  case  in  Ophiurids. 

Animals  in  which  half  the  disc  is  retained  can  regenerate  the  rest  of  the  body 
both  among  the  Ophiuroidea  and  the  Asteroidea. 

Defects  both  great  and  small  in  the  disc  are  repaired. 

In  Linckia  multifora,  an  Asteroid  distinguished  by  an  extraordinary  capacity  for 
regeneration,  cases  have  been  known  in  which,  after  the  animal  has  lost  the  greater 
part  of  an  arm,  two  new  tips  have  been  formed  by  the  wounded  surface,  and  in  one 
case  regeneration  led  to  the  formation  of  a  complete  new  Asteroid  at  such  a 
point.  This  latter  case  is  illustrated  in  outline  in  Fig.  397  C.  The  new  animal 
consists  of  two  discs  with  their  arms,  connected  by  the  regenerating  stump  of  the 
arm. 

Holothurioidea. — Here  also  the  capacity  for  generation  seems  to  be  very  great. 
Not  only  are  tentacles  and  integumental  defects  repaired,  but  the  ejected  viscera 
(intestine,  respiratory  trees,  and  even  the  calcareous  ring,  the  water  vascular  ring, 
and  the  gonads)  can  be  regenerated.  In  Synapta,  after  the  body  has  been  completely 
cut  to  pieces,  its  anterior  portion  can  regenerate  the  whole.  In  a  Cucumaria,  the 
two  separate  halves  can  grow  into  complete  animals. 

Increase  in  the  capacity  for  regeneration  is  accompanied  by  increased  irritability. 
Many  Holothurioidea,  especially  Aspidochirotce,  when  strongly  stimulated,  contract 


EGHINODERMATA— REGENERATION,  ETC. 


505 


so  violently  that  the  intestine  is  torn  out  (usually  behind  the  calcareous  ring),  and 
together  with  the  right  respiratory  tree  is  ejected  through  a  rent  in  the  cloacal 
wall. 

In  certain  Holothurioidea,  the  integument,  when  strongly  irritated,  easily  dis- 
solves into  slirne.  A  Stichopus  was  observed  to  come  entirely  out  of  its  skin,  i.e. 
the  whole  integument  dissolved  into  slime,  so  that  only  the  dermomuscular  tube 


FIG.  397.— A,  OpMdiaster  diplax.  A  specimen  with  the  anus  (3,  4,  5)  in  the  act  of  being  re- 
generated ;  and  two  (1  and  2)  being  constricted  off  (after  Haeckel).  B,  Linckia  (Ophidiaster)  multi- 
fora,  a  "comet"  form.  One  arm  is  in  the  act  of  regenerating  the  disc  and  the  other  four  anus 
(after  Haeckel).  C,  The  case  given  in  the  text  of  a  specimen  of  Linckia  multifora  (after  P.  and  F. 
Sarasin). 

enclosing  the  viscera  remained.  That  regeneration  follows  such  a  phenomenon  has 
not  yet  been  established  by  observation. 

The  SynaptidcB  react  on  stimuli  (often  quite  slight)  by  falling  to  pieces,  the 
circular  musculature  being  at  certain  points  so  strongly  contracted  that  the  parts 
thus  constricted  break  off. 

It  will  no  doubt  be  proved  in  time  that  all  these  manifestations  of  increased 
irritability  which  coincide  with  increased  capacity  for  regeneration  are  of  use  to  the 
animal. 

Asexual  reproduction  (schizogony).  In  certain  Echinoderms,  the  strongly 
developed  capacity  for  regeneration  has  had  as  a  consequence  asexual  reproduction. 
It  is,  indeed,  not  certain  that,  in  the  cases  to  be  quoted  below,  the  division  into 


506  COMPARATIVE  ANATOMY  CHAP. 

parts  is  purely  voluntary  (i.e.  results  from  causes  entirely  within  the  animal  itself) 
and  not  to  some  extent  due  to  external  stimuli,  however  slight.  In  any  case,  the 
final  result  of  the  regeneration  which  follows  is  the  same — the  multiplication  of 
individuals. 

Fission  of  the  body  into  two  halves  of  approximately  equal  size  with  subsequent 
regeneration  has  been  observed  in  Ophiuroidea,  Asteroidea,  and  Holothurioidea. 
In  the  two  former  classes  the  plane  of  fission  passes  through  the  middle  of  the  disc 
(through  the  mouth  and  digestive  sac),  in  the  Holothurioidea  (Cucumaria)  it  passes 
transversely  through  the  tubular  body,  dividing  it  into  an  anterior  (oral)  and  a 
posterior  (apical)  half. 

In  the  Ophiuroidea,  reproduction  by  means  of  fission  has  been  observed  in  the 
following  genera  :  Ophiactis  (Miilleri,  Savigny.  virens),  Ophiocnida  (sexradia},  Ophio- 
coma  (pumila,  Valencies},  Ophiothcla  (isidicola,  dividua). 

Among  the  Asteroidea,  schizogony  is  specially  characteristic^  many  species  of  the 
genus  Asterias  (acutispitia,  atlantica,  calamaria,  microdiscus,  tenuispina],  and  is  also 
found  in  Asterina  Wega,  Cribrella  sexradiata,  Stichaster  albulus. 

Another  kind  of  asexual  reproduction  seems  to  be  very  common  in  the  family  of 
the  Linckiidce.  In  these  Asteroids,  the  arms  become  constricted  off  at  their  bases, 
after  which  not  only  does  the  disc  regenerate  the  arms  which  have  been  cast  off,  but 
each  individual  arm  regenerates  the  disc  and  the  other  arms  ("comet"  forms  of 
Asteroids,  Fig.  397  A,  B). 

Asexual  reproduction  does  not,  as  a  rule,  appear  to  take  place  simultaneously  with 
sexual  reproduction,  but  there  are  exceptions  to  this  rule. 


XXI.  Ontogeny. 

In  all  Echinoderms,  except  those  few  forms  in  which  care  of  the  brood  occurs,  the 
fertilised  eggs  develop  into  free-swimming,  bilaterally  symmetrical  larvae,  which  are 
transformed  into  the  radially  built  Echinoderm  after  passing  through  an  often  very 
complicated  metamorphosis. 

The  larvse  of  the  different  classes  of  Echinoderms  will  first  be  compared  ex- 
clusively according  to  their  external  characteristics. 


A.  The  various  Larval  Forms  of  the  Echinodermata. 

We  shall  first  construct  a  hypothetical  larval  form,  and  then  deduce  the  various 
larval  forms  from  it  (Fig.  398,  A). 

The  body  of  the  larva  is  egg-shaped,  and  concave  on  the  ventral  side.  In  the 
base  of  the  concavity  lies  the  larval  mouth.  Near  one  of  the  poles  (i.e.  near  the 
posterior  end),  but  still  on  the  ventral  side,  there  is  a  second  aperture  (proceeding 
from  the  blastopore  of  the  gastrula  larva)  ;  this  is  the  larval  anus.  A  ciliated  band 
which  runs  back  upon  itself  surrounds  the  mouth  along  the  edge  of  the  ventral  con- 
cavity ;  posteriorly  it  runs  over  the  ventral  side  in  front  of  the  anus,  and  is  the 
circumoral  ciliated  ring.  The  aperture  of  the  mouth  and  its  immediate  surround- 
ing are  also  ciliated  (adoral  ciliated  band). 

1.  Holothurioidea.— The  Holothurid  larva  known  as  Auricularia  (Fig.  398,  A) 
differs  but  little  from  the  hypothetical  form.  The  ventral  concavity  becomes  more 
complicated,  lengthening  on  each  side  posteriorly  and  anteriorly,  while  a  posterior 
median  portion  -\\ith  the  anus  forms  a  ventral  prominence,  the  anal  area,  and  a 
median  portion  in  front  of  the  mouth  forms  another  prominence,  the  preoral  area. 
The  ciliated  band  which  runs  longitudinally  along  the  ventral  depression  assumes 


ECHINODERMA  TA— ONTOGENY  507 

in  consequence  a  more  complicated  form,  and  takes  a  winding  course.  This  descrip- 
tion will  be  elucidated  by  the  figures. 

Here,  as  in  all  other  Echinoderms,  the  ciliated  rings  are  mere  remains  of  the 
cilia  which  covered  the  whole  body  in  an  earlier  stage,  i.  c.  in  the  gastrula. 

2.  Asteroidea  (Fig.  398,  B). — The  Asteroid  larvae  are  known  as  Bipinnarise  and 
Brachiolariae.  The  chief  distinction  between  them  and  the  Auricularia  is  the 
preoral  ciliated  ring.  This  is  a  circle  on  the  preoral  area,  and  within  the  circumoral 
ciliated  ring,  from  which  it  is  altogether  distinct. 

The  comparison  of  a  Bipinnaria  with  an  Auricularia  led  to  the  conjecture  that 
the  preoral  ciliated  ring  of  the  former  corresponds  with  a  preoral  portion  of  the 
common  circumoral  ciliated  ring  of  the  latter,  which  has  become  distinct  and  has 
closed  to  form  a  ring.  Direct  observation  of  the  ontogenetic  development  of  the 
ciliated  ring  of  the  Asteroid  larva  has  entirely  confirmed  this  conjecture. 


FIG.  398. — A,  B,  C,  Auricularia,  Bipinnaria,  and  Tornaria  (Enteropneustan  larva),  from  the 
right  side,  diagrammatic.  1,  Pretrochal  area  ;  2,  oral  area  ;  3,  postoral  area  ;  4,  anal  area ;  I,  pre- 
oral; II,  circumoral  :  III,  anal  or  principal  ciliated  ring;  5,  neural  plate;  os,  mouth,  OH,  anus. 

The  Bipinnaria  passes  through  an  Auricularia  stage.  The  general  ciliation  of 
the  body,  belonging  to  an  early  stage,  disappears  first  from  the  ventral  side,  which 
becomes  depressed,  then  from  the  dorsal  side,  in  such  a  way  as  to  leave  a  band 
running  back  on  itself  at  the  edge  of  the  ventral  depression  ;  this  corresponds  entirely 
with  the  course  of  the  circumoral  ciliated  band  in  the  Auricularia.  In  the  frontal 
region,  however,  where  the  two  lateral  strips  of  the  circumoral  band  approach  each 
other  in  the  median  line,  a  ciliated  island  is  for  a  time  retained  connecting  them 
(Asterias  rube/is}.  The  covering  of  cilia  thus  forms  an  X-like  cross  on  the  frontal 
region.  By  the  disappearance  of  the  ciliation  from  the  centre  of  the  X,  the  preoral 
section  of  the  ciliated  ring  is  separated  from  the  rest,  and  forms  the  distinct  preoral 
ring  enclosed  within  the  circumoral  ring. 

The  process  in  Asterias  vulgaris  seems  to  take  a  somewhat  different  course,  but 
has  the  same  final  result.  On  the  frontal  region,  where,  in  A.  rubens,  an  isolated 
ciliated  area  remained  to  form  a  connection  between  two  portions  of  the  circumoral 
ciliated  band,  this  connection  arises  only  secondarily  by  the  approximation  of  the 
two  portions  in  the  middle  line.  The  further  process  of  separation  of  the  preoral 
ring  from  the  rest,  which  latter  then  represents  the  secondary  circumoral  ciliated 
ring,  agrees  with  that  in  A.  rubens. 

The  ventral  depression  (in  which  the  mouth  lies)  which,  in  the  Auricularia,  runs 
forward  to  the  right  and  left  of  the  preoral  portion  of  the  circumoral  ciliated  ring, 
is  now  able,  after  the  latter  has  become  constricted  off  as  a  ring,  entirely  to  surround 


508 


COMPARATIVE  ANATOMY 


CHAP. 


that  portion  ;  it  forms  a  moat  round  the  preoral  area,  which  becomes  raised  up  like  a 
shield. 

An  adoral  ciliated  ring,  closely  encircling  the  mouth  and  extending  some  way 
into  the  buccal  cavity,  is  also  present. 

The  body  is  produced  into  longer  or  shorter  processes  or  arms,  in  the  regions  of  the 
preoral  and  circumoral  ciliated  rings.  An  anterior  unpaired  frontal  process,  belong- 
ing to  the  ciliated  ring,  is  distinguished  by  its  constant  occurrence  and  its  greater 
length. 

In  some  species,  the  ciliated  band  disappears  on  this  frontal  process,  which,  on 


20- 


FIG.  399.— Older  Auricularia,  seen  diagonally  from  the  lower  and  left  side  (after  Semon).  1, 
Circumoral  ciliated  ring ;  2,  hydropore  ;  3,  hydrocoel ;  4,  adoral  ciliated  ring  ;  5,  median  or  stomach 
intestine ;  6,  nerve  band  ;  7,  hind-gut ;  8,  left  enterocrel ;  9,  calcareous  wheel ;  10,  fore-gut,  oeso- 
phagus ;  11,  right  enterocoel. 

the  other  hand,  divides  into  three  branches,  apparently  covered  with  protuberances 
at  their  tips.  Such  larvae  are  known  as  Brachiolariae. 

There  are,  further,  Asteroids  whose  larvae  do  not  at  all  resemble  the  Bipinnarian 
and  Brachiolarian  larvae,  or  else  show  only  a  superficial  resemblance  to  them  ;  cf. 
below  the  account  of  the  larva  of  Asterina  gibbosa  (p.  525). 

3.  Ophiuroidea. — The  Ophiurid  larva  is  known  as  the  Pluteus,  and  can  be 
just  as  easily  deduced  from  the  hypothetical  larval  form  of  the  Echinoderms,  sketched 
above,  as  the  Auricularia  and  the  Bipinnaria.  The  gastrula  stage  is  followed  by 
the  bilateral  stage  with  depressed  ventral  surface,  in  the  centre  of  which  lies  the 
larval  mouth.  A  circumoral  ciliated  band  is  retained,  running  along  the  edge  of  this 
ventral  depression.  This  band  always  remains  single.  While  the  preoral  area  (the 
larva  being  viewed  from  the  ventral  side)  remains  very  small  or  is  even  indistinguish- 
able, the  anal  area  appears  very  large.  The  body  is  produced  into  processes  or  arms, 
which  may  become  very  long,  and  are  supported  by  calcareous  rods.  These  pro- 


VIII 


ECHINODERMATA— ONTOGENY 


509 


cesses  are  of  two  kinds.  One  kind,  which  belong  to  the  region  of  the  circumoral 
ciliated  ring,  are  paired,  and  diverge  in  a  forward  direction.  Two  arms  are 
distinguished  by  their  constant  occurrence  and  special  length  ;  these  belong  to  the 
posterior  and  lateral  region  of  the  circumoral  ciliated  band.  Opposed  to  these  paired 
processes  of  the  circumoral  ciliated  band  pointing  anteriorly  is  an  unpaired,  posterior, 
postanal  process  projecting  backwards  from  the  posterior  end  of  the  anal  area  ;  its 
tip  may  carry  a  cap  of  cilia. 

In  Ophiuroidea  in  which  care  of  the  brood  occurs,  the  typical  larval  forms  are  not 
developed. 

4.  Echinoidea  (Figs.  400  and  401). — The  larva  of  Echinoidea  agrees  to  a  great 
extent  with  that  of  the  Ophiuroidea,  and  is,  like  it,  known  as  the  Pluteus.  The  only 


FIG.  400.— Larva  of  an  Echinid  (Pluteus)  from 
the  ventral  side.  1,  Ciliated  "epaulettes "  ;  ant,  an- 
terior ;  post,  posterior;  dex,  right ;  sin,  left. 


FIG.  401.— Spatangid  larva  (Pluteus) 
from  the  ventral  side.  1,  The  three  processes 
of  the  anal  area. 


important  difference  is  that  the  two  lateral  arms  which,  in  the  Ophiurids,  are  the 
most  constant  and  the  longest,  seem  to  be  altogether  wanting  in  the  Echinoidea. 

The  Pluteus  of  Echinus  has  eight  arms  or  processes,  and  at  the  bases  of  each  ot 
the  four  posterior  arms  a  ciliated  li  epaulette  "  (Fig.  400). 

The  larvre  of  Arbacia  and  Spatangus  (Fig.  401)  have  no  ciliated  "epaulettes," 
but  Arbacia  has  two  and  Spatangus  three  long  posterior  processes  of  the  anal  area, 
which,  like  all  the  other  processes,  are  supported  by  calcareous  rods. 

Echinoids  in  which  care  of  the  brood  occurs  develop  direct  without  meta- 
morphosis. 

5.  Crinoidea  (Fig.  402). — The  free-swimming  larva  of  Antedon  is  long  and  egg- 
shaped.  At  the  frontal  pole,  the  thickened  but  somewhat  depressed  ectoderm  (the 
neural  pit  or  plate)  carries  a  tuft  of  long  flagella.  The  larva,  in  swimming,  has  the 
frontal  pole,  which  corresponds  with  the  anterior  end  of  other  Echinoderms,  directed 
forwards. 


flp" 

OF  THE 

UNIVERSITY 


510  COMPARATIVE  ANATOMY  CHAP. 

The  body  is  surrounded  by  five  ciliated  rings,  distinct  from  one  another  ;  these 
cannot  be  ontogenetically  derived  from  one  continuous  ciliated  ring. 
The  most  anterior  ring  is  interrupted  on  the  ventral  side. 

The  second  ring  runs  somewhat  diagonally  from  above  downwards  and  forwards, 
the  third  is  directed  downwards  and  backwards,  so  that  there  is  a  large  interval 

between  the  second  and  third  rings  ventrally. 

In  this  region,  the  ventral  side  sinks  in  to 
,     u»|,,,  form  a  large  ciliated  vestibular  depression. 

A  smaller  depression  on  the  ventral  side 
between  the  first  and  second  ciliated  ring  is 
known  as  the  adhesive  pit.  The  larva  attaches 
itself  at  this  point,  by  means  of  a  special 
secretion  yielded  by  the  glandular  cells  of  the 
depression. 

To  the  left,  between  the  third  and  fourth 
rings,  there  is  a  small  aperture,  the  primary 
madreporite  (water  pore). 

The  intestine  lies  as  an  entirely  closed  sac  in 

J  :-..-  /  the  posterior  part  of  the  larva.     The  free-swim - 

Hpfe.  ming  larva  has  neither  larval  mouth  nor  larval 

\  anus.     The  definitive  mouth  breaks  through  the 

floor  of  the  vestibular  depression  later. 


The  whole  anterior  part  of  the  larva,  as  far 
FIG.  402.— Free -swimming  larva  of   as  the  third  ciliated  ring,  becomes  the  stalk,  and 
Antedon,  from  the  right  lower  side  (after    the    posterior   part   the   calyx  of  the  attached 
Bury).    I-  V,  The  five  ciliated  rings ;  1,  the    larya 
neural  tuft;  2,  the  adhesive  pit;  3,  the 

vestibular  depression;  d,  dorsal;  v,  The  free-swimming  Crmoid  larva  cannot  with 
ventral.  certainty  be  derived  from  the  same  hypothetical 

form  as  other  Echinoderm  larvae.     The  difficulty 

consists  in  the  varying  number  and  arrangement  of  the  ciliated  rings,  which  most 
recall  the  condition  in  the  Holothurid  larva  (pupa),  to  be  described  later.  The 
vestibular  depression  of  the  Antedon  larva  may,  however,  be  compared  without 
forcing  to  the  ventral  depression  of  the  other  Echinoderm  larvse.  A  thickening  of 
the  ectoderm,  comparable  with  the  neural  plate  of  the  Antedon  larva,  also  occurs,  as 
we  shall  see  in  other  Echinoderm  larvse. 


B.  Ontogeny  of  the  Holothurioidea. 

The  segmentation  of  the  ovum  is  total  and  equal,  and  leads  to  the  formation  of  a 
coeloblastula,  whose  unilaminar  cell  wall  usually  consists  on  one  side  of  somewhat 
larger  cells.  By  invagination  of  this  part  of  the  blastula  wall,  a  ccelogastrula  is 
formed.  The  invaginated  part,  i.e.  the  archenteron,  is  a  blindly  ending  tube,  with 
narrow  lumen  (archenteric  cavity),  which  is  far  from  filling  the  segmentation  cavity. 
This  latter  is  filled  with  an  albuminous,  fluid  or  semifluid,  mass,  the  gelatinous 
nucleus. 

The  ectoderm  and  the  endoderm  are  ciliated. 

During  the  process  of  invagination  (occasionally  even  during  the  blastula  stage) 
cells  arise  by  division  from  the  ectoderm,  but  more  especially  from  the  endoderm, 
which,  as  mesenchyme  cells,  wander  into  the  enclosed  jelly-like  substance,  multiply 
by  division  and,  in  ever-increasing  numbers,  occupy  the  blastocoel.  From  them  is  pro- 
duced all  the  connective  tissue  of  the  Holothurian  body.  The  calcareous  corpuscles 
arise  exclusively  in  the  mesenchyme. 


VIII 


ECH1NODERMATA— ONTOGENY 


511 


The  blind  end  of  the  lengthening  archenteron  bends  to  that  side,  which  becomes 
the  dorsal  side  of  the  larva  as  it  rapidly  grows  bilaterally  symmetrical  (Fig.  403,  A), 
and  at  the  same  time  it  moves  somewhat  to  the  left  side.  As  the  hydro-enteroccel 
vesicle,  it  soon  becomes  entirely  constricted  off  from  the  rest  of  the  archenteron, 
which  opens  outward  through  the  blastopore  (Fig.  403,  B,  C,  D,  4,  5). 

This  constriction  from  the  archenteron,  the  hydro -enterocoelomic  vesicle,  is  of 
the  greatest  importance,  because  out  of  its  wall  arises  the  whole  musculature  of  the 
body  and  all  the  internal  epithelia,  i.  c.  the  coelomic  and  water  vascular  epithelia. 

The  hydro-enterocrelomic  vesicle  increases  in  length,  alongside  of  the  intestine, 
in  the  direction  of  the  blastopore,  and  again  divides  into  two  vesicles  by  means  of  a 


FIG.  403.— Formation  of  the  larval  mouth 
and  the  hydro  -  enteroccelomic  vesicle  in  the 
gastrula  larva  of  Synapta  digitata,  diagram- 
matic (after  Selenka).  .4.  Gastrula,  the  archen- 
teron bent  towards  the  dorsal  side ;  B,  archen- 
teron, opening  outward  through  the  hydropore  ; 

C,  hydro-enteroccel,  constricted  from  the  intestine ; 

D,  intestine,  opening  outward  through  the  larval 
mouth  ventrally.    1,  Segmentation  cavity,  blasto- 
coel ;  2,  archenteron  ;  3,  blastopore ;  4,  hydropore ; 
5,  hydro-enteroccel ;  6,  intestine  ;  7,  esophagus  ; 
8,  mouth;  ant,  anterior;  post,  posterior;  r,  ven- 
tral ;  d,  dorsal. 


FIG.  404.— Auricularia  with  the  left  half  of 
the  ectoderm  removed,  from  the  left  (after 
Ziegler's  model).  The  organs  lying  in  the 
segmentation  cavity  (11)  are  seen.  1,  Cut  edge 
of  the  ectoderm ;  2,  mouth ;  3,  oesophagus ; 
4,  mesenchyme  cells ;  5,  mid-gut  or  stomach 
intestine  ;  6,  anus  ;  7,  hind-gut ;  8,  left  entero- 
coel,  still  connected  with  the  hydroccel  10,  the 
latter  showing  slight  indications  of  the  first 
radial  outgrowths  ;  9,  dorsal  pore  or  hydro- 
pore  ;  11,  blastoccel,  segmentation  cavity. 


transverse  constriction.  The  anterior  vesicle  (that  further  from  the  blastopore)  is 
the  hydrocffilomic  vesicle,  which  at  once  sends  off  a  canal  to  the  dorsal  side,  which 
opens  outwards  through  a  pore  on  the  left  of  the  middle  line.  The  canal  is  the 
primary  stone  canal,  and  the  pore  the  primary  madreporite.  The  hydroccelomic 
vesicle  is  the  rudiment  of  all  the  rest  of  the  water  vascular  system,  and  in  the  first 
place,  of  course,  of  the  circular  canal  (Fig.  404). 

The  first  appearance  of  the  various  structures  just  described  does  not  occur  in  the 
same  order  in  all  species  of  Holothurioidea  examined  on  this  point.  The  hydro- 
enteroccelomic  vesicle  may  become  connected  with  the  exterior  through  a  stone  canal 
before  it  has  divided,  or  even  (a  unique  condition  found  in  Synapta  digitata}  before 


512 


COMPARATIVE  ANATOMY 


CHAP.  VIII 


it  has  itself  separated  from  the  archenteron  (Fig.  403).  In  the  last  case,  it  can  be 
established  that  the  archenteron  which  begins  with  the  blastopore  opens  outward  for 
a  time  through  a  second  aperture,  the  primary  madreporite. 

After  the  hydro-enterocoelomic  vesicle  has  become  constricted  from  the  archen- 
teron, the  intestine  grows  further,  its  blind  end  bending  to  the  ventral  side  (that  lying 
opposite  to  the  water  pore),  which  commences  to  become  depressed  and  to  sink  in. 

The  blind  end  of  the  intestine  soon  becomes  applied  to  the  ectoderm  of  the 
depressed  ventral  side  of  the  larva,  about  half  way  down  the  body,  or  a  little  in 


\ 


OS- 


—  2 


20- 


U^- 


CCJl 

FIG.  405.— Young  Auricularia  of 
Synapta,  from  the  ventral  side  (after 
Semon).  1,  Circumoral  ciliated  band  ; 
2,  entero  -  hydrocoel ;  3,  calcareous 
wheel ;  4,  adoral  ciliated  ring ;  os, 
mouth  ;  an,  anus ;  5,  mid  -  gut  or 
stomach  intestine ;  6,  larval  nerve 
band. 


FIG.  406.— Older  Auricularia,  seen  diagonally  from  the 
left  lower  side  (after  Semon).  1,  Circumoral  ciliated  band  ; 
2,  hydropore ;  3,  hydrocoel ;  4,  adoral  ciliated  band ;  5,  mid- 
gut  or  stomach  -  intestine  ;  6,  nerve  band;  7,  hind -gut; 
8,  left  enterocoel ;  9,  calcareous  wheel ;  10,  fore-gut,  oaso- 
phagus  ;  11,  right  enterocoal. 


front  of  the  middle  point.  Where  the  two  touch  one  another,  an  aperture,  the  mouth, 
breaks  through. 

The  median  portion  of  the  intestine  (the  mid-gut)  swells  up  and  becomes  distinct 
both  from  the  fore-gut  and  from  the  hind-gut. 

In  the  meantime,  the  larva  has  undergone  a  change  of  shape  through  which  it 
reaches  the  Auricularia  stage,  the  depression  of  the  ventral  side  being  the  most 
important  part  of  this  change.  The  general  ciliation  has  disappeared  ;  of  the 
complete  covering  of  cilia,  only  the  circumoral  ring  and  the  adoral  band  are 
retained,  and  the  region  immediately  around  the  mouth  has  become  depressed  to 
form  the  oral  vestibule  (Figs.  404-407). 

The  transformation  of  the  Auricularia  into  the  barrel-shaped  larva  (Figs.  408- 
413). — The  Auricularia  does  not  change  direct  into  a  young  Holothurid,  but  passes 
through  an  intermediate  stage,  which  was  formerly  known  as  the  pupal  stage, 
becfinse  during  it  no  nourishment  is  taken. 


Fi<;.  407.— Older  Auricu- 
laria  (after  Semon).  an, 
Anus ;  os,  mouth  ;  1,  out- 
growths (primary  and 
secondary)  of  the  primitive 
lior.se  -  shoe  -  shaped  hydro- 
ccel ;  2,  stone  canal ;  3,  left, 
4,  right  enterocoelomic  sac, 
which  have  become  closely 
applied  to  the  mid-gut. 


FIG.  40S.  —  Auricularia, 
in  which  the  circumoral 
ciliated  ring  is  beginning 
to  break  up  into  lengths 
(after  Semon).  The  horse- 
shoe -  shaped  hydroco?!  is 
growing  round  the  intestine. 
The  first  pieces  of  the  cal- 
careous ring  (1)  have  ap- 
peared. 


VOL.  II 


2  L 


514 


COMPARATIVE  ANATOMY 


CHAP. 


The  Auricularia  assumes  the  shape  of  a  barrel.  The  circnmoral  ciliated  ring 
atrophies  in  sixteen  places,  which  are  indicated  in  the  diagram  (Fig.  413).  The 
sixteen  lengths  of  the  ring  which  remain  continue  to  grow  and  join,  as  indicated 


FIG.  409.— Old  Auricularia. 
Transition  to  the  barrel  -  shaped 
pupa,  the  whole  body  decreasing 
considerably  in  size.  1,  The  nerve 
bands,  in  the  act  of  forming  the 
nerve  ring ;  2,  primary  tentacle. 


FIG.  410.— Intermediate  stage  be- 
tween Auricularia  and  the  barrel- 
shaped  pupa  of  Synapta  (after 
Semon).  I-V,  the  rudiments  of  the 
five  ciliated  rings.  1,  The  oral  funnel  ; 
2,  the  primary ;  3,  the  secondary  out- 
growths of  the  water  vascular  ring ; 
4,  pieces  of  the  calcareous  ring ;  5, 
coelomic  vesicle  ;  6,  water  vascular  ring. 


by  dotted  lines  (Fig.  413),  to  form  five  ciliated  rings  entirely  encircling  the  barrel  - 
shaped  body.  The  centre  of  the  former  oral  area  becomes  surrounded  by  four  lengths 
of  the  ciliated  band  which  join  together  to  make  a  square.  The  part  of  the  oral 


FIG.  411. — Young  barrel  -  shaped 
larva  (pupa)  (after  Semon).  1,  Oral 
funnel ;  2,  tentacles  ;  3,  pieces  of  the 
calcareous  ring  ;  4,  Polian  vesicle  ; 
5,  left  coelom  ;  6,  hind-gut ;  7,  audi- 
tory vesicles ;  8,  secondary  out- 
growths of  the  hydrocoel  ring. 


FIG.  41-2.— Barrel- shaped  larva, 
with  the  tentacles  (1)  beginning  to 
project  from  the  opening  oral  funnel 
(after  Semon).  2,  Water  vessels  of 
the  body — secondary  outgrowths  of 
the  circular  canal ;  3,  the  rapidly 
swelling  enterocoel. 


area  enclosed  by  this  ring  sinks  below  the  surface,  and  thus  increases  the  size  of  the 
oral  vestibule.  The  ciliated  square  itself  becomes  depressed  and  forms  the  oral 
shield.  The  spacious  oral  vestibule  becomes  cut  off  from  the  exterior,  with  the 
exception  of  a  very  narrow  aperture,  and  shifts  quite  to  the  front,  so  that  now  the 


ECHINODERMA  TA— ONTOGENY 


515 


three  apertures,  that  of  the  oral  vestibule,  the  mouth  itself  lying  in  its  floor,  and  the 
anus,  lie  almost  in  the  axis  of  the  barrel-shaped  body. 

In  the  older  Auricularia  stages  and  during  the  transformation  into  the  barrel  - 
shaped  larva  important  internal  processes  take  place. 

Calcareous  corpuscles  appear  early  (even  in  the  younger  Auricularia)  in  the 
mesenchyme.  In  the  best  known  Auricularia,  that  of  Synapta  digitata,  these 
bodies  appear  iu  the  form  of  wheels  in  the  two 
posterior  tips  of  the  larva  (cf.  Fig.  404,  p.  511, 
and  following). 

The  hydroccelomic  vesicle  assumes  the  form 
of  a  horse-shoe  with  the  curve  towards  the  dorsal 
side.  On  the  convex  side  of  this  horse-shoe- 
shaped  vesicle  five  outgrowths  appear.  The  two 
arms  of  the  horse-shoe  then  close  round  the  fore- 
gut,  growing  towards  each  other  round  it  until, 
finally,  they  meet  and  fuse  (probably  in  the 
right  half  of  the  body).  The  horse-shoe-shaped 
hydroccel  is  now  the  closed  water  vascular  ring 
.surrounding  the  fore-gut  It  continues,  as  before, 
to  communicate  with  the  exterior  through  the 
primary  stone  canal  and  the  dorsal  water  pore. 

The  five  outgrowths  of  this  hydroccel  ring 
now  become  more  distinct.  They  are  originally 
directed  forwards,  but  very  soon,  with  further 
growth,  bend  backward,  and,  as  the  rudiments 
of  the  radial  canals  of  the  water  vascular 
system,  grow  further  back  under  the  body  wall, 
in  the  five  radii.  The  rudiments  of  the  tentacle 
canals  appear  very  early  on  the  rudiments  of  culuria  (after  Ludwig).  The  pieces  of 

the  radial  canals  as  orally  directed  lateral  out-    the  ciliated  band  are  uaarked  b*v  broad 
,  black  lines,  the  interruptions  being  left 

:hs-  clear.     8,  The  preoral ;  9,  the  postoral 

The  above  account  of  the  first  processes  of   intermediate  piece  of  the  ciliated  ring ; 
differentiation  in  the  hydroccel  vesicle  are  those    os,  mouth.    The  dotted  lines  give  the 
found    in    Cv.cuiiinna   Pland,    the   ontogeny   of    direction  in  which  the  pieces  of  the  ring 
which  has  recently  been  carefully  investigated.    '^Sf&^V^^' 
Iii  other  Holothurioidea,   at  least   in   Synapta 

diqitata,  according  to  former  authors,  the  ontogenetic  processes  differed  essentially 
from  these.  The  first  five  outgrowths  of  the  hydroccel  in  Synapta  develop  exclusively 
into  the  tentacle  canals,  and  only  after  the  appearance  of  these  and  alternately 
with  them,  five  other  outgrowths  form  the  rudiments  of  the  radial  canals. 

This  and  certain  other  discoveries  led  to  the  conclusion  that  the  radial  canals 
in  the  Holothurians  arise  interradially  and  only  shift  into  the  radii  secondarily, 
hence  it  was  inferred  that  the  tentacle  canals  of  the  Holothurioidea  were  homologous 
with  the  radial  canals  of  other  Echinoderms,  and  that  the  radial  canals  of  the  Holo- 
thurioidea are  not  represented  in  other  classes.  The  above  discoveries  in  the  larva 
of  Cucn.iiw.ria  Planci  dispose  of  this  suggestion,  which  must  always  have  appeared 
improbable  to  comparative  anatomists. 

It  is  a  very  noteworthy  fact  that,  in  Synapta,  the  radial  canals  appear  onto- 
geuetically,  whereas  they  are  wanting  in  the  adult. 

The  Polian  vesicle  also  arises  as  an  outgrowth  of  the  circular  canal  ;  in  Cucu- 
nuiri'A  Plfiiid.  it  forms  at  the  point  where  it  lies  in  the  adult,  i.e.  in  the  left  dorsal 
interradius. 

The  tube-feet  arise  as  outgrowths  of  the  radial  canals,  which  push  the  ectoderm 


FIG.  413.— Diagram  illustrating  the 
rise  of  the  five  ciliated  rings  of  the 
Holothurian  pupa  from  the  pieces  of  the 
ciliated  bands  1-7  and  l'-7'  of  the  Auri- 


516 


COMPARATIVE  ANATOMY 


CHAP. 


out  in  front  of  them  (Fig.  415).  The  first  two  tube-feet  in  Cucumaria  Planci  arise 
simultaneously  at  the  posterior  end  of  the  body.  Both  these  feet  belong  to  the 
medioventral  radial  canal. 

The  differentiation  of  the  enteroccel  vesicle.  —  After  the  hydro-enteroca-1 
vesicle  has  divided  into  the  hydrocoel  and  the  enteroccel  vesicles,  the  latter  grows 
backwards  longitudinally,  the  growing  posterior  end  pushing  its  way  gradually  over 
the  intestine,  on  to  its  right  side  (i.e.  into  the  right-hand  portion  of  the  segmenta- 
tion cavity).  The  anterior  part  of  the  enterocoel  vesicle  now  lies  to  the  left,  the 

posterior  to  the  right,  near  the  intestine  (Fig. 
404).  These  parts  become  completely  separated 
by  a  constriction  which  stretches  transversely  over 
the  intestine,  into  a  left  and  a  right  enteroccel 
vesicle. 

Each  of  these  vesicles  becomes  applied  to  the 
intestine,  and,  increasing  in  size,  takes  the  shape 
of  a  hollow  disc  resembling  a  watch  glass. 

The  nervous  system  of  the  larva.  —On  each 
side  of  the  Auricularia  larva,  on  the  ventral  side, 
in  the  oral  area,  there  is  a  ciliated  ectodermal 
ridge,  beneath  the  surface  of  which  ganglion  cells 
lie  and  longitudinal  nerve  fibres  run.  The  ridge 
consists  of  two  limbs,  meeting  in  an  obtuse  angle 
which  is  open  towards  the  mouth.  From  the  ends 
and  angles  of  these  ridges  nerve  fibres  go  off  to 
the  circumoral  ciliated  band. 

Formation  of   the   tentacles. — The   tentacle 


vessels  of  the  water  vascular  system  ; 
7,  hind -gut;  8,  calcareous  wheels; 
9,  mid-gut ;  10,  madreporite  ;  11,  stone 
canal. 


FIG.  414.— Young  Synapta  (Pentac- 
tula)  (after  Semon).  1,  Oral  tentacles  ; 
2,  auditory  vesicles  ;  3,  pieces  of  the 
calcareous  ring  ;  4,  water  vascular 
ring  ;  5,  Poliau  vesicle  ;  6,  radial  canals,  whether  as  lateral  outgrowths  of  the  radial 

canals  or  direct  outgrowths  of  the  water  vascular 
system,  grow  towards  the  oral  vestibule,  and  press 
out  the  ectodermal  wall  of  the  latter  before  them. 
The  ectodermal  covering  thus  afforded  them  is 
derived  from  the  oral  shield,  i.e.  indirectly  from  parts  of  the  original  circumoral 
ciliated  ring  of  the  Auricularia  larva.  The  tentacles  (five  of  which  form  first) 
remain  hidden  in  the  oral  vestibule  during  the  pupal  stage. 

Transformation  of  the  barrel- shaped  larva  into  the  young  Holothurian  (Figs. 
414  and  415). — The  external  changes  are  as  follows.  The  ciliated  rings  atrophy. 
The  tentacles  project  freely  from  the  expanding  and  widely  opening  vestibule,  and 
increase  in  number.  In  the  Actinopoda  tube-feet  are  formed  in  all  the  five  radii. 

It  is  an  important  fact  that,  in  the  comparatively  simple  transformations  of  the 
Holothurian,  not  only  does  the  whole  of  the  larval  epithelium  pass  into  the  body 
epithelium  of  the  adult,  but  none  of  the  larval  organs  are  eliminated. 

The  following  description  applies  to  Cucumaria  Planci. 

The  tentacles. — It  is  somewhat  remarkable  that  the  five  tentacles  first  formed  do 
not  each  arise  from  a  single  radial  canal.  Two  of  the  five  tentacles,  on  the  contrary, 
receive  their  canals  from  the  medioventral,  and  two  others  from  the  left  dorsal 
radial  canal.  The  fifth  tentacle  belongs  to  the  right  dorsal  radial  canal.  Other 
tentacles  appear  only  at  a  very  late  stage,  two  more  being  added  first,  a  sixth  and  a 
seventh.  These  belong  to  the  two  lateral  ventral  radii,  which  up  to  this  time  have 
been  without  tentacles. 

The  stone  canal. — An  anterior  outgrowth  forms  on  the  primary  stone  canal  ;  the 
epithelium  of  this  outgrowth  flattens,  giving  rise  to  the  madreporitic  vesicle.  On 
the  wall  of  this  vesicle  the  mesenchyme  forms  an  incomplete  shell,  perforated  like  a 
lattice 


viii  ECHINODERMATA— ONTOGENY  517 

The  water  pore  lying  on  the  right  side  of  the  mesentery  (now  formed)  disappears 


19 


FIG.  415.— Longitudinal  section  of  a  larva  of  Cucumaria  doliolum  (aftp r  Selenka).  1,  Frontal 
prominence  with  enclosed  jelly-like  substance;  -2,  tentacle  vessels;  3,  6,  15,  14,  13,  radial 
vessels  ;  5,  stone  canal ;  8,  madreporite  ;  4,  Polian  vesicle  cut  off ;  7  and  12,  coelom,  enterocoel 
vesicle  ;  9,  anus  ;  10  and  11,  the  first  two  tube-feet ;  16,  circular  canal  of  the  water  vascular 
system  ;  17,  oesophagus  ;  18,  mouth  ;  19,  mesenchyme  cells  ;  II,  III,  IV,  V,  ciliated  rings. 

later,  and  still  later  the  madreporitic  vesicle  opens  into  the  body  cavity,  and  thus 
becomes  the  secondary  inner  madreporite. 


518  COMPARATIVE  ANATOMY  CHAP. 

The  radial  canals. — The  five  radial  canals  do  not  develop  with  equal  rapidity, 
nor  indeed  do  the  radial  nerves  and  the  radial  longitudinal  muscles.  The  medio- 
ventral  organs  (radial  canal,  radial  nerve,  longitudinal  muscle)  in  all  cases  arise  first, 
then  follow  the  organs  of  the  two  dorsal  radii,  and  only  at  a  later  stage,  those  of 
the  two  lateral  ventral  radii. 

The  tube-feet. — In  agreement  with  the  order  of  appearance  just  described,  the 
two  first  tube-feet,  already  mentioned  above,  belong  to  the  ventral  radius  (Fig. 
415).  The  two  next  in  order  also  belong  to  the  medioventral  radial  canal,  and, 
according  to  the  rule  which  applies  to  all  the  newly  developing  tube-feet,  arise  in 
front  of  those  already  present.  The  fifth  tube-foot  belongs  to  the  left  dorsal  radial 
canal.  (The  correspondence  of  this  order  with  that  of  the  rudiments  of  the  tentacles 
should  be  noted. ) 

According  to  observations  which  have  been  made,  it  appears  that 
when,  in  a  Holothurian,  the  tube-feet  are  scattered,  this  arrangement 
is,  ontogenetically,  secondary.  In  the  same  way  animals  which  have 
several  rows  of  tube-feet  in  each  radius  have  only  two  rows  in  a  young 
stage,  or  a  zigzag  row  of  alternating  feet. 

The  nervous  system. — The  first  part  of  the  nervous  system  to 
appear  is  the  oral  circular  nerve,  and  this  arises  as  an  ectodermal 
circular  ridge  on  the  floor  of  the  oral  vestibule  in  the  larva.  It 
sends  out  five  band-like  processes  in  the  direction  of  the  rudiments  of 
the  radial  canals ;  these  are  the  rudiments  of  the  radial  nerves. 

In  that  the  rudiments  of  the  circular  nerve  and  the  radial  nerves 
become  subepithelial,  there  arises  between  them  and  the  body 
epithelium  which  closes  over  them  a  narrow  space ;  this  is  the 
epineural  canal. 

The  rudiments  of  the  five  radial  nerves  grow  backwards  together 
with  those  of  the  radial  canals. 

In  Cucumaria  Planci  there  seems  to  be  no  larval  nervous  system. 
In  Synapta,  on  the  contrary,  the  larval  nervous  system  yields  the 
rudiments  of  the  definitive  system.  The  two  lateral  nerve  ridges  of  the 
Auricularia  larva,  when  the  oral  vestibule  of  the  barrel-shaped  larva  is 
formed,  shift  into  it.  Their  free  ends  then  become  connected  from 
the  two  sides  to  form  a  ring  encircling  the  mouth,  which  is  the  rudi- 
ment of  the  nerve  ring. 

The  intestine  shows,  at  an  early  stage,  the  coils  characteristic  of  the  adult. 

The  first  portions  of  the  calcareous  ring  to  appear  are  the  five  radial  pieces  :  these 
arise  on  the  radial  canal  and,  like  all  the  calcareous  structures,  are  produced  by  the 
mesenchyme.  The  medioventral  calcareous  piece  is  from  the  first  the  largest. 

The  enteroccel. — The  right  and  the  left  enteroc<el  vesicles  groAv  round  the  intes- 
tine. At  the  point  where  they  meet  ventrally  they  open  into  one  another. 
Dorsally  they  press  the  mesenchyme  cells  together  in  a  vertical  lamina.  In  this 
way  the  dorsal  (anterior)  mesentery  arises.  The  middle  and  the  posterior  mesen- 
teries probably  arise  in  consequence  of  the  two  enteroccel  vesicles  twisting  round  the 
intestine  posteriorly. 

The  visceral  wall  of  the  enteroccel,  which  becomes  applied  to  the  intestine, 
presses  the  mesenchyme  cells,  which  have  greatly  increased  in  number,  against  the 
endodermal  intestinal  wall,  till  they  form  a  layer,  which  becomes  the  connective 


vin  ECHINODERMATA— ONTOGENY  519 

tissue  layer  of  the  intestine.  In  a  similar  way,  the  parietal  wall  of  the  enterocctd 
presses  the  peripheral  mesenchyrae  cells  to  form  a  layer  below  the  ectodermal  body 
epithelium  ;  this  layer  forms  the  cutis  of  the  body  wall. 

Whilst  these  processes  have  been  going  on  (i.e.  while  the  enterocoel  vesicles  are 
increasing  in  size  till  they  surround  the  intestine,  the  parietal  wall  becoming  applied 
to  the  body  wall,  and  the  visceral  wall  to  the  intestinal  wall  of  the  young  animal, 
and  the  two  vesicles  opening  into  one  another  ventrally),  the  narrow  slit-like  cavities 
of  the  watch -glass -shaped  enteroca-l  vesicles  in  the  larva  have  become  the  large 
spacious  body  cavity. 

The  visceral  wall  of  the  enteroecel  yields  the  intestinal  musculature  and  the 
endothelial  covering  of  the  intestine  ;  the  parietal  wall  of  the  enterocoel  gives  rise 
to  the  longitudinal  and  circular  musculature  of  the  body  wall  and  its  endothelial 
covering.  Since  the  musculature  of  all  parts  of  the  water  vascular  system  is 
formed  from  the  epithelial  wall  of  that  system,  it  follows  that  the  whole  of  the 
musculature  of  the  Holothurian  body  is  of  epithelial  origin. 

The  blood  lacunar  system  arises  in  the  form  of  cavities  in  the  connective  tissue 
(mesenchymatous)  layer  of  the  different  organs  (integument,  intestine). 

The  rudiments  of  the  genital  organs,  of  the  respiratory  trees,  and  of  the  Cuvierian 
organs  are  unknown. 

Xot  all  the  Holothurids  pass  through  a  distinctly  marked  Auricularia  stage. 
The  gastrula  larva  of  Cucurnaria  Planci,  for  example,  passes  direct  into  the  stage  of 
the  barrel-shaped  larva.  Nevertheless,  "  when  the  oral  depression  of  the  Cucumaria 
begins  to  form,  it  is  beset  along  its  edge  with  wreath-like  ectodermal  protuberances 
(ciliated  protuberances)  which,  taken  together,  may  be  compared  with  the  ciliated 
ring  of  the  Av.ricularifi  larva."  One  trace  of  the  Auricularia  stage  seems  thus  to 
be  retained. 

The  preoral  region  in  the  Cucumaria  larva  rises  up  sharply  as  the  frontal 
prominence,  and  in  its  gelatinous  nucleus  an  excessive  growth  and  multiplication  of 
the  mesenchyme  cells  early  takes  place.  This  is  resorbed  at  a  later  stage  as 
nourishment  for  the  formation  or  further  development  of  the  different  organs. 

The  plane  of  symmetry  of  the  young  Holothurid  does  not  coincide  with  that  of 
the  larva,  but  deviates  from  the  latter,  anteriorly  to  the  left,  and  posteriorly  to  the 
right.  The  longitudinal  axis  of  the  young  Holothurian  deviates  anteriorly  ventrally, 
and  posteriorly  dorsally,  from  that  of  the  larva. 


C.  Ontogeny  of  the  Echinoidea. 

The  following  description  is  an  epitome  of  the  results  gained  by  various  investi- 
gators in  the  examination  of  different  Echinoids. 

Segmentation  is  total,  and,  in  a  peculiar  manner,  unequal.  The  inequality 
between  the  blastomeres,  however,  soon  almost  entirely  disappears.  A  spherical, 
egg-shaped,  or  (Ecliinocyamns  pusillus]  long  elliptical  coeloblastula  arises,  with  a 
imilaminar  wall,  which  becomes  covered  with  long  flagellate  hairs  (one  on  each  cell). 

The  wall  of  the  blastula  becomes  thickened  at  the  vegetative  pole.  At  this 
thickened  part,  the  blastoderm  cells  divide  actively,  so  that  the  wall  becomes  bi-  or 
trilaminar.  The  deeper  cells  pass  in  succession  into  the  segmentation  cavity, 
become  auueboid,  and  are  the  first  mesenchyme  cells.  At  this  same  point  the 
blastula  wall  sinks  in  to  form  the  archenteron.  The  blastula  becomes  a  gastrula. 
During  this  process  of  invagination,  the  wandering  of  mesenchyme  cells  out  of  the 
wall  of  the  archenteron  into  the  blastoccel  continues. 

In  Echinocyamus  (and  other  Echinoids?)  the  cells  of  the  blastula  wall  at  the 
animal  pole  (the  point  opposite  the  later  blastopore)  are  longer  than  the  rest  even  at 


520 


COMPARATIVE  ANATOMY 


CHAP. 


the  blastula  stage,  and  their  flagella  are   less  movable.     This  differentiated  part 
(larval   sensory   organ  ?    neural  plate  ?)  can  also  be  recognised  in  the  subsequent 


First  Pluteus  stage. — The  gastrula  becomes  concave  on  the  ventral  side  ;  on  the 
opposite  (dorsal)  side  it  becomes  convex.  The  larva  is  now  bilaterally  symmetrical. 
The  blastopore  at  first  indicates  the  posterior  end  ;  then  it  shifts  somewhat  on  to 
the  ventral  side  on  to  a  mound-like  bulging  of  the  body  (the  anal  area),  which  lies 
posteriorly  to  the  ventral  depression.  The  anterior  edge  of  this  anal  area  becomes 
drawn  out  into  two  anteriorly  diverging  processes,  the  two  posterior  ventral  arms 


a.  nt 


FIG.  416.— Echinocyamus  pusillus,  gas- 
trula, forty  hours  after  fertilisation  (after 
Thdel).  1,  BlastoccBl ;  2,  frontal  thickening 
of  the  ectoderm  ;  3,  mesenchyme  cells ; 
4,  formation  of  these  wandering  cells  at  the 
base  of  the  archenteron ;  5,  the  first  two  cal- 
careous spicules ;  6,  archenteron ;  7,  primitive 
mouth,  blastopore. 


<le:r 


FIG.  417.— Echinocyamus  pusillus,  young 
Pluteus,  about  forty -eight  hours  after  fertil- 
isation (after  Thdel),  from  the  ventral  side. 
1,  Rudiment  of  the  larval  mouth  ;  2,  the  first 
arms ;  3,  rudiment  of  the  hydro-enterocu-1 
at  the  base  of  the  archenteron  ;  4,  larval 
skeleton  ;  4i,  dorsal  branches  of  the  same : 
5,  archenteron  ;  6,  primitive  mouth,  blasto- 
pore. 


(Fig.  417).  The  circumoral  ciliated  ring,  which  is  continued  on  to  the  arms, 
becomes  raised  above  the  general  ciliated  covering  of  the  body. 

During  the  first  larval  stage  the  following  important  internal  changes  take  place. 
The  two  first  lateral  calcareous  spicules  develop  in  the  niesenchyme  and  send  sup- 
porting rods  into  the  only  two  arms  present  at  this  stage,  the  posterior  ventral  arms. 
(The  first  rudiments  of  these  two  spicules  can  be  made  out  even  in  the  gastrula 
larva,  Fig.  416.) 

Formation  of  the  hydro-enteroccel. — The  anterior  blind  end  of  the  archenteron 
has,  on  each  side,  a  small  outgrowth,  which  lengthens  posteriorly.  The  archenteron 
becomes  constricted  immediately  behind  these  outgrowths,  which  finally  become 
separated  from  it  in  the  form  of  a  horse-shoe-shaped  vesicle,  with  two  limbs  directed 
posteriorly  and  applied  to  the  archenteron.  This  hydro-enteroccel  vesicle  at  once 
divides  into  its  two  lateral  limbs,  forming  two  lateral  hydro-enteroca'l  vesicles.  In 


VIII 


]•:<_  'HINODERMA  TA— ONTOGENY 


521 


ant 


certain  species  this  division  seems  to  take  place  as  early  as  the  constriction  of  the 
hydro-en  terocu-1  from  the  archenteron. 

From  the  very  commencement  of  the  formation  of  the  hydro-enteroccel,  the  three 
sections  of  the  intestinal  canal,  the  hind-gut,  the  mid-gut,  and  the  oesophagus,  begin 
to  be  marked  off  by  constrictions,  which  become  more  distinct.  All  three,  especially 
the  middle  section  (the  stomach  intestine),  begin  to  widen  out  like  vesicles.  The 
blastopore  having  shifted,  as  larval  anus,  far  from  the  vegetative  pole  ventrally  to 
the  centre  of  the  anal  area,  the  hind-gut  is  drawn  up  and  forms  an  angle  with  the 
mid-gut  (Fig.  418).  After  the  hydro-enteroccel  has  been  constricted  off,  the  new 
blind  end  of  the  intestine  bends  veutrally 
and  soon  meets  a  small  imagination  of  the 
ectoderm  of  the  depressed  ventral  surface. 
These  break  through  into  one  another,  and 
the  larval  mouth  thus  enters  into  open 
communication  with  the  lumen  of  the 
fore-gut. 

During  the  latter  part  of  the  first  larval 
stage,  contractile  fibres  appear  in  the  wall 
of  the  cesophagus,  and  enable  this  section 
of  the  intestine  to  contract  strongly. 

Of  the  two  hydroccel  vesicles  lying  at 
the  sides  of  the  posterior  portion  of  the 
fore -gut,  that  on  the  left  opens  outward 
by  a  water  pore  at  the  centre  of  the  dorsal 
surface. 

Second  larval  stage. — The  two  posterior 
dorsal  arms  grow  out.  They  are  supported 
by  the  rod-like  processes  of  two  new  cal- 
careous bodies,  which  have  appeared  in  the 
mesenchyme.  On  the  left  side,  in  the 


—      dors 


r 


post 


FIG.   418.  —  Echinocyamus  pusillus,  the 
same  larva  from  the  left  side  (after  Thdel). 


angle  between  the  posterior  dorsal  and  the    *>  The  ttrst  larval  arms;  2'  rudiraent  of  tLe 
,  ,  larval  mouth ;  3,  ectoderm ;  4,  hvdro-enterocoel 

posterior  ventral  arms,  an  ectodermal  in-    rudiment .    5>  ^enteron  ;    6,  blastopore  ; 
vagination    appears    (Fig.    419,    3)  ;     this    7,  larval  skeleton. 
sinks  into  the  blastoccel  in  the  shape  of  a 

flask.     This  invagination  plays  an  important  part  in  the  transformation  of  the  larva 
into  the  young  Echinoderm. 

Third  stage  (fully-grown  Pluteus  larva). — The  two  anterior  dorsal  and  the  two 
anterior  ventral  arms  continue  to  grow  (cf.  Figs.  400  and  401,  p.  509).  On  the  dorsal 
side,  a  fifth  unpaired  calcareous  spicule  arises,  and,  in  the  immediate  neighbourhood 
of  the  water  pore,  sends  off  processes,  two  of  which  enter  the  anterior  dorsal  arms 
and  support  them.  The  body  has  shortened,  and  its  posterior  region  has  become 
rounded  off. 

Further  differentiation  of  the  hydro-enteroccel.— We  resume  the  description  of 
this  organ  from  the  stage  in  which  it  consisted  of  two  lateral  vesicles  applied  to  the 
intestine.  Each  vesicle  now  becomes  divided  by  a  constriction  into  an  anterior 
and  a  posterior  vesicle.  The  two  anterior  vesicles  lie  at  the  sides  of  the  posterior 
portion  of  the  cesophagus,  the  two  posterior  at  the  sides  of  the  stomach  intestine. 
The  left  anterior  vesicle  opens  outward  through  a  water  pore  ;  the  other  three  are  in 
no  way  connected  with  the  future  water  vascular  system  ;  they  are  distinguished  as 
the  right  anterior,  right  posterior,  and  left  posterior  enteroccel  vesicles.  Some- 
what later,  three  vesicles  are  seen  on  the  left  side  (Fig.  419,  2,  4,  5).  The  anterior 
and  middle  vesicles  are  in  communication,  whereas  the  posterior  is  distinct,  and 
becomes  applied  to  the  middle  vesicle,  assuming  a  crescent  shape.  The  left 


522 


COMPARATIVE  ANATOMY 


CHAP. 


anterior  (enteroccel)  vesicle  is  the  one  which  opens  outward  through  the  water 
pore  ;  it,  however,  does  not  become  the  hydrocoel,  but  the  middle  vesicle  (which  is 
in  communication  with  it)  now  represents  the  rudiment  of  the  hydrocoel.  It  is 
probable  that  this  is  produced  by  constriction  from  the  left  anterior  hydro-enterocu-1 
vesicle.  . 

On  the  left  side,  the  development  of  the  hydro-enterocoel  is  now  as  follows. 
The  water  pore  leads  into  the  posterior  end  of  a  left  anterior  enteroccel  vesicle, 
which,  again,  communicates,  by  means  of  a  constricted  portion,  with  the  hydroccel 


ler 


post 

FIG.  419.— Dorsal  aspect  of  an  EcMnoid 
Pluteus,  to  illustrate  the  relations  of  the  hydro- 
enterocoel  (after  Bury),  ant,  Anterior ;  post,  pos- 
terior ;  sin,  left ;  dex,  right ;  1,  larval  oesophagus  ; 
2,  left  anterior  enterocoel ;  3,  ectodermal  in- 
vagination  ;  4,  rudiment  of  the  hydrocoel ;  5,  left 
posterior  enterocoel  vesicle  ;  6,  stomach  intes- 
tine ;  7,  right  posterior  enterocoel ;  8,  hydro- 
pore  ;  9,  unpaired  dorsal  skeletal  piece ;  10,  right 
anterior  enteroccel.  The  arms  are  not  fully 
represented. 


FIG.  420.— Rudiment  of  the  Echinoid 
in  the  Pluteus  larva  of  Echinocyamus 
pusillus  (after  Theel).  The  Pluteus  is  seen 
from  the  dorsal  side,  and  only  the  left  side 
is  completely  drawn.  1,  Arms  of  the 
Pluteus  ;  2,  aperture  of  imagination  of  the 
sac  (3),  whose  floor  will  form  the  oral  body 
wall  of  the  Echinoid  ;  4,  outgrowths  of  the 
hydrocoel,  which  push  this  wall  before  them 
and  form  the  first  ambulacral  tentacles; 
skeletal  rods  of  the  Pluteus  ;  6,  hydrocoel ; 
7,  hydropore  ;  9,  portion  of  a  skeletal  piece 
lying  in  the  neighbourhood  of  the  hydro- 
pore,  which  will  probably  become  the 
madreporite  ;  8,  stomach-intestine. 


vesicle.  This  latter  is  embraced  posteriorly  by  the  horse-shoe-shaped  left  posterior 
enteroca-1.  The  stone  canal  does  not  arise  out  of  the  water  pore,  but  out  of  the 
connecting  piece  between  the  left  anterior  enterocoel  vesicle  and  the  hydroccel 
vesicle,  which  becomes  drawn  out  into  a  canal.  The  left  anterior  enteroccel 
seems  to  become  the  madreporitic  ampulla. 

(The  above  description  of  the  differentiation  of  the  hydro-enteroccel  must  not  be 
considered  as  fully  established.  The  observations  are  not  quite  continuous,  and 
do  r  >t  all  agree.) 


VIII 


ECHINODERMA  TA— ONTOGENY 


Transformation  of  the  Pluteus  larva  into  the  young  Echinoid. — This  meta- 
morphosis is  far  from  having  been  satisfactorily  described,  its  investigation  being 
exceedingly  difficult. 

An  important  part  in  the  shaping  of  the  Echinoid  body  is  played  by  the  above- 
mentioned  flask-like  invagination  of  the  ectoderm  on  the  left  side.  The  thicken- 
ing floor  of  this  invagination  grows  towards  the  hydroccel,  and  becomes  externally 
applied  to  this  latter  as  the  "Echinoid  disc"  (Fig.  420).  The  thin  lateral  walls  of 
the  capacious  flask,  which  is  still  connected  by  its  neck  with  the  larval  ectoderm, 
are  known  as  the  amnion. 

The  hydroccel  vesicle  assumes  the  horse-shoe-shape,  and  at  the  same  time  puts 
forth  five  outgrowths  which  push  before  them  the  Echinoid  disc,  i.e.  the  floor  of 


dea 


FIG.  421.— Dorsal  aspect  of  a  larva  of  Echi- 
nocyamus pusillus,  about  forty-five  days  old 
(after  Theel).  1,  The  larval  arms,  with  their 
calcareous  rods  ;  2,  unpaired  calcareous  rod, 
taking  part  in  surrounding  the  dorsal  pore  (3)  ; 
4,  spines  ;  5  and  6,  primary  tentacles  of  the 
young  Echinoid  ;  ant,  anterior ;  post,  posterior  ; 
si/i,  left ;  dex,  right. 


FIG.  422.— Lateral  view  of  a  very 
young  Echinoid  (Echinocyamus 
pusillus\  forty  -  five  days  old  (after 
Theel).  The  first  tube-feet  and  spines 
of  the  Echinoid  are  seen,  and,  attached 
to  its  back,  the  remains  of  the  calcar- 
eous rod  of  a  larval  arm. 


the  flask-like  invagination.  Five  hollow  tubes  thus  now  project  into  the  cavity  of 
the  flask,  which  continually  becomes  more  and  more  spacious  ;  these  are  the  five 
primary  tentacles,  which  receive  their  covering  from  the  spreading  Echinoid  disc. 
This  Echinoid  disc  forms  the  oral  wall  (that  is,  no  doubt,  only  the  epithelium  and 
the  nerves  ?)  of  the  young  Echinoid,  while  the  apical  wall  is  formed  direct  from  the 
larval  dorsal  ectoderm  of  the  Pluteus. 

The  fate  of  the  amnion  is  differently  described  for  different  forms.  Sometimes 
it  is  said  to  pass  over  into  the  young  Echiuoid,  the  amnion  sac  opening  and  spread- 
ing out,  and  yielding  the  circular  integumental  region  between  the  apical  and  oral 
surfaces  of  the  body.  At  other  times,  again,  the  amnion  sac  is  said  to  remain  closed 


524 


COMPARATIVE  ANATOMY 


CHAP. 


and  the  anmion,  together  with  part  of  the  larval  integument,  are  lost  when  the 
larva  changes  into  the  young  Echinoid. 

The  larval  arms  disappear,  and  their  spicules  are  for  the  most  part  absorbed. 
As  a  rule,  one  or  other  of  the  arms  of  the  Pluteus  still  adheres  to  the  quite  young 
Echinoid  (Fig.  422). 

The  intestine,  at  least  the  whole  stomach,  the  spreading  enterocoel,  and  the 
growing  hydrocoel  are  taken  over  into  the  young  Echinoid  ;  the  latter,  however, 

has,  at  first,   neither  mouth  nor 

<vtvl  anus.      In   Echinoids,    therefore, 

the  larval  mouth  and  anus  do  not 
pass  direct  into  the  corresponding 
organs  of  the  adult. 

Formation  of  the  mouth  and 
the  definitive  resophagus. — Ac- 
cording to  one  account,  the  oeso- 
phagus  only  grows  out  from  the 
intestine  after  the  horse  -  shoe- 
shaped  hydroccel  has  closed  ;  it 
then  passes  through  the  water 
vascular  ring  and  opens  outward 
at  the  centre  of  the  Echinoid  disc 
through  the  definitive  mouth. 

The  pedicellarise  arise  very 
early.  They  are  even  occasionally 
seen  on  the  dorsal  side  of  an  older 
Pluteus  larva. 

The  water  pore  becomes  the 

FIG.  423.-Echinocyamus  pusillus,  young  Echinoid,  madreporite,  and  the  unpaired 
about  forty-five  days  old,  from  the  oral  side  (after  Thdel).  spicule  which,  in  the  older  Pluleus, 
ant,  Anterior  unpaired  ambulacrum ;  post,  posterior  un-  arose  in  its  immediate  neighbour- 
paired  interambulacrum  ;  1,  tentacle  ;  2,  spines  ;  3,  sphser-  hood  chanHng  into  a  lattice-like 
idiain  their  niches;  4,  pieces  of  the  masticatory  apparatus :  ,  ,  ,  .,- 

5,  teeth  ;  6,  oral  integument,  the  mouth  is  not  yet  formed  ;  Plate>  Becomes  the  madrepontlC 
7,  radial  skeletal  plates ;  8,  interradial  skeletal  plates.  basal.  Four  other  plates,  which 

arise   over  the  right  enterocoel 

of  the  larva,  become  the  other  basals.  In  their  centre  the  dorso-central  is  soon 
distinguishable.  On  the  oral  side,  in  the  peripheral  part  of  the  original  Echinoid 
disc,  where  the  primary  tube-feet  developed,  the  first  ambulacral  and  interambu- 
lacral  plates  appear,  with  the  rudiments  of  the  spines  and  the  sphseridia,  both 
of  which  form  independently  over  the  plates  (Fig.  423).  In  the  future  oral  area, 
which  is  surrounded  by  a  circle  of  ambulacral  and  interambulacral  plates,  thirty 
small  calcareous  centres  form,  three  in  each  radius  and  three  in  each  interradius  ; 
these  are  the  rudiments  of  the  plates  of  the  masticatory  apparatus.  The  middle 
calcareous  plates  of  the  interradii  become  the  teeth. 

Little  or  nothing  is  known  of  the  ultimate  fates  of  the  other  twenty-five  pieces,  or 
of  the  enterocoel,  of  the  hydroccel  (e.g.  the  order  of  appearance  of  the  tube-feet),  or  as 
to  the  appearance  of  the  nervous  system,  the  origin  of  the  radial  plates,  etc. 


D.  Ontogeny  of  the  Asteroidea. 

Segmentation  is  total,  and  leads  to  the  formation  of  a  coeloblastula,  through  the 
iir.  igination  of  which  a  ccelogastrula  arises.  The  formation  of  the  mesenchyme 
takes  place  in  the  manner  already  described  for  the  Holothurioidea  and  the  Echin- 


VIII 


ECHINODERMATA— ONTOGENY 


525 


oidca,  and  commences  either  in  the  blastula  stage  or  not  until  the  gastrula  stage. 
In  the  former  case,  that  part  of  the  blastoderm  which  becomes  iuvaginated  to  pro- 
duce the  archenteron  yields  the  mesenchyme,  and,  after  invagination  has  taken 
place,  continues  to  produce  it.  In  the  second  case,  also,  the  endoderm  is  the  place 
of  formation  of  the  mesenchyme  cells,  which  wander  into  the  blastoccel.  Such 
observations,  however,  seem  to  point  to  the  fact  that,  although  most  mesenchyme 
cells  arise  from  the  endoderm,  the  ectoderm  also  takes  part  in  their  formation. 

In  the  older  gastrula  stage  of  Asterias  vulgaris,  the  ectoderm  seems  to  be 
thickened  at  the  (aboral)  pole  opposite  to  the  blastopore.  This  may  be  the  rudi- 
ment of  the  neural  plate. 

As  a  further  illustration  of  the  development  of  the  Asteroidea,  we  shall  utilise 
the  observations  made  on  Asterina  gibbosa,  in  which  form,  however,  a  typical 


FIG.  424.— Asterina  gibbosa, 
gastrula  four  days  old ;  ap- 
proximately horizontal  longi- 
tudinal section,  from  the  ventral 
side  (after  Ludwig).  ant,  An- 
terior ;  post,  posterior ;  dex,  right ; 
sin,  left ;  1,  segmentation  cavity ; 
•2  and  3,  right  and  left  coelomic 
outgrowths  of  the  archenteron  ; 
4,  blastopore. 


FIG.  425.— Asterina  gib- 
bosa, larva  at  the  end  of  the 
fourth  day,  horizontal  longi- 
tudinal section  seen  from  the 
ventral  side  (after  Ludwig). 
The  enterocoel  outgrowths 
have  grown  in  length.  2,  right 
enterocrel  outgrowth  ;  3,  left 
or  hydro  -  enterocoel  out- 
growth ;  5,  intestine ;  6,  an- 
terior unpaired  coelom.  The 
coelom  is  still  in  open  com- 
munication with  the  intestine. 


FIG.  420.—  Asterina  gib- 
bosa, larva  at  the  com- 
mencement of  the  fifth 
day,  horizontal  longitudi- 
nal section  (after  Ludwig). 
The  enteroccel  has  become 
constricted  off  from  the 
intestine.  Lettering  as 
before. 


Bipinnaria  larva  does  not  attain  development.  In  the  course  of  the  description 
observations  made  on  other  Asteroids  will  be  referred  to. 

In  the  ovoid  gastrula  of  Asterina,  the  blastopore  does  not  lie  altogether  at  the 
posterior  pole,  but  is  shifted  somewhat  on  one  side,  which  in  the  further  develop- 
ment of  the  animal  becomes  distinguishable  as  the  ventral  side.  Two  sections  can  be 
made  out  in  the  archenteron,  a  short  cylindrical  commencement  (posterior  section), 
and  a  vesicular  blind  terminal  part  (anterior  section).  This  description  applies  to 
the  gastrula  on  the  second  day. 

Third  day. — The  rudiment  of  the  hydro- enterocoel  vesicle.— The  anterior 
vesicular  section  of  the  archenteron,  which  represents  the  rudiment  of  the  hydro- 
enterocoel  vesicle,  bulges  out  posteriorly  on  each  side,  while  its  wall  becomes 
thinner  (Fig.  424).  The  two  bulgings  grow  out  longitudinally  backwards,  at  the 
sides  of  the  posterior  part  of  the  archenteron,  and  become  the  two  hydro-enteroccel 


526 


COMPARATIVE  ANATOMY 


CHAP. 


vesicles,   which  continue  to  grow  backward  in  proportion  as  the  posterior  part  of  the 
archenteron,  the  larval  intestine,  grows  anteriorly  (Fig.  425). 

Fourth  day.— The  whole  hydro -enteroccel  becomes  constricted  off  from  the 
larval  intestine,  and  is  now  found  as  a  large  vesicle  occupying  the  anterior  part  of 
the  larval  body,  and  continued  posteriorly  in  the  two  long  hydro-enteroccel  vesicles, 
the  one  on  the  left  being  longer  than  the  one  on  the  right  (Fig.  426). 

An  imagination  of  the  ectoderm,  somewhat  anteriorly  to  the  middle  of  the 
ventral  side,  represents  the  rudiment  of  the  larval  mouth  and  oesophagus,  and. 
towards  the  end  of  the  fourth  day,  breaks  through  into  the  larval  intestine. 

Anteriorly  a  cushion-like  thickening  of  the  body  appears  encircling  a  depression. 


post 


FIG.  428.— Larva  of  As- 
terina  gibbosa  four  days 
Old,  just  hatched,  from 
the  ventral  side  (after 
Ludwig).  1,  Larval  organ  ; 
2,  blastopore.  Here,  and 
in  the  following  figures, 
ant  =  anterior ;  post  =  pos- 
terior. 


FIG.  429.  —  Asterina 
gibbosa,  larva  six  days 
Old,  from  the  left  side 
(after  Ludwig).  v,  Ven- 
tral side  ;  d,  dorsal 
side  ;  1,  larval  organ. 


FIG.  427.  —  Asterina  gibbosa, 
larva  five  days  old,  horizontal 
longitudinal  section  seen  from  the 
ventral  side.  First  rudiment  of  the 
hydroccel  outgrowth  (7)  on  the  left 
of  the  hydro-enterocoel  vesicle  (3). 
The  two  enterocoel  vesicles  have 
opened  into  one  another  posteriorly 
at  8. 

This  circular  cushion,  the  rudiment  of  the  larval  organ,  slants  from  above  anteriorly 
to  below  posteriorly  (Figs.  428-430). 

At  the  end  of  the  fourth  day  the  embryo  leaves  the  egg-envelope,  and  swims 
about  freely  by  means  of  the  cilia  covering  its  entire  surface. 

Fifth  day.—  The  two  hydro-enterocoel  vesicles  grow  round  the  larval  intestine 
above  and  below.  —  Where  they  meet  below,  somewhat  to  the  left  of  the  median  line, 
they  form  a  ventral  mesentery,  which,  however,  rapidly  disappears,  the  two  vesicles 
opening  into  one  another  at  this  point.  Above  the  intestine  a  dorsal  mesentery, 
lying  to  tlu  right  of  the  median  line,  arises  in  a  similar  manner,  and  persists. 

The  left  hydro-enterocoel  vesicle  bulges  out  laterally  somewhat  behind  its  middle 
point.  This  bulging  is  the  rudiment  of  the  hydroccel  (Fig.  427). 

At  this  stage,  therefore,  the  hydro-enterocoel  consists  of  the  following  sections  in 
widely  open  communication  with  one  another. 

1.  Anterior  unpaired  enterocoel  (6  in  Figs.  426,  427),  lying  in  the  larval  organ. 

2.  Right  enterocoel  vesicle  (2  in  the  figs.),  anteriorly  in  wide  open  communica- 
tion with  No.  1,  and  ventrally  in  open  communication  with  — 


ECHINODERMA  TA—ONTOGEN  Y 


527 


3.  The  left  enterocoel  vesicle  (3  in  the  figs.).     This  vesicle,  again,  has  an  out- 
growth (7)  on  the  left,  which  is — 

4.  The  hydroccel  vesicle. 

Simultaneously  with  the  formation  of  the  hydroecel  rudiment,  that  of  the  water 
pore  appears  dorsally,  somewhat  to  the  left  of  the  median  line,  as  an  invagination 
of  the  ectoderm,  which  grows  towards  the  left  enterocoel,  and  breaks  through 
into  it. 

Sixth  and  seventh  day. — The  outer  form  of  the  larva  has  been  considerably 
modified  on  the  fifth  day.  The  larval  organ  has  increased  in  size,  and  its  sloping 
circular  ridge  projects  considerably  beyond  the  surface  of  the  larval  body. 

The  rudiment  of  the  hydrocoel  has  grown  out  further  backward,  but  is  still 
anteriorly  in  open  communication  with  the  left  enteroccel.  Five  outgrowths  (Nos. 
I-V  in  Fig.  435)  now  appear  at  its  posterior  edge  ;  these  are  the  rudiments  of  the 
five  radial  vessels.  The  water  pore  (dorsal  pore,  madreporite)  still  leads  into  the 


post 

FIG.  430.  —  The  same 
specimen  of  Asterina  gib- 
bosa  viewed  from  the 
left  and  from  the  ventral 
Side.  1,  Larval  organ  with 
ir>  dorsal  and  ventral 
lobes  ;  2,  larval  mouth. 


FIG.  431.— Asterina  gibbosa,  larva  at  the 
beginning  of  the  eighth  day,  from  the  left 
side;  the  larval  organ  is  very  largely  de- 
veloped (after  Ludwig). 


left  enteroccel.  "A  channel  develops  on  that  wall  of  the  hydrocoel  which  faces  the 
interior  of  the  body,  which  soon  closes  to  form  a  canal. "  This  canal  runs  towards 
the  point  where  the  dorsal  pore  opens  into  the  left  enteroccel.  One  end  of  this 
canal  remains  in  open  communication  with  the  hydroccel,  while  the  other  enters 
the  enterocoel  quite  near  the  aperture  of  the  dorsal  pore.  This  canal  is  the  stone 
canal  of  the  future  Asteroid.  The  dorsal  pore  of  the  larva  does  not  thus  lead 
direct  into  the  stone  canal,  but  enters  it  through  the  left  enteroccel  (Fig.  436). 
Only  at  a  later  stage  does  the  dorsal  pore  come  into  direct  connection  with  the  stone 
canal. 

Formation  of  the  hydro-enterocoel  in  other  Asteroids. — In  the  larva  of  Asterias 
fulf/aris  also,  the  entero-hydroccel  arises  in  the  form  of  two  lateral  diverticula  of 
the  blind  and  somewhat  swollen  archenteron,  whose  wall  has  become  thinner.  The 
two  diverticula  soon  become  constricted  from  the  archenteron,  and  become  distinct 
vesicles.  Each  sends  off  an  outgrowth  towards  the  dorsal  surface,  a  growth  of  the 
ectoderm  running  in  towards  it.  The  two  meet  and  fuse,  become  hollow,  and  form 
the  stone  canal  with  the  water  poro.  Thus  in  the  young  Bipinnaria  larva  of 
Asterias  vulgaris,  the  bilateral  symmetry  is  so  marked  that  a  right  as  well  as  a 
left  stone  canal  attains  development  (Fig.  432).  The  right  pore,  however,  soon 
disappears,  and  the  right  canal  somewhat  later. 


528 


COMPARATIVE  ANATOMY 


CHAP. 


The  two  lateral  mesoderm  vesicles  lengthen  and  fuse  in  front  of  and  above  the 
mouth,  and,  further,  surround  the  intestine.  On  trie  left  vesicle  (hydro-enteroccel 
vesicle)  a  transverse  constriction  appears,  which  finally  divides  it  into  two  vesicles, 
an  anterior,  which  at  its  posterior  end  opens  outward  through  the  stone  canal  and 
water  pore,  and  a  posterior  (Fig.  433). 

Further  development  of  the  hydroccel  in  Asterina  gibbosa. — After  the  seventh 

day  the   five  outgrowths  of  the  hydrocrel  (Figs.   437-440)  become  trilobate,  and 

later  have  five  lobes.    The  unpaired  terminal  lobe  of  each  outgrowth  is  the  rudiment 

of  the  terminal  tentacle,  the  paired  lobes  are  the  rudiments  of  the  first  two  pairs 

ant 


2 1- 


1- — 


pott 

FIG.  432. —Larva  01 
Asterias  vulgaris,  about 
four  days  old,  from  the 
dorsal  side  (after  Field). 
1,  Circumoral  ciliated 
band  ;  2,  n.outh  ;  3,  right 
and  left  hydro-enterocoel 
vesicles,  with  their  hydro- 
pores  (4)  ;  5,  oesophagus;  6, 
mesenchymatous  muscle 
fibres;  7,  stomach  intes- 
tine ;  8,  anus.  The  mouth 
and  the  anus  lie  on  the 
side  turned  away  from 
the  reader. 


FIG.  433.— Dorsal  aspect  of  a 
Bipinnaria  larva  to  illustrate  the 
developmentoftheliydro-enteroccel 
(after  Bury).  1,  Larval  oesophagus  ; 
2,  left  anterior  enterocoel ;  3,  hydro- 
pore  ;  4,  rudiment  of  the  hydrocoel ; 
5,  stomach  intestine  ;  6,  terminals  ; 

7,  left  posterior  enterocoel  vesicle ; 

8,  dorsal  mesentery ;  9,  right  posterior 
enterocoel  vesicle  ;  10,  madreporite  ; 

11,  blood  vesicle,  pulsating  vesicle ; 

12,  right  anterior  enterocoel. 


FIG.  434.— Asterina  gib- 
bosa, larva  six  days  old, 
horizontal  longitudinal  sec- 
tion from  the  ventral  side 
(after  Ludwig).  The  hydro- 
coel  (7)  has  become  con- 
stricted posteriorly  from  the 
left  enterocoel.  An  out- 
growth of  the  intestine  (8)  is 
the  first  indication  of  the 
future  oesophagus  of  the 
Asteroid. 


of  tube-feet.  Each  new  pair  of  feet  arises  between  the  terminal  tentacle  and  the 
foot  last  formed. 

The  five  outgrowths  of  the  hydroccel  become  outwardly  visible,  bulging  out  the 
body.  On  the  left  side  of  the  seven-days-old  larva  there  are  thus  visible  five  flat 
protuberances  arranged  in  a  convex  arch  directed  upward  and  backward  ;  these 
protuberances  become  more  marked  on  the  eighth  day,  and  are  then  divided  either 
into  three  or  five  lobes  each  (Figs.  438-440).  These  are  the  first  indications  of  the 
young  Asteroid,  the  rudiments  of  its  ambulacral  arms. 

The  rudiment  of  the  definitive  oesophagus  appears  in  the  form  of  a  bulging  of 
the  left  side  of  the  archenteron,  that  facing  the  hydroccel.  This  arises  in  the  region 
which  corresponds  with  the  anterior  part  of  the  gastrula  intestine  (Fig.  434,  8),  and 
has  nothing  to  do  with  the  larval  oesophagus.  This  latter  degenerates  on  the 
ei&i  th  or  ninth  day,  and  the  larval  anus  also  disappears. 


VIII 


ECHINODERMATA— ONTOGENY 


529 


The  larval  organ  attains  its  greatest  development  on  the  eighth  or  ninth  day,  it 
diminishes  in  size  later,  and  finally  is  altogether  resorbed,  without  giving  origin  to 
any  organ  of  the  young  Asteroid.  Its  wall  consists  of  three  layers :  (1)  an  outer 
ciliated  larval  epithelium,  (2)  the  inner  epithelium  of  the  unpaired  section  of  the 
enteroccel  which  fills  up  the  whole  cavity  of  the  larval  organ,  and  (3)  between  these 
two  layers,  one  of  mesenchyme  cells  differentiated  into  muscle  fibres.  The  larva  uses 
the  organ  for  locomotion  and  for  temporary  attachment. 

Soon  after  the  ambulacral  (oral)  rudiments  of  the  arms  have  appeared  on  the  left 
side  in  the  form  of  the  five  protuberances  (1-5,  Fig.  437)  mentioned  above,  five 
mesenchyme  thickenings  form,  which  also  bulge  out  the  ectoderm,  and  represent 
the  antiambulacral  (apical,  dorsal)  arm  rudiments  (I-V).  Three  of  these  are 


22 


FIG.  435.— Asterina  gibbosa.  larva  six 
days  old,  seen  from  the  left  (after  Ludwig). 
I-V,  The  five  primary  bulgings  of  the 
hydroccel  (7) ;  3,  the  left  enterocoel,  opening 
outward  through  the  hydropore  or  dorsal 
pore  (11)  on  the  dorsal  side  ;  5,  intestine; 
6,  anterior  enterocoel,  enterocoel  of  the 
larval  organ ;  9,  larval  mouth ;  10,  mesen- 
tery;  11,  dorsal  pore  ;  12,  ectoderm  of  the 
larval  organ. 


FIG.  436.— Asterina  gibbosa,  larva  eight 
days  old,  seen  from  the  dorsal  and  somewhat 
from  the  left  side,  optical  longitudinal  section 
(after  Ludwig).  II,  V,  second  and  fifth  primary 
bulgings  of  the  hydrocoel ;  3,  left  enterocoel ; 
5,  intestine ;  7,  hydroccel ;  11,  dorsal  pore ; 
13,  oesophagus  of  the  Asteroid ;  14,  rudiment  of 
the  stone  canal. 


found  on  the  left  ventral  side,  and  two  somewhat  to  the  left  of  the  median  line,  on 
the  dorsal  side  of  the  larva.  The  five  stand  in  a  curved  row,  the  curve  opening 
anteriorly,  the  plane  in  which  they  lie  making  an  angle  with  that  of  the  ambulacral 
arm  rudiments. 

These  two  sets  of  arm  rudiments  then  shift  towards  one  another,  until  their 
planes  are  nearly  parallel. 

Appearance  of  the  skeletal  plates. — As  early  as  the  time  when  the  hydrocoel 
bulgings  begin  to  become  trilobate,  a  small  calcareous  body  appears  in  the  mesen- 
chyme on  each  side  of  each  primary  bulging,  on  the  proximal  side  of  the  lateral 
secondary  bulgings  (i.e.  of  the  rudiments  of  the  first  tube-feet).  These  are  the 
rudiments  of  the  first  five  pairs  of  ambulacral  plates.  "When  a  second  pair  of  lateral 
lobes  appear  distally  to  the  first  pair  on  each  hydroccel  bulging,  a  second  pair  of 
VOL.  II  2  M 


530 


COMPARATIVE  ANATOMY 


CHAP. 


calcareous  bodies  form  between  this  and  the  first  pair ;  these  are  the  rudiments  of  a 
second  pair  of  ambulacral  plates,  and  so  on. 


FIG.  437.— Asterina  gibbosa,  larva  at  the  end 
of  the  eighth  day,  from  the  left  side  (after 
Ludwig).  1-5,  The  ambulacral  rudiments  of  the 
arms  over  the  primary  hydroccel  bulgings ;  I-V,  the 
antiambulacral  rudiments  of  the  arms. 


FIG.  438.— Asterina  gibbosa,  larva  ten 
days  old,  seen  from  the  left  and  some- 
what from  the  ventral  side  (after  Ludwig). 
The  ambulacral  arm  rudiments  (1-5)  now 
have  five  lobes,  ft,  Terminal  lobe,  terminal 
tentacle. 


As  early  as  the  seventh  day,  the  rudiments  of  the  apical  skeletal  plates,  eleven 
in  number,  appear.     These  eleven  rudiments  lie  superficially  below  the  ectoderm  of 


FIG.  439.— Asterina  gibbosa,  young 
Asteroid,  with  much  reduced  lateral 
organ  (fc,)  at  the  end  of  the  tenth  day, 
from  the  left  side  (after  Ludwig). 
The  first  rudiments  of  the  ambulacral 
skeleton  have  appeared  (five  pairs  of 
ambulacral  plates).  The  mouth  of 
the  Asteroid  has  not  yet  formed. 


FIG.  440.— Asterina  gibbosa,  young  Asteroid 
eleven  days  old,  horizontal  section  immediately 
below  the  oral  surface  (after  Ludwig).  1-5,  The  five 
five-lobed  outgrowths  of  the  hydroccel  ring  (aa)  which 
has  not  yet,  closed  ;  ax,  the  two  outgrowths  at  the  two 
ends  of  the  horse-shoe-shaped  hydroccel,  which  by 
growing  out  towards  one  another  and  opening  out 
into  one  another  close  the  hydroccel ;  lo,  interradius  of 
the  larval  organ  ;  m,  interradius  of  the  madreporite. 


the  apical  region.   Five  of  them  appear  in  the  mesenchyme  of  the  five  apical  brachial 
ru.liments,  and  become  the  terminals  of  the  Asteroid  arms,  always  remaining  at  the 


VIII 


ECHINODERMA  TA—OKTOGENY 


531 


tips  of  the  growing  arms  (Fig.  441,  ^  - 15).  Five  others  appear  within  the  anteriorly 
open  curve  made  by  the  five  terminals,  and  alternate  with  these  latter  ;  these  are 
the  primary  interradials  (basals)  of  the  dorsal  surface  of  the  Asteroid  disc  (b^  -  ba5). 
One  of  these  (ba5)  always  lies  on  the  right  near  the  dorsal  pore,  and,  growing  round 
this  pore  later,  becomes  the  madreporitic  plate.  The  eleventh  plate  lies  in  the 
centre  of  the  two  curves  just  mentioned,  and  is  the  rudiment  of  the  central  plate  (ce). 

The  basals  and  the  central  appear  on  the  right  side  of  the  larva  over  the  right 
enterocoel.  The  relation  of  the  terminals  to  the  enterocoel  has  not  yet  been 
certainly  ascertained.  It  has 
been  proved  that  in  the  Bipin- 
iin /'in  they  appear  even  before 
the  rudiment  of  the  five  hydro- 
coel outgrowths,  above  the  left 
enterocoel. 

Metamorphosis  of  the  larva 
into  the  young  Asteroid. — 
This  is  throughout  a  continuous 
process.  Only  two  parts  of  the 
larva  are  not  taken  over  by 
the  young  Asteroid,  viz.  the 
larval  organ  and  the  larval 
oesophagus,  which  are  gradually 
resorbed.  The  anus  of  the 
Asteroid  does  not  indeed  de- 


40, 


FIG.  441.— Asterina  gibbosa,  young  Asteroid  ten  days 
old,  dorsal  view  (after  Ludwig).  I-V,  The  antiainbulacral 
arm  rudiments  ;  I,  interradius  of  the  larval  organ  (lo) ;  m,  in- 
terradius of  the  madreporite  (mp) ;  bc^  -  ba5,  the  five  basals  ; 
h~^5,  the  five  terminals  ;  ce,  central. 


velop  out  of  that  of  the  larva, 
but  at  the  same  point. 

The  last  remains  of  the 
larval  organ  are  found  on  the 
ventral  side  of  the  young 
Asteroid  lying  excentrically 
in  that  interradius  in  which 
the  hydrocoel  closes  to  form 
the  water  vascular  ring  ;  view- 
ing the  body  apically  this  interradius  follows  the  madreporitic  interradius  on  the 
right  (cf.  Figs.  440,  441  ;  the  arrows  indicate  these  interradii). 

The  mouth  and  oesophagus  of  the  young  Asteroid  arise  by  the  outgrowth  from 
the  left  side  of  the  archenteron,  mentioned  above,  reaching  the  body  wall  and  finally 
breaking  outward  through  it  (thirteenth  or  fourteenth  day).  The  oesophagus  is  then 
grown  round  by  the  hydrocoel,  which  closes  to  form  the  water  vascular  ring.  Only 
shortly  before  this  takes  place  does  the  hydrocoel  become  entirely  constricted  off 
from  the  enterocoel,  and  the  dorsal  pore  comes  into  direct  connection  with  the 
stone  canal. 

The  intestine  widens  into  a  sac,  five  radially  placed  outgrowths  appearing  in  it, 
directed  towards  the  rudiments  of  the  arms.  At  the  point  where  the  larval  anus 
formerly  lay,  in  the  interradius  between  the  first  and  second  apical  brachial  rudi- 
ments, the  definitive  anus  breaks  through. 

The  two  curves  formed  by  the  five  apical  (antiambulacral)  and  the  five  oral 
(anibulacral)  arm  rudiments  approach  one  another  more  and  more,  the  zone  of  the 
body  wall  which  separates  them  (and  which  is,  with  regard  to  the  animal,  equatorial) 
becoming  continually  narrower.  Finally,  the  edges  of  the  apical  and  those  of  the 
oral  rudiments  touch  to  form  the  young  Asteroid.  During  this  process  the  arm 
rudiments  unite  in  the  following  peculiar  manner :  Number  1  unites  with  II. 
'2  with  III,  3  with  IV,  4  with  V,  and  5  with  I. 


532  COMPARATIVE  ANATOMY  CHAP. 

In  the  meantime  new  pairs  of  outwardly  projecting  lateral  lobes  (rudiments  of 
tube-feet)  have  appeared  on  the  hydroccel  bulgings,  which  have  developed  into  the 
radial  water  vascular  trunks  ;  these  new  growths  always  appear  distally  to  those 
already  formed  and  proximally  to  the  median  terminal  lobe  (terminal  tentacle). 

The  nervous  system  develops  as  an  epithelial  circular  cushion  in  the  oral  area, 
even  before  the  mouth  has  broken  through  its  centre. 

The  skeleton  receives  the  addition  of  fifteen  new  plates  on  the  apical  side  outside 
of  the  basals,  five  being  radial  and  five  pairs  interradial. 

In  each  interradius  orally  (on  the  thirteenth  day)  a  plate  forms  between  the 
separate  proximal  pairs  of  ambulacral  plates.  These  five  plates  are  the  rudiments 
of  the  orals  (odontophores). 

At  the  sides  of  the  ambulacral  plates  the  adambulacral  plates  appear.  The 
remaining  pairs  of  ambulacral  and  adambulacral  plates  arise  in  the  same  order  as 
the  pairs  of  tube-feet,  always  proximally  to  the  terminals  of  the  arms,  and  distally 
to  those  already  formed. 

The  five  first  and  the  five  second  pairs  of  ambulacral  plates  unite  with  the  five 
first  pairs  of  adambulacral  plates  to  form  the  oral  skeleton. 

The  five  radial  outgrowths  of  the  intestine  quickly  grow  into  the  arms,  forking, 
and  thus  producing  the  ten  brachial  diverticula  of  the  digestive  sac.  Five  pairs  of 
small  interradial  outgrowths  on  the  water  vascular  ring  represent  the  rudiments  of 
Tiedemann's  bodies.  None  of  the  tube-feet  at  first  have  suckers.  The  formation 
of  the  nerve  ring  is  followed  by  that  of  the  radial  nerve  ridges,  which,  like  the 
former,  are  epithelial  in  position,  persisting  as  such  even  in  the  adult  Asteroid.  The 
continuous  ciliated  covering  of  the  larva  is  at  no  time  interrupted,  but  passes 
direct  into  the  ciliated  covering  of  the  Asteroid. 

We  shall  not  enter  upon  the  accounts  given  of  the  rise  and  development  of  the 
blood  vascular  system,  since  there  is  nothing  more  problematical  in  the  anatomy 
of  the  adult  Asteroids  than  this  system. 

Where,  among  Asteroids,  a  typical  Bipinnaria  larva  is  developed,  the  meta- 
morphosis which  produces  the  young  Asteroid  seems  to  resemble  in  essentials  that 
of  Asterina.  The  rudiment  of  the  young  Asteroid  is  found  in  the  posterior  part  of 
the  larva  which  contains  the  swollen  mid-gut.  At  first,  as  in  Asterina,  this 
rudiment  is  double,  i.e.  it  consists  of  an  oral  rudiment,  arising  close  to  the  hydroccel, 
and  an  apical  rudiment,  the  two  uniting  round  the  intestine.  The  larger  anterior 
portion  of  the  larval  body,  together  with  the  ciliated  rings  of  the  Bipinnaria,  are, 
like  the  larval  organ  of  Asterina,  gradually  resorbed. 


E.  Ontogeny  of  the  Ophiuroidea. 

According  to  the  present  state  of  our  knowledge  the  development  of  the  Ophiur- 
oidea does  not  appear  to  differ  so  greatly  from  that  of  the  Asteroidea,  in  spite  of 
the  difference  in  shape  of  the  larvte,  as  to  need  detailed  description.  We  shall 
therefore  limit  ourselves  to  a  few  points. 

Development  of  the  hydro  -  enteroccel.—  The  first  rudiment  of  the  hydro- 
enterocoel  has  not  been  observed  with  as  much  certainty  as  could  be  desired.  In 
the  quite  young  Pluteus  larva  an  enteroccel  vesicle  lies  at  each  side  of  the  oesophagus. 
Somewhat  later  the  larva  possesses,  besides  this  pair  of  vesicles,  another  pair  of 
enteroccel  vesicles  at  the  sides  of  the  stomach-intestine,  these  latter  having,  as  it 
appears,  been  constricted  off  from  the  former.  The  left  anterior  vesicle  at  this 
stage  enters  into  communication  with  the  exterior  through  the  dorsal  pore  (water 
pore).  On  the  left  side  there  now  arises,  between  the  anterior  and  the  posterior 
•mteroccel  vesicles,  apparently  by  constriction  from  the  latter,  a  third  new  vesicle, 


VIII 


ECHINODERMA  TA—ONTOGEX  Y 


533 


the  hydroccel  vesicle  (Fig.  442).  This  at  once  becomes  entirely  distinct,  and 
lengthens  out  anteriorly  below  the  left  anterior  enteroccel  vesicle.  At  its  outer  left 
edge  it  then  produces  five  outgrowths,  the  rudiments  of  the  radial  portions  of  the 
water  vascular  system.  Between  the  fourth  and  fifth  outgrowths  (reckoning  from 
before  backward)  a  dorsally  directed  diverticulum  further  grows  out  from  the 
hydroccel  vesicle,  which,  after 

a  very  short  course,  comes  in  t^""^  "nt 

contact  with  the  left  anterior 
enteroccel  vesicle,  and  opens 
into  it  immediately  below  the 
aperture  of  the  water  pore. 
This  diverticulum  is  the  rudi- 
ment of  the  stone  canal.  Its 
connection  with  the  dorsal  pore 
(madreporite)is  thus  secondary, 
and  is  bi'ought  about  by  means 
of  the  left  anterior  enteroccel, 
which  no  doubt  becomes  the 
ampulla. 

The  long  hydroccel  vesicle, 
with  its  five  outgrowths,  then 
clasps  the  larval  cesophagus 
like  a  halter,  and  grows  round 
it  ;  this  larval  oesophagus 
apparently  becomes  the  defini- 
tive cesophagus,  while  no 
definitive  anus  replaces  that 
of  the  larva. 

First  appearance  of  the 
plates  of  the  skeleton. — Soon 
after  the  formation  of  the  stone 
canal,  ten  skeletal  plates  appear 
on  the  Plutev.s  larva,  five  on 
the  left  and  five  on  the  right 
side,  i.e.  above  the  left  and 

right  posterior  coelomic  vesicles.  The  five  on  the  right  side  are  the  radials  of  the 
apical  system  ;  the  five  on  the  left  are  the  terminals.  In  the  middle  of  the  right 
side  the  rudiment  of  the  central  plate  then  appears,  and  on  the  left  side,  immediately 
in  front  of  the  water  pore,  another  plate  appears,  which  is  the  fifth  oral,  the  one 
which  becomes  the  madreporitic  plate.  The  inadreporite  thus  belongs  ontogenetically 
to  the  oral  system  of  plates.  The  other  parts  of  the  skeleton  form  only  after  the 
metamorphosis. 


pott 

FIG.  442.— Dorsal  aspect  of  a  young  Ophiuroid  Pluteus. 
to  illustrate  the  hydro  -  enteroccel  (after  Bury).  1,  Larval 
cesophagus  ;  2,  left  anterior  enteroccel ;  3,  hydropore ; 
4,  hydroccel ;  5,  left  posterior  enteroccel  vesicle  ;  6,  stomach- 
intestine  ;  7,  right  posterior  enteroccel  vesicle ;  8,  right 
anterior  enteroccel  vesicle. 


F.  Ontogeny  of  the  Crinoidea. 
The  Ontogeny  of  Antedon  alone  has  been  investigated. 


1.  Embryonic  Development. 

Here  also  a  coelogastrula  is  formed  by  the  invagination  of  a  cceloblastula.  The 
transverse  slit-like  blastopore  indicates  the  posterior  end  of  the  larva,  which  at  a 
later  stage  becomes  bilaterally  symmetrical.  The  segmentation  cavity  is  filled  by 
an  albuminiferous  gelatinous  mass  (gelatinous  nucleus). 


534 


COMPARATIVE  ANATOMY 


CHAP. 


After  the  process  of  invagination  has  begun,  the  formation  of  mesenchyme  also 
commences,  proceeding  from  the  blind  end  of  the  archenteron,  which  here  is  bi- 
laminar.  The  cells  of  the  layer  which  is  turned  to  the  segmentation  cavity  Avander 
into  that  cavity,  i.e.  into  the  gelatinous  nucleus  Avhich  fills  it,  and  become  mesen- 
chyme cells  (Fig.  443).  The  formation  of  mesenchyme  proceeds  actively  during 
the  whole  process  of  invagination  along  the  Avhole  archenteron.  but  chiefly  at  its 
base.  Here,  indeed,  the  formation  of  mesenchyme  is  observed  long  after  important 
processes  of  separation  and  differentiation  have  been  accomplished  in  other  parts 
of  the  archenteron. 

The  formation  of  mesenchyme  takes  place  here  more  actively  than  in  any  other 
Echinoderm  in  which  it  has  been  observed,  so  that  the  large  segmentation  cavity 
soon  appears  to  be  croAvded  with  mesenchyme  cells. 

The  ectoderm  becomes  covered  with  cilia. 

The  blastopore  closes  completely  in  the  course  of  the  second  stage  of  develop- 


FIG.  443.— A,  Horizontal  longitudinal  section  through  an  embryo  (gastrula)  of  Antedon 
twenty-six  hours  old  ;  B,  the  same  of  one  forty-eight  hours  old,  in  which  the  archenteron,  which 
has  become  constricted  off,  is  divided  into  two  sections  (after  Seeliger).  1,  Ectoderm ;  2,  mesen- 
chyme cells  ;  3,  place  of  formation  of  the  mesenchyme  cells  at  the  base  of  the  archenteron ;  4, 
blastopore ;  5,  endoderm  ;  6,  archenteric  cavity ;  T,  mesentero-hydrocoel  A^esicle ;  8,  enteroccel 
vesicle. 


ment.  The  archenteron  then  lies  as  a  closed  vesicle  in  the  posterior  region  of 
the  segmentation  cavity. 

An  important  process  soon  takes  place.  The  archenteric  vesicle  or  archenteron 
becomes  constricted  by  a  circular  furrow  (Fig.  443  B).  This  constriction  leads  to 
a  complete  division  of  the  archenteron  into  an  anterior  and  a  posterior  vesicle. 
The  anterior  is  someAvhat  larger  than  the  posterior  ;  the  formation  of  the  mesen- 
chyme continues  actively  on  its  Avail  (Fig.  444). 

From  the  anterior  vesicle  are  derived  the  intestine  and  the  hydroccel ;  from  the 
posterior  the  ccelom  with  the  chambered  sinus,  etc.  We  here  have  a  remarkable 
difference  between  Antedon  and  other  Echinoderms,  in  Avhich  latter,  as  above 
described,  the  anterior  blind  end  of  the  archenteron  ahvays  yields  the  ccelom. 

The  anterior  vesicle  is  in  close  proximity  to  the  ectoderm,  on  the  ventral  side. 

The  posterior  vesicle  becomes  a  transversely  placed  tube,  whereas  the  anterior 
is  produced  into  a  horn,  both  dorsally  and  ventrally.  These  two  horns  clasp  the 
posterior  vesicle  from  its  anterior  side.  The  larva  is  now  distinctly  bilaterally 
symmetrical  (Fig.  444). 

The  next  changes  to  occur  are  the  following : — 


VIII 


ECHINODERMA  TA— ONTOGENY 


535 


The  two  horns  of  the  anterior  vesicle  grow  out  towards  one  another  round  the 
posterior  vesicle  until  they  touch,  and  so  form  a  hollow  ring  surrounding  the 
posterior  vesicle,  but  not  closed  posteriorly. 

The  posterior  vesicle  (enteroccel  vesicle)  assumes  the  shape  of  a  dumh-bell,  its 
two  lateral  parts  swelling  up,  while  the  transverse  connecting  piece  becomes 
narrower.  It  is  this  connecting  piece  which 
becomes  encircled  by  the  anterior  vesicle 
(Fig.  444  . 

The  ectoderm  thickens  on  the  ventral  side. 

The  germ,  which  till  now  is  approximately 
spherical,  begins  to  lengthen  from  before  back- 
ward (in  the  direction  of  the  principal  axis). 

The  anterior  vesicle  forms  a  large  ventrally 
directed  outgrowth,  the  first  rudiment  of  the 
hydrocoel  (Fig.  445,  3).  A  small  outgrowth 
of  its  anterior  wall  is  the  rudiment  of  a  sinus, 
which  has  been  called  by  some  the  parietal 
cavity,  and  by  others  the  anterior  enterocoel  (2). 
The  circular  anterior  vesicle  itself  becomes  the 
intestine  (5,  7). 

In  the  posterior  (enteroccel)  vesicle  the  two 
lateral  swellings  increase  in  size,  while  ^the 
connecting  piece  becomes  thinner  and  thinner, 
and  finally,  at  a  later  stage,  entirely  disappears. 
The  enteroccel  vesicle  is  thus  divided  into  a 
right  and  a  left  enterocoel  sac. 


FIG.  444.  —  Horizontal  longitudinal 
section  through  an  embryo  of  Antedon, 
fifty -seven  hours  old  (after  Seeliger). 
1,  Point  at  which  the  neural  plate  becomes 
differentiated  ;  2,  ectoderm  ;  3,  mesen- 


Duriug  the  next  period,  which  more  or  less    cnyme  ;    4,   place   of  formation   of  the 


corresponds  with  the  fourth  day  of  develop- 
ment, the  embryo  increases  somewhat  more  in 
length.  Anteriorly,  in  the  frontal  region,  i.e. 


meseuchyme ;  5,  rudiment  of  the  intes- 
tine ;  6,  rudiment  of  the  left  coelom ; 
7,  ventral  outgrowth  of  the  mesentero- 
hydroccel  vesicle  ;  8,  rudiment  of  the  right 


at  the  end  of  the  embryo  diametrically  opposite    coelom .  9>  transverse  duct,  connecting  the 

to  the  point  where  the  now  vanished  blastopore    two  rudiments  of  the  coelom. 

lay,    a   ciliated    tuft    forms.      Ciliated   rings 

appear  in  the  arrangement  characteristic  of  the  free-swimming  larva  (cf.  Fig.  402, 

p.  510). 

The  ectoderm  which  carries  the  neural  tuft  thickens  (neural  plate),  becomes 
multilaminar,  and  at  the  same  time  appears  to  be  slightly  depressed  (Figs.  446  and 
447  .  The  deep  cells  become  ganglion  cells,  and  nerve  fibrillae  also  appear  below 
the  surface,  closely  applied  to  the  neural  plate  and  formed  by  the  ectoderm  ;  these 
are  the  rudiments  of  the  larval  nervous  system. 

Ventrally  from  the  neural  plate,  close  behind  it  in  the  median  line,  a  pit-like 
depression  forms  ;  this  is  the  adhesive  pit,  so  called  because,  at  a  later  stage,  the 
free-swimming  larva  attaches  itself  by  means  of  it. 

Another  depression,  which  rapidly  deepens  and  increases  in  circumference,  lies 
in  the  thickened  ventral  ectoderm,  and  is  the  rudiment  of  the  vestibule,  whose  signi- 
ficance will  be  explained  later. 

The  two  coelom  sacs  have  become  completely  detached,  the  connecting  piece 
having  disappeared.  That  on  the  right  spreads  chiefly  dorsally  forward  into  the 
segmentation  cavity  and  then  over  the  intestine,  even  crossing  the  median  line  on  to 
the  left  side.  The  left  coelom  sac,  however,  spreads  chiefly  backward  and  grows 
round  the  intestine  posteriorly  like  a  cap,  until  it  touches  the  posterior  wall  of  the 
right  sac.  Dorsally  it  touches  the  latter  somewhat  to  the  left  of  the  median  line, 
and  a  mesentery  is  thus  formed  which  runs  dorsally  somewhat  to  the  left  of  the 


536  COMPARATIVE  ANATOMY  CHAP. 

median  line,  but  on  the  posterior  side  shifts  the  more  to  the  right  the  nearer  it 
approaches  the  ventral  side.  This  is  the  principal  mesentery.  Ventrally  the  two 
coelom  sacs  still  remain  far  apart. 

In  the  anterior  vesicle,  the  hydroccel  rudiment,  together  with  the  rudiment  of 
the  parietal  sinus,  becomes  separated  from  the  rudiment  of  the  intestine.  After 
this  separation  has  taken  place,  the  hydrocoel  vesicle  still  remains  for  a  short  time 
in  open  communication  with  the  parietal  sinus. 

The  hydroccel  vesicle  lies  close  below  the  thickened  ventral  ectoderm,  shifted 
somewhat  far  to  the  left. 

The  rudiment  of  the  parietal  sinus  becomes  a  transverse  tube. 
The  rudiment  of  the  intestine  changes  shape.     It  is  no  longer,  as  before,  an 
incomplete  hollow  ring,  placed  vertically,  through  which  the  connecting  piece  of 

the  two  coelom  vesicles  passed  (Fig.  445). 
The  connecting  piece  having  degenerated, 
the  lumen  of  the  tubular  ring  has  room 
to  expand  from  before  backward,  until 
the  hollow  ring  becomes  a  vesicle. 

Towards  the  close  of  embryonic 
development,  in  the  fifth  stage,  the  first 
rudiment  of  the  calcareous  skeleton 
appears.  In  an  embryo  one  hundred 
hours  old,  the  rudiments  of  the  following 
plates  were  found  :  5  orals,  5  basals,  3-5 

infrabasals,   and   about   11    segments    of 
FIG.  445.— Posterior  end  of  an  embryo  of    ,,       ,   ,, 

Antedon  sizty  hours  old,  seen  from  the  right    r 

side  (after  Seeliger).    1,  The  outline  of  the  right  Tlie    five    orals    liave    a    superficial 

coelom  sac ;  2,  rudiment  of  the  parietal  sinus ;  position  at  the  posterior  part  of  the 
3,  rudiment  of  the  hydroccel ;  4,  mesenchyme ;  embryo,  making  a  horse  -  shoe  -  shaped 
6,  ventral,  and  7,  dorsal  process  of  the ,  mesentero-  arch  which  ig  anteriorly  and  down- 

hydroccel  vesicle  ;  6,  connecting  duct  between  the  ,     ' 

right  and  the  left  coelom  sacs. ,  wards-      The  left  end  of  the  arch  reaches 

further  forward  than  the  right. 

As  a  rule  (with  the  exception  of  the  first  oral,  which  indicates  the  end  of  the 
left  side  of  the  arch)  the  five  orals  lie  round  the  left  coelom  sac. 

The  five  basals  have  exactly  the  same  arrangement  as  the  five  orals,  merely  lying 
somewhat  further  forward  than  the  latter.  All  of  them,  except  the  first  basal,  lie 
above  the  right  ccelomic  vesicle. 

The  3-5  infrabasals  again,  which  are  still  extremely  small,  lie  in  front  of  the 
basals,  but  further  below  the  surface  of  the  embryo. 

In  the  anterior  half  of  the  embryo  the  infrabasals  are  followed  by  the  row  of 
stalk  plates.  This  row  forms  an  arch  which  is  concave  towards  the  ventral  surface, 
so  that  the  most  anterior,  the  pedal,  plate,  lies  near  the  floor  of  the  vestibular 
invagination. 

The  newly-arising  skeletal  plates  of  the  stalk  appear  at  the  posterior  end  of  the 
row,  generally  (but  not  exclusively)  immediately  in  front  of  the  future  centrodorsal, 
between  it  and  the  last  formed  most  posterior  stalk  plate. 

Up  to  this  time  the  embryo  has  lain  enclosed  in  the  egg-membrane,  on  the  pin- 
nule of  the  mother,  but  it  is  now  ready  to  be  hatched.  Even  at  this  stage  its 
organisation  leads  to  the  conjecture  that  the  calyx  will  be  produced  from  the  larger 
posterior  half,  which  alone  contains  the  internal  rudiments,  while  the  stalk  of  the 
future  attached  larva  will  be  produced  from  the  anterior  half. 


VIII 


ECHINODERMA  TA— ONTOGENY 


537 


nrd". 


2.  The  Free-swimming  Larva  (Figs.  402  (p.  510),  446,  447,  448). 

The  external  form  of  the  free-swimming  larva  has  already  been  described  on 
p.  510. 

The  duration  of  this  manner  of  life  differs  greatly  in  individuals  of  the  same 
brood,  varying  from  a  few  hours  to  several  days. 

Ectoderm. — For  the  ciliated  bands,  see  above,  p.  510. 

In  the  intermediate  zones,  which  are  free  from  cilia,  a  fine  cuticle  becomes 
differentiated  from  the  ectodermal  epithelium,  the  cells  of  which  later  begin  to 
secrete  a  homogeneous  substance  which  separates  cell  from  cell,  so  that  the  epi- 
thelium comes  to  resemble  a  connective  tissue. 

The  neural  plate,  the  neural  tuft,  and  with  them  the  larval  nervous  system  attain 
their  highest  development  at  this  stage,  but  undergo  complete  degeneration  in  the 
next.  The  ganglion  cells  below  the  neural  plate 
increase  in  number,  and  the  layer  of  nerve  fibres 
spreads  out  over  the  whole  anterior  end  of  the  larva. 
Fine  nerve  trunks  run  to  the  ciliated  rings,  and  two 
specially  strong  ventral  nerve  trunks  run  back  at 
the  sides  of  the  vestibular  invagination,  their  an- 
terior parts  being  beset  with  isolated  ganglion  cells. 

The  adhesive  pit  becomes  larger  and  deeper, 
and  towards  the  end  of  this  period  loses  its  ciliation 
and  assumes  a  glandular  character. 

The  vestibular  invagination  spreads  over  the 
greater  part  of  the  ventral  side.  It  closes  and 
becomes  a  tube,  the  lateral  edges  of  the  invagination 
growing  towards  one  another  and  fusing  in  the 
median  line.  This  process  takes  place  from  behind 
forwards,  and  is  not  fully  completed  during  this 
period,  a  small  aperture  being  retained  anteriorly. 
The  ciliation  of  the  vestibule  disappears. 

The  intestine  alters  its  shape.  It  spreads  out  FlG-  440.  -Free-swimming  larva 
somewhat,  assuming  tirst  the  form  of  a  hollow  plate,  £?£%  2KS5J8 
with  the  concavity  directed  ventrally  and  the  con-  j_v  The  five  ^uated  rjngS  .  j>  the 
vexity  dorsally.  In  the  ventral  concavity  lies  the  parietal  sinus ;  2,  the  vestibule, 
hydroccel  vesicle.  At  a  later  stage  the  intestine  already  closed  posteriorly;  3,  the 
again  becomes  rounded  and  vesicular.  hydrocoel ;  4,  the  hydropore ;  5,  left 

The  two  enterocal  sacs  continue  to  change  their  ^n 
positions  and  to  spread  out.  The  right  vesicle  pro- 
duces anteriorly  five  tubular  outgrowths,  which  become  grouped  round  the  principal 
axis.  These  five  tubes  arise,  widened  like  funnels,  from  the  right  coelom,  then 
narrow  anteriorly,  and,  losing  their  lumina,  run  out  as  strands.  They  are  the 
rudiments  of  the  chambered  sinus. 

The  skeletal  pieces  of  the  stalk  are  at  this  time  horse-shoe-shaped,  and  tend  to 
enclose  the  five  tubes  of  the  chambered  sinus.  When  they  become  complete  rings 
the  chambered  sinus  passes  through  them. 

The  hydroccel  vesicle  becomes  completely  constricted  from  the  parietal  sinus, 
flattens  in  the  dorsoventral  direction,  and  at  once  assumes  the  horse-shoe  shape. 
The  gape  of  the  horse-shoe  points. at  first  backwards  and  to  the  left,  and  finally  to 
the  left  and  forwards.  Five  ventrally  directed  outgrowths  appear  on  it,  out  of  each 
of  which,  at  a  later  stage,  three  tentacle  vessels  arise. 


538 


COMPARATIVE  ANATOMY 


CHAP. 


The  rudiment  of  the  primary  stone  canal  appears  at  the  blind  end  of  the  left 
limb  of  the  hydroccel,  as  a  dorsal  process,  running  to  the  left. 

The  tubular  parietal  sinus,  which  is  now  completely  isolated  from  the  hydroccel, 


FIG.  447.— Median  longitudinal  section 
through  a  free-swimming  larva  of  An- 
tedon,  twenty -eight  hours  old,  in  the 
act  of  becoming  attached  (after  Seeliger). 
[-V,  The  ciliated  rings  ;  1,  neural  plate  with 
nerve  fibres  (2),  and  ganglion  cells  (3) ; 

4,  gelatinous    nucleus,    the    mesenchyme 
cells  which  crowd  it  are  not  represented  ; 

5,  the  tubes  of  the  chambered  organ  ;  6,  in- 
testine ;    7,    right    coelom ;    8,   vestibule  ; 
9,   parietal    sinus ;    10,   right    enterocoel ; 
11,  hydroccel ;    12,  adhesive  pit ;  13,  left 
enterocoel. 


FIG.  448.— Free-swimming  larva  of 
Antedon,  forty -eight  hours  after 
being  hatched,  from  the  left  side, 
specially  to  illustrate  the  rise  of  the 
skeletal  plates.  I-V,  The  ciliated  rings  ; 
?>«l-bag,  the  five  basals  ;  orror5,  the  five 
orals,  those  lying  on  the  right  side  re- 
presented as  discs  ;  1,  vestibule  ;  2,  in- 
testinal vesicle  ;  3,  right  enterocoel  ; 

4,  calcareous    joints    of    the    stalk  ; 

5,  pedal  plate. 


has  shifted  to  a  position  in  front  of  and  above  the  latter.  Its  posterior  end  grows 
out  till  it  touches  the  ectoderm  immediately  in  front  of  the  fourth  ciliated  ring 
ventrally  and  to  the  left,  and  finally  breaks  out  through  the  hydropore  at  this  point. 


3.  Attachment  of  the  Larva  and  its  Transformation  into  the  Stalked  Form 

(Figs.  449-453). 

Attachment  takes  place  by  means  of  the  adhesive  pit,  which  yields  a  sticky 
secretion  ;  and  since  this  pit  lies  ventrally  at  the  anterior  part  of  the  body,  the 
attached  larva  has  at  first  a  position  parallel  to  the  surface  to  which  it  is  fastened, 
and  the  vestibule  lies  immediately  above  that  surface.  The  body,  however,  soon 
becomes  erect,  and  the  adhesive  pit  takes  up  a  terminal  position. 

. « ry  soon  after  attachment  the  ciliated   rings  disappear,   and   so   does  the 


VIII 


EGHINODERMA  TA— ONTOGENY 


539 


neural  tuft ;  the  neural  plate  flattens  out,  and  the  larval  nervous  system 
completely  degenerates. 

The  ectoderm  cells  continue  to  yield  an  intermediate  substance.  Many  of  them 
sink  below  the  surface,  the  consequence  being  that  the  distinction  between  the 
body  epithelium  and  the  mesenchymatous  cntis  is  entirely  obliterated. 

The  vestibule  becomes  completely  constricted  off,  the  last  remains  of  the  aperture 
of  imagination  closing.  At  the  same  time  it  shifts  entirely  to  the  posterior  end 
of  the  larva,  the  end  which  now  freely  projects,  twisting  through  an  angle  of  90°,  so 
that  its  thickened  epithelial  floor,  which  before  lay  parallel  to  the  principal  axis,  now 

eat 


FIG.  440.—  Young  attached  larva  of  An- 
tedon.  forty-eight  hours  old,  from  the  left 
side  (after  Seeliger).  The  vestibule  has  be- 
come entirely  constricted  off,  but  the  distinc- 
tion between  the  calyx  and  the  stalk  is  not  yet 
pronounced.  601-603,  Basals  ;  orj-org,  orals  ; 
il>,  infrabasals  ;  1,  pedal  plate ;  2,  parietal 
canal ;  3,  hydrocoel  outgrowths  ;  4,  vestibule  ; 
5,  intestinal  vesicle ;  6,  left  crelom  sac ;  7,  right 
ccelom  sac  ;  8,  calcareous  joints  of  the  stalk. 


FIG.  450. —  Young  attached  larva  of 
Antedon,  forty -eight  hours  after  being 
hatched,  from  the  left  side  (after  Seeliger). 
co,  Stalk ;  co,  calyx ;  601-603,  basals ;  ori-org, 
orals  ;  ib,  infrabasals  of  the  left  side  ;  1,  pedal 
plate  ;  2,  hydropore ;  3,  left  coelora ;  4,  right 
crelom  ;  5,  joints  of  the  stalk. 


lies  at  right  angles  to  it.  The  larval  body  becomes  club-shaped,  the  anterior  body 
forming  the  handle  of  the  club.  The  vestibule,  which  continues  to  increase  in  size, 
comprises  the  entire  posterior  part  of  the  club  (the  calyx)  ;  it  becomes  pentagonal, 
and  imprints  the  same  shape  upon  the  whole  posterior  part  of  the  body,  and  thus 
first  determines  the  radiate  structure  (Figs.  450,  451,  and  452). 

The  anterior  end  of  the  larva  becomes  the  apical  end  of  the  stalk,  the  pos- 
terior end  becoming  the  oral  side  of  the  calyx  of  the  attached  Pentacrinus-like 
larva. 

The  hydrocoel  undergoes  the  same  twisting  and  shifting  as  the  vestibule,  beneath 


540 


COMPARATIVE  ANATOMY 


CHAP. 


whose  floor  it  lies  after  as  before  the  process.  It  has  passed  from  the  horse-shoe 
shape  to  the  circular,  but  the  hydroccel  ring  still  remains  unclosed  for  a  long  time 
at  the  point  where  the  gape  of  the  horse-shoe  formerly  was.  Its  five  outgrowths 


-Li 


d. 


FIG.  451. —stalked  larva  of  Antedon, 
eighty-four  hours  old,  with  twenty-five 
tentacles,  from  the  right  side  (after 
Seeliger).  Calcareous  plates  not  repre- 
sented. 1,  Right  ccelom  sac  ;  2,  stomach 
intestine ;  3,  left  coelom  sac ;  4,  sacculi ; 
5,  vestibule,  still  closed ;  6,  the  fifteen 
primary  tentacles ;  7,  the  five  pairs  of 
secondary  interradial  tentacles  ;  8,  oeso- 
phagus ;  9,  hind-gut ;  10,  axial  organ ; 
11,  fibrous  strands  in  the  stalk,  continua- 
tions of  the  axial  sinus. 


FIG.  45-_'.— Transverse  section 
through  the  region  of  the  left 
or  oral  ccelom  of  an  attached 
larva  of  Antedon,  108  hours 
old  (after  Seeliger).  I-V,  The 
five  radii ;  1,  left  oral  coelom ; 
2,  oesophagus ;  3,  stone  canal ; 
4,  parietal  canal. 


Fir,.  453. — Diagrammatic 
transverse  section  through  the 
region  of  the  aboral  ccelom  in 
an  Antedon  larva,  108  hours  old 
(after  Seeliger).  I-V,  The  five 
radii;  b«i-&«g,  the  five  basal.s ; 
1,  right  or  aboral  coelom  ;  2,  hind- 
gut  ;  3,  axial  organ  ;  4,  parietal 
sinus ;  5,  oesophagus. 


push  up  the  ectoderm  of  the  floor  of  the  vestibule  into  the  vestibular  cavity  ;  they 
soon  appear  to  be  trilobate,  so  that  in  all  5  x  3  tentacles  are  present,  ten  more  being 
added  to  them,  which  arise  in  pairs  at  the  bases  of  the  primary  outgrowths. 


viii  ECHIXODERMATA— ONTOGENY  541 

The  stone  canal  breaks  through  into  the  parietal  sinus. — The  point  at  which 
tliis  occurs  does  not,  however,  correspond  with  the  point  at  which  the  hydroccel  and 
the  parietal  sinus  originally  were  in  open  communication. 

The  parietal  sinus  also  takes  part  in  the  shifting  just  mentioned.  In  the  free- 
swimming  larva  it  lay  in  front  of  the  hydroccel.  This  position  it  retains,  while 
shifting  backward  (towards  the  oral  end)  together  with  the  hydroccel.  It  thus 
approaches  its  external  pore.  Compared  with  the  other  growing  organs,  it  remains 
small  and  stationary. 

The  hydroccel  is  thus  connected  by  the  stone  canal  with  the  parietal  sinus,  and 
this  latter  is  in  open  communication  with  the  exterior  through  the  hydropore. 

The  intestine. — An  extraordinary  process  goes  on  in  the  intestinal  vesicle. 
Numerous  cells  become  detached  from  its  wall,  and  wander  into  its  cavity,  which 
they  finally  completely  fill.  They  fuse  for  the  most  part  into  a  large  yolk-like  mass, 
which  is  entirely  resorbed  at  a  later  stage  as  nutritive  material. 

The  floor  of  the  vestibule  deepens  at  the  centre,  and  is  produced  anteriorly  like  a 
funnel  towards  the  intestinal  vesicle.  This  funnel,  which  passes  through  the  hydro- 
ccel ring,  becomes  the  oesophagus,  and  joins  a  posterior  process  of  the  intestinal 
vesicle  which  grows  out  to  meet  it. 

The  intestinal  vesicle  divides  into  a  stomachal  section  to  the  left  of  the  larva 
and  a  narrower  portion  running  dorsoventrally  on  the  right  side.  This  latter  part, 
the  hind-gut,  rises  with  a  broad  base  out  of  the  former  and  ends  blindly.  The  blind 
end  of  the  hind -gut  then  grows  over  to  the  left  ventrally. 

The  ccelom. — The  two  ccelom  sacs  shift  and  spread  out  in  a  peculiar  manner. 
The  left  sac  shifts  quite  posteriorly,  and  becomes  the  oral  ccelom,  which  grows  round 
the  oesophagus  on  all  sides  from  above  downward,  thus  assuming  the  shape  of  a 
hollow  horse-shoe  which  clasps  the  oesophagus,  the  stone  canal,  and  the  parietal 
canal  (counting  these  in  order  from  within  outward).  The  gape  of  this  horse-shoe 
is  directed  ventrally  to  the  left,  and  since  the  tips  of  its  two  arms  grow  towards  one 
another,  a  short  longitudinal  accessory  mesentery  arises.  The  right  ccelom  vesicle 
passes  through  changes  of  form,  expansion,  and  shifting  which  are  difficult  to  describe 
briefly,  and  becomes  the  aboral  or  apical  ccelom.  In  consequence  of  the  shiftings  of 
these  vesicles  the  longitudinal  principal  mesentery  which  separated  the  originally 
right  from  the  originally  left  coelom  vesicle  becomes  a  transverse  mesentery, 
separating  the  oral  from  the  apical  ccelom,  and  surrounding  the  cesophagus  like  a 
diaphragm.  Xear  the  right  (now  aboral)  ccelom  also,  a  longitudinal,  somewhat 
diagonal  accessory  mesentery  develops,  which  runs  somewhat  to  the  right  of  the 
ventral  median  line.  The  walls  of  the  apical  (originally  right)  ccelom  are  continued 
anteriorly  into  the  Avails  of  the  five  tubes  which  form  the  chambered  sinus,  but  at 
the  point  where  they  pass  into  one  another  they  are  so  pressed  together  that  no 
open  communication  exists  between  the  two  sinuses. 

The  axial  organ  (genital  stolon)  of  the  calyx  arises  as  a  thickening  in  the  left 
epithelial  wall  of  the  longitudinal  accessory  mesentery  of  the  aboral  ccelom,  at  its 
most  anterior  (apical)  end,  where  the  chambered  organ  commences.  As  the  genital 
strands  of  the  arms  and  pinuulse  most  probably  arise  as  outgrowths  of  the  axial 
organ,  it  might  thus  be  proved  that  in  the  Crinoids  also  the  genital  cells  are  derived 
in  the  last  instance  from  the  endothelium  of  the  body  cavity.  The  cushion-like 
thickening  increases  in  length,  becoming  partly  constricted  from  the  mesentery ; 
posteriorly,  it  reaches  to  the  oral  ccelom  ;  anteriorly,  the  axial  organ  passes  into  the 
stalk  up  the  centre,  between  the  five  tubes  of  the  chambered  sinus. 

The  formation  of  trabeculse  begins  in  the  aboral  ccelom.  Single  endothelial 
cells  lengthen  and  project  like  pillars  into  the  ccelomic  cavit}'.  A  similar  process 
takes  place  in  the  hydroccel. 

The  skeleton. — When  the  vestibule  shifts  to  the  posterior  end  of  the  larva  the 


542  COMPARATIVE  ANATOMY  CHAP. 

skeletal  plates  are  also  shifted.  The  horse-shoes  formed  by  the  five  orals  and  the 
five  basals  close,  and  form  two  flat  rings  or  circles.  The  circle  of  the  orals  shifts 
backward  and  on  to  the  roof  of  the  vestibule  in  such  a  way  that  the  five  plates  together 
form  a  pyramid,  the  truncated  tip  of  which  forms  the  centre  of  the  vestibular  roof,  at 
the  extreme  posterior  end  of  the  larva.  The  orals  have  thus  shifted  away  from  the 
region  of  the  left  (oral)  ccelom. 

The  circle  of  basals  which  lies  in  front  of  (apically  to)  that  of  the  orals  forms  a 
pyramid  in  the  body  wall  of  the  calyx,  around  the  aboral  ccelom,  the  truncated  end 
of  this  pyramid  lying  at  the  commencement  of  the  stalk,  or  at  the  anterior  (apical) 
end  of  the  calyx.  The  orals  and  the  basals  together  form  a  pentagonal  double 
pyramid,  truncated  at  both  ends.  At  the  truncated  end  of  the  basal  pyramid,  round 
the  uppermost  (most  posterior)  joints  of  the  stalk,  lie  the  four  or  five  small  infra  - 
basals.  The  number  of  joints  in  the  stalk  increases,  and  the  anterior  body  of  the 
larva  becomes,  as  the  stalk,  more  and  more  distinctly  demarcated  from  the  posterior 
body  or  calyx,  which  has  now  become  five-rayed. 

The  orals  and  basals  alternate  with  the  primary  outgrowths  of  the  hydroccel. 
i.e.  they  are  interradially  placed.  If  we  indicate  that  primary  outgrowth  of  the 
hydroccel  which,  when  the  larva  is  viewed  from  the  oral  side,  comes  next  in  the 
direction  followed  by  the  hands  of  the  clock,  to  the  hydropore  (which  lies  ventrally 
to  the  right)  as  No.  I,  and  those  which  follow  in  this  direction  as  II,  III,  IV,  and 
V,  and  if,  again,  we  indicate  that  oral  or  basal  which  lies  in  the  interradius  between 
radii  I  and  V  as  the  first,  and  those  which  follow  in  this  direction  as  orals  (or  basals) 
2  to  5,  it  can  be  proved  that  the  hydropore,  in  the  older  stages  of  the  attached  larva, 
is  enclosed  by  the  basal  part  of  the  first  oral  plate.  In  these  stages  it  is  also  seen 
that  the  infrabasals  fuse  to  form  a  single  plate,  the  centrodorsal,  at  the  centre  of 
which  there  is  an  aperture  for  the  passage  of  the  chambered  organ. 

During  the  first  developmental  periods  which  follow  the  attachment  of  the  larva, 
the  sacculi  appear.  Five  of  these  first  arise  exactly  radially  at  the  bases  of  the 
middle  tentacle  of  each  group  of  tentacles,  on  the  outer  side  of  the  circular  canal. 
These  sacculi  can  be  ontogenetically  derived  from  groups  of  mesenchyme  cells. 


4.  The  Stalked  Larva  after  the  Vestibule  has  been  Perforated. 
(From  five  days  to  the  sixth  week  after  hatching,  Fig.  454.) 

The  calyx  becomes  more  and  more  distinct  from  the  stalk. 

The  roof  of  the  vestibule  becomes  ever  thinner  at  its  centre,  an  aperture  finally 
forming.  Radial  incisions  run  from  this  central  aperture  towards  the  peripheral  base 
of  the  roof,  so  that  this  latter  becomes  divided  into  five  interradial  lobes  or  valves, 
each  of  which  contains  an  oral  plate.  This  pyramid  of  valves  can  open 'and  shut. 
The  vestibule  has  opened  outward. 

The  five  tentacles  meanwhile  lengthen  and  receive  their  papillse.  They  usually 
project  outwards  from  between  the  five  oral  valves. 

The  definitive  nervous  system  (which  is  oral  and  superficial)  rises  quite  inde- 
pendently of  the  larval  nervous  system,  which  entirely  disappears.  The  first  ap- 
pearance of  the  nerve  ring  was  observed  very  late,  long  after  the  perforation  of  the 
vestibule.  The  ectoderm  of  the  oral  disc,  i.e.  the  peripheral  portion  of  the  original 
vestibular  floor  (the  central  part  having  sunk  in  to  form  the  oesophagus),  becomes 
thickened  in  a  ring  which  is  bordered  by  tentacles,  and  here  becomes  multilaminar. 
The  cells  of  the  deeper  layer  yield  the  nerve  tissue. 

Neither  the  rudiments  of  the  deeper  oral  nervous  system  nor  those  of  the  apical 
sy.stc  m  have  as  yet  been  certainly  observed. 


VIII 


ECHINODERMA  TA— ONTOGENY 


543 


16 


Alimentary  canal.— The  mouth  from  the  very  first  does  not  lie  exactly  at  the 
centre  of  the  oral  disc,  but  somewhat  eccentrically  in  the  interradial  area  bordered 
by  the  first  and  fifth  radii. 

The  stomach  becomes  a  capacious  sac,  and  the  yolk-like  mass  contained  in  it  is 
gradually  absorbed.  The  hind-gut  arises  out  of  it  (in  interradius  III-Y),  being 
broad  at  the  base,  and  then  thinning  into  a 
tube  which  has  the  following  course.  Viewing 
the  larva  from  the  oral  pole,  the  hind-gut  runs 
(in  the  direction  of  the  hands  of  the  clock)  in 
the  horizontal  mesentery,  near  the  body  Avail, 
through  the  interradial  space  IV- V,  then  runs 
across  radius  V,  and  immediately  after  opens 
outwards  in  the  interradial  space  V-I,  through 
the  anus  which  has  in  the  meantime  broken 
through  the  calyx  laterally.  This  is  the  same 
interradius  in  which  the  hydropore  lies,  on 
the  original  ventral  side  of  the  bilaterally 
symmetrical  larva.  The  ectoderm  takes  no 
part  in  the  formation  of  the  anus. 

In  the  ccelom  sacs  profound  changes  are 
going  on,  which  may  be  briefly  summarised 
as  follows. 

(a)  The   chambered   sinus   gives  up  all 
connection  with  the  original  right,  now  the 
aboral  coelom  sac. 

(b)  The  mesenteries  (both  the  principal 
and  the  longitudinal  accessory  mesentery)  are 
completely  resorbed,  and  as  a  consequence 
the  right  and  left  cceloms  unite  to  form  one 
large  body  cavity. 

(c)  The  trabeculae  (of  endothelial  origin) 
become  very  strongly  developed,  and  traverse 
the  body  cavity  in  all  directions  as  a  network. 

(d)  The  axial  organ  becomes  differentiated 
as  an  independent  solid  cell  strand,  lengthens 
till  it  reaches  the  tegmen  calycis  (oral  disc), 
and  at  a  later  stage  becomes  hollow. 

(e)  In  the  parietal  sinus,  which  comes  to 


post 


FIG.  454.— Calyx  of  a  decalcified  larva 
of  Antedon,  five  weeks  old,  with  extended 
tentacles,  from  the  left  and  lower  side  (after 
Seeliger).      I-V,   The    five   radially  placed 
primary  sacculi ;   1,  axial  organ ;   2,  right 
lie   quite   in    the    body   cavity,    two   sections    (aboral)   coelom ;    3,   principal    mesentery, 
become   more    and   more   distinct:    the    one    between  the  right  (aboral)  and  the  left  (oral) 
i  j    ,,         ,,  ,  ,.,        coelom;    4.    hind -gut    which    follows    the 

vesicular,  and  the  other  a  narrow  canal-like    stomach;  J>oralw5om.  6>  hydrocoel  ring. 

section  opening  outwards  through  the  hydro-  7,  two  of  the  ten  secondary  tentacles; 
pore.  The  former,  into  which  the  primary  8,  tentacle  papillae;  9,  primary  tentacles 
stone  canal  enters,  loses  its  independent  (°nly  seven  of  the  total  number,  fifteen,  are 
endothelium,  and  the  thin  wall  which  represented);  10,  oral  lobes ;  11,  stone  canal; 

12,  hydropore ;  13,  oesophagus ;  14,  stomach ; 

separates   it  from   the  ccelom  also  probably    15>  continuation  of  the  chambered  organ  in 
disappears,    so   that   it  ceases  to  exist  as  a    the  stalk  (16). 
separate  cavity.     The  stone  canal  now  opens 

into  the  general  body  cavity,  which  is  thus  in  communication  with  the  exterior 
through  the  narrower  section  of  the  original  parietal  sinus,  and  through  the 
hydropore  in  the  anal  interradius. 

In  the  hydrocoel,  the  water  vascular  ring  completely  closes.     The  whole  of  the 
musculature  of  the  hydroccel  is  formed  by  the  hydroccelomic  epithelium  itself.     The 


544  COMPARATIVE  ANATOMY  CHAP. 

trabeculse  within  the  system  of  canals  increase  in  number.  In  the  tentacles,  the 
following  changes  have  taken  place.  Formerly  the  twenty-five  tentacles  were 
arranged  in  five  radial  groups  of  five  each.  The  five  tentacle  canals  of  a  group  were 
connected  by  a  common  tentacle  canal  rising  with  the  circular  canal.  Now  all  the 
five  tentacle  canals  of  a  group  rise  separately  from  the  water  vascular  ring. 

Further,  during  the  period  of  the  attachment  of  the  stalked  larva,  four  new  stone 
canals  appear,  and  four  new  calyx  pores  form  in  the  other  interradii.  These  and 
all  that  arise  later  cannot,  of  course,  form  in  the  same  way  as  the  primary  calyx 
pore. 

The  stage,  the  development  of  which  has  just  been  described,  has  been  called 
the  Cystid  stage,  owing  to  the  absence  of  arms,  to  there  being  no  clear  division 
of  the  calyx  into  dorsal  cup  and  tegmen  calycis,  and  to  the  occurrence  of  the 
rudiment  of  the  genital  organs  as  an  axial  organ,  whereas  later  the  genital  glands  He 
in  the  arms  and  especially  in  the  pinnules. 

In  opposition  to  the  above  view  it  may  be  remarked  : 

1.  That  neither  the  want  of  arms  nor  the  absence  of  division  of  the  calyx  into  a 
dorsal  cup  and  an  ambulacral  disc  is  characteristic  of  the  Cystidea. 

2.  That  the  skeletal  system  of  the  attached  larva  of   Comatula  is  altogether 
radiate,  consisting  of  the  three  circles,  the  radials,  the  basals,  and  the  infrabasals. 
On  the  other  hand,  the  irregular  arrangement  of  the  skeletal  plates  is,  as  a  rule, 
characteristic  of  the  Cystidea.      Those  Cystids  which  most  resemble  the  larva  of 
Comatula  in  the  number  and  radial  arrangement  of  the  skeletal  plates  are  also  those 
which  are,  of  all  Cystids,  the  most  nearly  related  to  the  Crinoidea. 

3.  The  hydroccel  of  the   stalked  Comatula  larva  consists  simply  of  the  water 
vascular  ring  and  a  circle  of  tentacles,  which  receive  their  canal  direct  from  the  water 
vascular  ring.     In  the  Cystidea,   radial  canals  must  have  run  out  from  the  water 
vascular  ring,  below  the  food  grooves  of  the  ambulacra,  giving  off  tentacle  canals  to 
right  and  left,  and  also  probably  penetrating  into  the  arms. 

4.  The  appearance  of  the  first  rudiments  of  the  genital  glands  in  the  body  merely 
proves  that  the  definitive  position  in  the  arms  is  secondary,  and  this  applies  to  all 
Echinoderms. 

The  position  of  the  anus,  indeed,  agrees  in  both. 


5.  Last  Stage  of  the  Attached  Stalked  Larva— Pentacrinus  Stage. 

(Of.  Fig.  326,  p.  375.) 

This  stage  is  distinguished  by  the  rise  of  the  arms,  which  begin  to  grow  out  in 
the  radii  between  the  circle  of  the  orals  and  that  of  the  basals.  Each  rudiment  of 
an  arm  is,  from  the  very  first,  supported  on  its  apical  side  by  a  newly  arising  skeletal 
plate.  These  plates  are  the  five  radials  of  the  dorsal  cup.  Distally  from  each 
radial  on  the  growing  arm,  two  new  skeletal  plates  follow  one  another,  the  first  and 
second  costals.  The  growing  rudiment  of  the  arm  then  forks,  the  distichals  form, 
and  so  on. 

During  the  formation  of  the  arms  the  five  middle  and  strictly  radially  arranged 
tentacle  canals  of  the  five  tentacle  groups  become  the  radial  vessels,  which  fork  with 
the  arms.  Fresh  investigation  of  this  point  is,  however,  much  needed. 

The  interval  between  the  oral  pyramid  and  the  bases  of  the  arms  increases,  and 
the  tegmen  calycis  thus  comes  into  existence.  The  pyramid  of  five  oral  valves  in 
the  middle  of  the  latter  does  not  grow  further,  and  the  valves  with  their  skeletal 
plates  finally  disappear.  Round  the  anus,  which  comes  to  lie  in  the  tegmen  calycis, 
an  anal  plate  develops  temporarily. 

At  this  stage  the  resemblance  of  the  attached  and  stalked  larva  of  Antedon 


vin  ECHINODERMATA— PHYLOGEXY  545 

to  the  Inadunata.  especially  to  the  so-called  Larviformia  (cf.  pp.  303,  328,etc.), 
is  so  striking  as  to  be  at  once  recognisable. 

The  calyx,  with  the  arms,  sooner  or  later  breaks  away  from  the  stalk,  and 
can  either  move  by  using  the  arms  as  paddles  or  catch  on  to  objects  by  means  of  its 
cirri.  When  it  breaks  loose  from  the  stalk,  some  of  the  uppermost  whorl  joints  on 
which  cirri  have  formed  remain  connected  with  it ;  these  fuse  with  one  another,  and 
with  the  centrodorsal.  The  basals,  again,  fuse  to  form  a  rosette,  which  is  soon  over- 
grown on  all  sides  by  the  large  apical  centrodorsal  plate. 


XXII.  Phylogeny. 

Xo  other  phylum  of  the  animal  kingdom  is  so  sharply  marked  off  from  all  others 
as  the  Echinoderms.  Their  organisation  is  in  all  points  strange  ;  even  the  radiate 
structure  is  strange,  in  so  far  as  it  is,  unlike  that  of  many  Ccelenterata,  only  a  mask 
which  hides  a  complicated  and  hitherto  inexplicable  asymmetry.  We  are  not  in  a 
position  to  compare  an  adult  Echinoderm  with  the  adult  representative  of  any  other 
phylum  from  a  phylogenetic  standpoint. 

The  difficulties  which  meet  us  in  attempting  to  reconstruct  the  phylogenesis  of 
the  Echinodermata  are  still  further  increased  by  the  fact  that  the  typical  charac- 
teristic Echinoderm  larva  cannot  at  any  stage  of  its  development  be  compared  with 
the  adult  or  larval  form  of  any  other  animal.  An  exception  to  this  statement  may, 
however,  perhaps,  be  made  in  favour  of  the  Enteropneusta,  which  will  be  described  in 
the  next  chapter. 

If,  taking  the  gastrsea  theory  as  a  foundation,  we  assume  for  the  Metazoa  a 
common  bilaminar  racial  form,  it  seems,  in  view  of  the  above-mentioned  difficulties, 
that  the  racial  form  of  the  Echinodermata  must  have  branched  off  extraordinarily 
early,  perhaps  at  a  stage  corresponding  phylogenetically  with  the  gastrula.  By  such 
an  assumption,  the  Echinoderms  and  their  larvae  would  be  removed  from  the  sphere 
of  comparative  anatomy  and  comparative  embryology,  except  in  so  far  as  such  com- 
parative enquiry  were  limited  to  the  Echinoderm  phylum  itself. 

It  appears  to  us,  however,  that  attempts  to  approximate  the  Echinodermata  to 
Metazoa  standing  higher  than  the  Coelenterata  should  not  be  abandoned.  Recent 
anatomical  and  ontogenetic  researches  have  brought  to  light  facts  which  open  up  new 
prospects.  We  may  mention  the  demonstration  of  a  neural  plate  and  of  a  larval 
nervous  system,  the  attempts  to  demonstrate  that  there  are  two  pairs  of  enterocoel 
vesicles,  the  proof  that  the  first  rudiments  of  the  gonads  proceed  from  the  endothe- 
lium  of  the  cu-lom,  the  suggestion  that  the  stone  canal  or  the  hydropore  should  be 
regarded  as  a  nephridial  canal,  etc. 

All  this,  of  course,  does  not  justify  us  in  closely  comparing  the  Echinoderm  larva 
with  other  definite  forms,  adult  or  larval,  belonging  to  Metazoan  classes  higher  than 
the  Ct.flenterata,  except  perhaps  the  Enteropneusta.  But  these  discoveries  and  new 
views  tend  to  make  the  Echinoderm  body  appear  somewhat  less  strange,  since  we 
find  in  its  organisation  important  points  in  which  it  is  fundamentally  in  agreement 
with  the  so-called  Triploblastica. 

It  cannot  be  doubted,  and  has  never  been  doubted,  that  the  Echinodermata  form 
a  distinct,  naturally  marked  out  phylum  of  the  animal  kingdom,  or,  in  the  language 
of  Phylogeny,  that  all  Echinoderms  have  had  a  common  racial  form. 

Within  the  phylum  of  the  Echinodermata,  further,  the  classes  are  again  quite  dis- 
tinct and  naturally  marked  off  from  one  another.  Among  known  Echinoderms, 
there  are  no  intermediate  forms  between  the  Pelmatozoa,  the  Holothurioidea,  the 
Ec/tinoidca,  the  Asteroidca,  and  the  Ophiuroidca.  Every  known  Echinoderm  can  at 
once  be  recognised  as  either  an  Echinoid,  an  Asteroid,  a  Holothurid,  etc.  The  Cystidea 
VOL.  II  2  N 


546  COMPARATIVE  ANATOMY  CHAP. 

alone,  perhaps,  form  an  exception  to  this  rule,  "showing  decided  resemblance  to 
the  Crinoids  on  the  one  hand,  with  an  oecasional  possible  approach  to  the  opposite 
extreme,  i.e.  the  Holothurioidea,  on  the  other.  It  is,  however,  very  difficult  to  judge 
of  the  Cystidea,  since  conclusions  as  to  the  inner  organisation  drawn  exclusively 
from  the  structure  of  the  skeleton  cannot  be  regarded  as  altogether  trustworthy. 

It  appears  to  us  that  there  is  not  the  least  justification  for  deducing  the  different 
Echinoderm  classes  in  any  definite  way  from  one  another,  nor  can  we  at  all  accept 
the  recently  urged  view  that  the  Holothurioidea  stand  nearest  to  the  racial  form. 
On  the  contrary,  the  morphology  of  the  genital  organs  leads  us  to  believe  that  the 
Holothurioidea  are  distinct  from  all  other  Echinoderms,  with  the  possible  exception 
of  the  Cystidea. 

If  we  review  the  whole  morphology  of  the  Echinodermata,  our  phylogenetic  specu- 
lations are,  first  of  all,  influenced  by  the  fundamental  fact  that  the  radiate,  but  at 
the  same  time  asymmetrical  Echinoderm  proceeds  ontogenetically  from  a  bi- 
laterally symmetrical  larva,  the  so-called  Dipleurula. 


The  Dipleurula  Larva. 

This  larva  is  regarded  from  two  opposite  points  of  view.  (1)  The  bilateral 
structure  is  thought  to  have  been  secondarily  acquired,  within  the  different  groups 
of  the  Echinoderms,  in  adaptation  to  the  free-swimming  manner  of  life.  (2)  The 
bilateral  structure  of  the  larva  has  been  inherited  from  the  common  racial  form 
of  the  Echinodermata,  or  from  the  larva  of  such  a  form.  The  first  view  is  now 
generally  abandoned.  The  manner  of  life  might  indeed  have  called  forth  external 
bilateral  symmetry  of  form,  but  certainly  not  the  marked  bilateral  symmetry  of 
structure  of  the  internal  organs. 

If  we  now  try  to  sketch  a  hypothetical  phylogenetic  stage  based  upon  a  com- 
parison of  the  various  Dipleurula  larva*  of  the  Echinodermata,  the  following  is  the 
result :  The  body  was  freely  movable,  ovoid,  and  bilaterally  symmetrical ;  the  mouth 
lay  anteriorly  on  the  ventral  side,  the  anus  at  the  posterior  end,  or  posteriorly  on 
the  ventral  side.  In  the  frontal  region  there  was  a  nerve  centre  below  the  surface 
of  the  ectodermal  epithelium  which  was  differentiated  into  a  sensory  organ  (neural 
plate).  Running  back  from  the  nerve  centre  along  the  ventral  side,  below  the  surface 
of  the  body  epithelium,  were  two  nerve  trunks  beset  with  ganglion  cells.  The  intes- 
tine was  divided  into  the  ectodermal(?)  oesophagus,  the  wider  endodermal  mid-gut, 
and  the  hind-gut,  which  was  also  endodermal.  At  the  sides  of  the  intestine  were 
two  pairs  of  coelomic  vesicles,  the  anterior  pairs  at  the  sides  of  the  (esophagus,  the 
posterior  at  the  sides  of  the  mid-  and  hind-guts.  The  two  anterior  coelomic  vesicles 
(or  their  posterior  portions)  were  connected  with  the  exterior  by  a  canal  laterally  or 
dorsally  (cf.  the  interesting  temporary  occurrence  of  a  hydropore  on  the  right  side  in 
Asteroids,  especially  in  Asterias  vwlgaris,  p.  527).  The  genital  products  developed 
out  of  the  endothelium  of  the  coelom. 

Such  an  organisation  has  nothing  strange  about  it.  It  has  almost  as  much  claim 
to  be  classed  with  the  Vermes  as  Sagiita  has,  for  which  latter  classification,  however, 
not  much  can  be  said.  It  is  further  possible  that  the  racial  form  possessed  special 
organs  for  locomotion,  respiration,  etc.,  about  which  nothing  can  now  be  positively 
affirmed,  since  they  have  in  all  cases  disappeared  from  the  ontogeny  of  the  Echinoderms. 
It  is  not  at  all  likely  that  the  ciliated  rings  have  any  phylogenetic  significance. 


vin  ECHIXODERMATA—PHYLOGEXY  547 


Metamorphosis  of  the  Dipleurula  Larva. 

The  larva  develops,  through  metamorphosis,  into  the  young  Echino- 
derm,  which,  under  its  radiate  mask,  is  asymmetrical.  The  radiate 
structure  is  amalgamated  with  an  asymmetrical  structure. 

Here  again,  we  are  not  altogether  without  light  as  to  the  phylo- 
genetic  significance  of  this  process.  We  agree  with  the  majority  of 
modern  authors  in  believing  that  a  radiate  structure  of  body  arises 
as  a  consequence  of  an  attached  manner  of  life.  We  are,  therefore, 
justified  in  assuming  that  the  radiate  Echinoderm  arose  from  a  free- 
moving  racial  form,  in  adaptation  to  a  newly -acquired,  attached 
manner  of  life. 

All  Eehmoderms  must,  therefore,  once  have  been  attached 
animals. 

If  now,  we  wish  to  ascertain  in  what  special  manner  attachment 
took  place,  we  unavoidably  turn  for  an  answer  to  this  special  question 
to  the  Crinoids.  These  are  the  only  Echinoderms  which,  in  all  prob- 
ability, never  again  gave  up  the  attached  manner  of  life.  That  the 
only  Crinoid  about  whose  ontogeny  we  know  anything,  Antedon,  is  a 
form  which  has  actually  once  more  become  free,  i.e.  has,  as  a.  secondary 
specialisation,  given  up  the  attached  manner  of  life,  detracts  in  no  way 
from  the  arguments  based  upon  its  ontogeny. 

All  other  Echinoderms  whose  ontogeny  we  can  investigate  have 
long  since  given  up  the  attached  manner  of  life,  and,  with  the  excep- 
tion of  certain  (analogous)  cases  among  the  Asteroids  (e.g.  Asterina\ 
do  not  any  longer  pass  through  an  attached  larval  stage.  Hence  the 
methods  of  development  of  other  Echinoderms,  even  when  simpler 
than  that  of  the  Crinoids,  must  in  comparison  with  the  latter  be  regarded, 
phylogenetic;illy,  with  some  suspicion. 

From  the  developmental  history  of  Antedon  then,  we  learn  that  the  attachment 
of  the  Dipleurula  larva  of  this  animal  took  place  by  means  of  the  ventral  side  of 
the  anterior  end  of  the  body.  In  a  similar  way  the  Dipleurula  larva  of  Aster  ina 
attaches  itself  by  means  of  the  larval  organ  which  develops  anteriorly. 

Authors  who  have  recently  attacked  this  problem  assume  that  attachment  took 
place  on  the  right  side ;  making  this  assumption  in  order  to  explain  the  asymmetry 
which  follows.  To  us  also  this  assumption  appears  necessary,  but  it  should  be 
specially  stated  that  the  attachment  took  place  on  the  right  anteriorly. 

When  this  assumption  is  made  we  must  further  ask  :  What  were  the  changes 
which  the  attached  manner  of  life  induced  ? 

It  is  difficult,  with  the  embryological  material  we  have  at  present,  to  obtain  an 
adequate  idea  of  the  resulting  processes,  and  only  a  very  tentative  explanation  can 
be  given. 

Judging  by  analogy  from  the  modifications  which,  in  other  parts  of  the  animal 
kingdom,  result  from  an  attached  manner  of  life,  it  may  be  assumed  that  the  arrange- 
ments for  conducting  food  were  the  first  to  become  adapted  to  the  new  condition  of 
existence.  The  mouth  left  its  unfavourable  position  and  wandered  along  the  ventral 
side,  first  to  the  left,  i.  e.  to  the  side  which  was  now  uppermost  (being  opposite  to  the 
point  of  attachment).  In  this  shifting  the  oesophagus  pushed  the  median  and 


548  COMPARATIVE  ANATOMY  CHAP. 

ventral  wall  of  the  left  anterior  coslom  in  front  of  it,  embedded  itself  to  a  certain 
extent  from  without  in  the  ccelomic  vesicle,  so  that  this  vesicle  surrounded  it  in  the 
shape  of  a  horse-shoe.  Round  the  mouth,  the  body  wall  (and  with  it  the  left  anterior, 
ccelomic  vesicle  which  lay  here)  grew  out  into  five  tentacles  which,  as  in  so  many 
attached  animals,  served  for  bringing  in  food,  for  the  sense  of  touch,  and  for  respira- 
tion. (Compare  the  tentacles  and  the  horse-shoe-shaped  tentacle -carrier  of  the 
Bryozoa  Cephalodiscus,  etc.)  Thus,  the  left  anterior  ccelom,  which  from  the  very 
first  was,  like  the  right,  connected  with  the  exterior  by  means  of  a  canal,  produced 
the  primary  horse- shoe-shaped  hydrocoel  with  the  primary  tentacles  and  the  hydro- 
pore  (stone  canal).  In  this  way  the  first  impulse  towards  the  development  of  the 
radiate  structure  was  given.  The  horse-shoe  finally  closed  to  form  the  circular  canal. 

The  right  anterior  side  of  the  body,  which  was  used  for  attachment,  could  be 
produced  into  a  stalk,  as  is  the  case  in  most  Pelmatozoa.  (The  larval  organ  of 
Asterina  may  be  a  modified  reminiscence  of  such  a  stalk.)  The  right  anterior 
ccelomic  vesicle,  which  lay  in  this  region,  now  serving  for  attachment,  lost  its  efferent 
aperture,  atrophied,  or  became  a  cavity  of  the  stalk  (chambered  sinus  and  its  con- 
tinuation in  the  Crinoids(1),  crelom  of  the  larval  organ  in  Asterina  (?). 

The  body  now  developed  principally  in  the  oral  and  tentacular  region  (on  the  left 
anterior  side).  The  posterior  portion  of  the  body  with  the  anus  near  its  end  was 
originally  like  a  lateral  outgrowth  or  shield  on  the  body,  which  gradually  subsided 
and  disturbed  less  and  less  the  radiate  appearance. 

According  to  this  view,  the  greater  development  of  the  anal  interradius  which  is 
found  in  many  Crinoids,  especially  in  palaeozoic  forms,  may  possibly  be  an  original 
condition,  in  connection  with  which  we  have  the  occurrence  of  special  anal  plates  in 
the  anal  interradius.  The  anus  also  may  originally  have  lain  outside  of  the  circle  of 
tentacles,  a  supposition  which  harmonises  with  its  position  in  the  Cystids  and  in 
the  ontogeny  of  Antedon. 

Concurrently  with  these  changes,  the  left  posterior  ccelom,  which  lay  nearer  than 
the  right  to  the  mouth,  which  had  shifted  to  the  left,  now  upper,  side,  grew  round 
the  oesophagus,  and  forming  a  vertical  mesentery,  became  the  oral  coelom.  The 
right  coelomic  vesicle,  however,  spread  out  chiefly  in  the  lower  (originally  the  right) 
region  of  the  body,  and  became  (also  forming  a  vertical  mesentery)  the  apical  coelom. 
The  mesentery  dividing  the  two  (oral  and  apical)  sections  of  the  ccelom  would  natur- 
ally be  horizontal  (transverse). 

In  the  vertical  mesentery,  the  rudiments  of  the  gonad  (the  axial  organ)  arose 
as  a  ridge-like  thickening  and  growth  of  the  endothelium  on  one  side  ;  in  mature 
animals,  this  opened  outward  through  a  genital  duct  and  aperture  in  the  region  be- 
tween the  mouth  and  the  anus. 

This  phyletic  stage,  deduced  as  a  result  of  the  attached  manner  of  life,  may  be 
called  the  Pentactsea. 

For  the  protection  of  the  body,  calcareous  plates  developed  in  the  mesenchyme 
below  the  integument,  at  first,  perhaps  quite  irregularly. 

From  the  Hypothetical  (unknown)  Pentactsea  Stage  to  the  known 
Echinoderm. 

Most  Echinoderms  gave  up  the  attached  manner  of  life  at  a  later  stage.  The 
known  case  of  Antedon,  in  which  an  animal  in  the  highest  degree  adapted  for  the 
attached  life  resumed  the  free  life,  is  specially  welcome  and  useful  in  this  connec- 
tion. 

The  ancestors  of  the  Holothurioidea  were  probably  the  first  to  renounce  the 
aci  ached  manner  of  life,  although  not  as  early  as  the  Pentactrea  stage. 


vin  ECHIXODEEMATA—PHYLOGENY  549 

The  organisation  of  the  Pentactrea  would  only  become  completely  adapted  to  the 
attached  manner  of  life,  when  the  number  of  tentacles  and  the  surface  for  taking  in 
nourishment  increased.  Such  increase  might  take  place  in  various  ways  ;  we  have 
many  examples  among  attached  animals  belonging  to  other  divisions  of  the  animal 
kingdom. 

The  Pentactaea  may  have  become  perfected  in  one  direction  as  follows  : — 

The  interval  between  the  bases  of  the  primary  tentacles  and  the  mouth  increased, 
while  the  (apical)  interval  between  the  primary  tentacles  and  the  attached  pole 
remained  the  same 'or  decreased.  By  the  shifting  of  the  circle  of  tentacles  away 
from  the  mouth,  the  basal  piece  of  each  tentacle  canal  would  be  drawn  out  radially 
below  the  oral  body  wall,  and  would  become  a  radial  canal,  from  which  new  lateral 
tentacle  canals  would  bud  out  alternately  to  right  and  left,  always  proximally  to 
the  shifting  primary  (now  terminal)  tentacle,  the  body  wall  projecting  in  the  form 
of  tentacles.  There  thus  arose  in  each  radius  a  double  row  of  tentacles,  which,  to 
speak  exactly,  stood  in  the  corners  of  a  zigzag  line.  Nutritive  particles,  descending, 
would  be  sent  on  to  the  mouth  between  these  rows  of  tentacles.  This  adaptation 
became  perfected  when  the  floors  between  these  rows  of  tentacles  sank  in  the  form  of 
furrows — the  food  grooves  or  ambulacral  furrows  ;  these  furrows  then  became  pro- 
vided with  means  of  transport,  in  this  case  with  cilia.  The  epithelial  cells  lining 
the  furrows  gradually  became  sensory  cells,  and  epithelial  nerve  ridges, — the  radial 
nerves, — would  arise,  which  would  meet  round  the  mouth  as  the  nerve  ring. 

This  rise  of  the  radial  nervous  system  of  the  Echinodermata  as  a  natural 
development  of  the  ambulacral  furrows,  and  of  the  palisades  of  sensory  tentacles  border- 
ing these  furrows  on  each  side,  may  be  without  difficulty  followed  out  in  detail, 
although  there  is  no  space  for  such  an  attempt  in  this  volume. 

At  this  stage  are  found  the  armless  Cystids  with  their  carapace  of  plates. 
The  genital  organ  is  enclosed  in  the  body,  and  opens  outward  through  a  single 
aperture  (the  "  third  "  aperture  of  the  Cijstidea). 

It  was,  perhaps,  at  a  similar  phyletic  stage  that  the  ancestors  of  the  Holothuri- 
oidca  gave  up  the  attached  manner  of  life.  For  locomotion,  they  used  the  tentacles, 
arranged  in  five  meridional  double  rows,  the  body  elongated,  and  the  anus  shifted 
to  the  apical  end,  which  became  free.  Food  was  taken  in  directly  at  the  mouth  ;  to 
assist  in  alimentation,  the  oesophagus  became  modified  into  a  pharynx,  and  the  ten- 
tacles lying  close  to  the  mouth  became  specialised.  The  food  grooves  lost  their 
function,  leaving,  however,  behind  them  the  epithelial  nerve  ridges,  which  continued 
to  innervate  the  tube-feet.  The  food  grooves  could  now  close  over  to  form  a  tube 
for  the  protection  of  the  nerve  ridges  ;  these  latter  became  the  subepithelial  radial 
nerves,  and  the  lumina  of  the  closed  tubes  the  epineural  canals.  According  to  this 
view,  therefore  (and  this  applies  also  to  the  Ophiuroidea  and  Echinoidea),  the  sub- 
epithelial  radial  nerves,  with  their  epineural  canals,  are  the  original  food  grooves 
closed  over  to  form  tubes.  They  gave  up  their  original  function  as  "  food  grooves  " 
in  proportion  as,  with  the  adoption  of  a  free  manner  of  life,  food  was  taken  in  direct 
at  the  mouth. 

It  is  a  question  of  subsidiary  importance,  in  the  derivation  of  the  Holothurid 
body,  which  still  possesses  the  single  genital  organ  and  the  single  aperture,  whether 
the  condition  of  its  skeleton  is  to  be  regarded  as  original,  or  whether  it  has  not  rather 
been  derived  from  the  carapace  of  plates  of  a  Cystid-like  animal  by  means  of  the 
multiplication  of  the  skeletal  pieces,  their  loose  arrangement,  and  their  decrease 
in  size. 

The  longitudinal  and  circular  musculature  may  be  new,  but  may  just  as  well  have 
been  inherited  from  an  attached  ancestral  stage,  in  which  they  could  be  functional, 
just  as  are  the  longitudinal  and  circular  muscles  of  Actinia. 

After  the  foregoing  description  it  is  obvious  that  the  Paractinopoda  (Synaptidce] 


550  COMPARATIVE  ANATOMY  CHAP. 

cannot  be  regarded  as  primitive  forms  of  Holothurioidea.  They  are,  on  the  contrary, 
highly  specialised  forms,  which,  in  adaptation  to  the  limicolous  life,  have  lost  the 
tube-feet  and  the  radial  canals,  which,  however,  still  occur  ontogenetically. 

We  have  thus  seen  how  the  Pentactrea  might  become  modified  in  the  direction 
of  certain  Cystids  and  of  the  Holothurioidea. 

By  remaining  attached,  the  Pentactsea  might  develop  in  another  direction. 

The  body  carried  by  the  stalk  might  remain  small,  but  become  drawn  out  into 
processes  or  arms  in  the  directions  in  which  the  primary  tentacles  travel  from  the 
mouth,  the  ends  of  these  arms  being  always  marked  by  the  possession  of  the  primary 
tentacles.  Secondary  tentacles  then  rose  out  of  the  radial  vessels  which  ran  along 
the  arms  (the  tentacle  canals  of  the  primary  tentacles)  in  the  way  above  described  ; 
the  food  grooves  and  their  nerve  ridges  also  fovmed  in  the  same  way.  A  still  more 
complete  adaptation  to  the  attached  manner  of  life  was  attained  by  the  branching  of 
the  arms  and  the  formation  of  pinnulre.  In  this  way  the  surface  for  capturing  food 
was  continually  increased. 

The  direction  of  adaptation  here  indicated  might  be  called  the  Crinoid  direction, 
the  Crinoids  having,  in  fact,  gone  furthest  in  this  direction. 

The  development  of  the  crown  of  arms  on  a  body  which  remained  small  had 
necessary  consequences.  The  body  (calyx,  disc)  and  the  stalk  (should  this  latter 
develop)  would  have  to  gain  the  necessary  stability  for  carrying  the  growing  arms. 
This  was  provided  for  by  the  formation  of  the  more  or  less  firm  carapace  of  plates. 
The  stalk  attained  firmness  by  the  development  of  joints  ;  the  calyx,  by  that  of  the 
dorsal  cup,  and  here  all  the  facts  seem  to  indicate  that  in  the  racial  form  of 
the  Crinoids,  the  dorsal  cup  had  a  definite  composition,  viz.  five  infrabasals,  five 
basals,  five  radials  arranged  in  the  typical  manner,  and  the  anals.  For  the 
protection  of  the  mouth,  five  orals  were  added,  forming  together  a  pyramid  which 
could  be  opened  and  closed.  For  the  support  of  the  arms,  and  in  connection  with 
the  developing  capacity  for  unfolding  and  closing  the  crown  of  arms,  the  jointed 
brachial  skeleton  formed. 

As  the  arms  grew  out  from  the  small  body,  the  ccelom  was  produced  into  them, 
and  processes  of  the  single  rudiment  of  the  gonad  (the  axial  organ)  spread  in  one 
way  or  another  into  them.  They  became  fertile  more  or  less  far  from  the  calyx  (or 
disc)  and  yielded  the  gonadial  bundles,  each  of  which  opened  outwards  through  one 
or  more  special  apertures. 

In  this  point  also,  the  Crinoids  are  the  most  extreme  forms. 

The  Echinoidea,  Ophiuroidea,  and  Asteroidea  appear  also  to  belong  as  lateral 
branches  to  this  Crinoid  development. 

First,  and  probably  very  early,  the  Echinoids  seem  to  have  branched  off.  They 
became  free,  used  their  tentacles  for  locomotion,  and  took  in  food  direct  through  the 
mouth,  the  food  grooves,  with  the  nerve  ridges,  becoming  the  subepithelial  radial 
nerves.  The  arms  were  again  incorporated  into  the  enlarging  calyx,  or  test ;  in  that 
the  apical  skeleton  of  the  arms  degenerated,  and  thus  brought  the  (ambulacral)  ends 
of  the  arms  close  up  to  the  continually  decreasing  apical  capsule.  As  this  latter 
was  free,  the  anus  could  shift  into  its  centre. 

We  have  come  to  hold  this  view  of  the  derivation  of  the  Echinoidea  from  attached 
ancestral  forms  with  arms,  a  view  which,  as  far  as  we  know,  has  never  before  been 
published,  chiefly  for  the  following  reasons  : — 

The  Echinoidea  possess  five  pairs  of  gonads,  which  are  at  first  connected  with  the 
axial  organ  by  means  of  an  aboral  circular  strand. 

This  important  distinction  from  the  Holothurioidea,  with  which  the  Echinoidea 
are  usually  compared  in  other  points,  can  only  be  explained  by  the  assumption  that 
th<i  Echinoidea  originally  possessed,  arms  which  contained  the  fertile  outgrowths  of 
the  central  genital  rudiment.  That  the  gonads  now  lie  interradially,  presents  no 


vni  EL'HIXODERMA  TA—PHYLOGEXY  551 

difficulty.  '  They  could  easily  have  been  in  the  short  thick  arms,  while  the  terminal 
portions  of  their  efferent  ducts  opened  outward  interradially. 

The  Ophiuroidea  branched  off  from  the  series  leading  to  the  Crinoids  by  the 
readoption  of  the  free  manner  of  life  later  than  the  Echinoids.  They  used  the 
free  arms  for  locomotion,  and  took  food  direct  into  the  mouth.  The  tentacles  never 
became  ambulatory  tube-feet,  but  only  retained  their  respiratory  functions.  The 
food  grooves  closed  to  form  tubes,  becoming  subepithelial  radial  nerves  with  their 
epineural  canals.  For  further  protection,  the  radial  longitudinal  rows  of  ventral 
shields  became  arranged  over  the  closing  food  grooves.  The  use  of  the  arms  almost 
exclusively  as  locomotory  organs  determined  their  slender  form,  which  makes  them 
appear  as  mere  appendages  of  the  body,  and  it  further  led  to  the  return  of  the 
gonads  into  the  disc. 

The  Asteroidea  were  the  last  to  branch  off  from  the  series  of  attached  Echino- 
derms  with  arms,  by  the  adoption  of  a  free  manner  of  life.  They  used  their  tube- 
feet  first  for  locomotion,  and  secondly  for  seizing  and  holding  fast  prey,  which  was 
enveloped  en  inasse  direct  by  the  evaginated  oral  portion  of  the  intestine  and  drawn 
through  the  mouth  into  the  stomach.  The  food  grooves  now  no  longer  serve  as 
such,  but  are  retained  as  deepened  ambulacral  furrows,  from  the  bases  of  which  the 
closely  crowded  tube-feet  rise,  and  into  which  they  can  withdraw.  Over  the 
tentacles,  withdrawn  within  the  furrow,  the  spines  which  border  the  furrow  can 
bend  protectively  together.  Deep  in  the  base  of  the  furrow,  the  radial  nerve  ridge 
is  found  still  in  its  epithelial  position. 

From  the  standpoint  of  the  foregoing  it  is  to  be  expected  that  the  ontogeny  of 
the  Holothurioidea,  which  earliest  gave  up  the  attached  life,  should  show  the  least 
trace  of  the  phyletic  stage  of  the  attached  animal,  and  that  by  the  avoidance  of  those 
complicated  rearrangements  which  attachment  caused,  their  development  should  be 
much  simplified.  The  facts  agree  with  this  expectation,  and  also  with  the  expecta- 
tion that  an  attached  stage  is  most  likely  to  be  ontogenetically  repeated  in  the 
•'•dds  (cf.  the  ontogeny  of  Astcrina,  with  its  temporary  fixation  by  means  of  the 
anterior  part  of  the  body,  the  larval  organ). 

In  the  foregoing  attempt  to  trace  the  phylogeny  of  the  Echinoderms,  we  have 
avoided  going  into  details,  and  we  have  also  avoided  all  reference  to  many 
important  points,  such  as  the  hydropore  and  the  stone  canal,  the  hydroccel  and  the 
left  anterior  enterocoel,  etc.  etc.  These  can  only  be  elucidated  by  renewed  research, 
which  must  be  both  extensive  and  thorough.  In  applying  our  views  to  explain 
special  points  in  F>chinoderm  morphology,  it  must  be  acknowledged  that  in  the 
majority  of  cases  it  does  not  suffice  for  a  full  explanation,  and  that  it  cannot  indeed 
at  present  be  reconciled  with  many  ontogenetical  and  anatomical  facts.  The  recent 
researches  in  Echinoderm  morphology  and  the  attempts  at  phylogenetic  explanations, 
which  are  continually  suggesting  new  points  of  view,  justify  us,  however,  in  hoping 
that,  little  by  little,  many  of  these  interesting  and  important  problems  will  receive 
a  satisfactory  solution. 


Review  of  the  most  important  Literature. 

Comprehensive  Works.      Text  Books.     Treatises  of  Wider  Scope.     Re- 
searches extending  over  some  or  all  of  the  Classes. 

A.  Agassiz.  Paleamtoloffieal  <>i>d  .••,,il>njolo</L<*al  development.  Address  before  the 
American  Association  for  the  Advancement  of  Science.  Boston  Meeting.  Cam- 
bridge, 1880.  Also  Ann.  and  Meg.  JVitf.  Hist.  (5).  Vol.  VI.  1880. 


552  COMPARATIVE  ANATOMY  CHAP. 

E.  Baudelot.     Contributions  &  Vhistoire  du  systeme  nerveux  des  Echinodermes.     Arch. 

deZool.  Exp.  (1).     Vol.1.     1872. 
P.  H.  Carpenter.     Several  important  treatises  especially  on  the  Skeletal  System,  from 

1870-1890,  in  various  Journals,  principally  in  Quart.  Journ.  Microsc.  Science, 

and  in  the  Ann.  and  Mag.  Nat.  Hist. 
L.  Cuenot.     Etudes  sur  le  sang,  son  r6le  et  sa  formation  dans  la  seric  animale.     Part 

II.     Invertebres.     Archives  Zoologie  exptrimentale  (2).     Tome  IX.     1891. 

Etudes  morphologiques  sur  les  Echinodermes.     Arch.   Biologic  (van  Benedcn}. 

Tome  XL     1891. 

Herb.  E.  Durham.  On  wandering  cells  in  Echinoderms,  etc.,  more  especially  with 
regard  to  excretory  functions.  Quart.  Journ.  Mic.  Sci.  Vol.  XXXIII.  1892. 

Jobs.  Frenzel.  Beitrdge  zur  vergleichenden  Physiologic  und  Histologie  der  Vcr- 
dauung.  I.  Mitth.  Der  Darmkanal  der  Echinodermen.  Arch.  f.  Anat.  u. 
Physiol.  Physiol.  AUh.  1892. 

Greeff.  Ueber  den  Bau  der  Echinodermen.  Sitzungsber.  d.  Gesellsch.  f.  Natunriss. 
zu  Marburg.  5  Parts.  1871,  1872,  1876,  1879. 

0.  Hamann.  Die  wandernden  Urkeimzellen  und  ihre  Rcifungsstdtten  bei  den  Echino- 
dermen. Zeitschr.  f.  wiss.  Zool.  46  Bd.  1887. 

M.  M.  Hartog.  The  true  nature  of  the  madreporic  system  of  Echinodermata,  wit  It 
remarks  on  Nephridia.  Ann.  and  Mag.  Nat.  Hist.  (5).  Vol.  XX.  1887. 

Herapath.  On  the  pedicellarice  of  the  Echinodermata.  Quart.  Journ.  Microsc. 
Science.  Vol.  V.  1865. 

Carl  Jickeli.     Ueber  den  Bau  der  Echinodermen.     Zool.  Anz.     7  Jahrg.     1884. 

A.  Kowalevsky.  Zur  Kenntniss  der  Excretionsorgane.    Biol.  Centralblatt.  9  Bd.   1889. 

H.  Ludwig.  Morphologische  Studien  an  Echinodermen.  Leipzig,  1877-1882. 
Reprints  from  the  Zeitschr.  f.  wiss.  Zool. 

Bronn's  Klassen  und  Ordnungen  des  Thierreichs.      2  Bd.,   3  Abth.   Echino- 
dermen.    Holothurioidea.     Asteroidea  in  progress. 

Joh.  Miiller.     Ueber  den  Bau  der  Echinodermen.     Berlin,  1854.     Abhandl.  Akad. 

Wissensch.     Berlin,  1853. 
M.    Neumayr.      Morphologische   Studien  iiber  fossile  Echinodermen.      Sitzungsber. 

Akad.  Wissensch.  Wien.  mathem.-naturw.  Gl.     84  Bd.     1  Abth.     1881. 

Die  Stdmme  des  Thierreiches,  etc.     1888. 

E.    Perrier.     Reclierches    sur    les  pedicellaires  et   les    ambulacres    des    Asteries    ct 

des    Oursins.      Annales    des    Sciences    natur.    (5).      Tomes    XII.    and    XIII. 

1869-1870. 

Echinodermes.     Part  L     Stellerides.     Exped.  du  Travailleur  et  du  Talisman. 

Paris.     1895. 

G.  J.  Romanes  and  J.  E.  Ewart.  Observations  on  the  locomotor  system  of  Echino- 
dermata. Proceed.  Roy.  Soc.  London.  Vol.  XXXII.  1881.  Philos.  Trans- 
actions London.  1881.  Part  III. 

C.  F.  and  P.  B.  Sarasin.  Ueber  die  Anatomic  der  Echinothuriden  und  die  Phylo- 
genie  der  Echinodermen.  Ergebnisse  nat.  Forschungen  Ceylon.  1  Bd.  1888. 

R.    Semon.     Die  Homologien  innerhalb  des  Echinodermenstammes.     Morph.    Jahr- 

bmh.     15  Bd.     1889. 

— Die  Entwickelung  der  Syna,pta  digitata  und  die  Stammesgeschichte  der  Echino- 
dermen.    JenaiscJie  Zeitschr.  f.  Naturwiss.     22  Bd.     1888. 

W.  P.  Sladen.  On  the  homologies  of  the  primary  larval  plates  in  the  test  of  Brachiate 
Echinoderms.  Quart.  Journ.  Microsc.  Science  (2).  Vol.  XXIV.  1884. 

Friedr.  Tiedemann.  Anatomic  der  Rohrenholothurie,  des  pomeranzenfarbigen 
Seesterns  und  des  Stein- Seeigels.  Landshut,  1816. 

K  A.  Zittel.  Handbuch  der  Paldontologie.  1  Bd.  Miinclien  and  Leipzig,  1876- 
1880. 


vni  EGHINODERMATA— LITERATURE  553 


Principal  Systematic  Works  and  Treatises  specially  dealing  with  the 
Morphology  of  the  Skeletal  System.     Pedicellaria. 

A.  Agassiz.  Revision  of  the  Echini  (Illustrated  Catalogue  of  the  Museum  of  Com- 
parative Zoology  at  Harvard  College.  No.  VII.).  Cambridge,  Mass.,  1872-1874. 

Xorth  American  Starfishes  (Mem.  of  the  Museum  of  Cornp.  Zool.  Vol.  V.  No.  I). 

Cambridge,  Mass.,  1877. 

—  Report   on    the   Echinoidea.      Report  on  the  scient.    results  of  the  voyage   of 
H. M.S.  "Challenger,"  Zool.     Vol.111.     Part  IX.     1881. 

-  Report  on  the  Echini  (Results  of  dredging  by  the  "Blake"  XXIV.     Part  I.). 
Memoirs  of  the  Museum  of  Comparative  Zoology  at  Harvard  College.     Vol.  X. 
No.  I.     Cambridge,  Mass.,  1883. 

F.  A.  Bather.     British  fossil  Crinoids.     A  series  in  progress  in  Ann.  and  Mag.  Nat. 

Hist,  beginning  (6).     Vol.  V.     1890. 
The  Crinoidea  of  Gotland.     Part  I.    The  Crinoidea  inadunata.    Kongl.  Svenska 

Vetcnskaps-Akademiens  Handlingar.     Bandet  25.     Stockholm,  1893. 
J.  F.  Brandt.     Prodromv.s  description-is  animalium  ab  H.  Mertensio  in  orbis  terrarum 

eit'cu//inacigatione  obscrvator-um.     Fasc.  1.     Petropoli,  1835. 
Leopold  von  Buch.     Ueber  Cystideen.     Abhandl.  Berl.  Akad.     1844. 
P.  H.  Carpenter.     Report  on  the  Crinoidea.     I.   The  Stalked  Crinoids.      Voyage  of 

the  "  Challenger ."    Vol.  XI.    Part  XXX II.    London,  1884.    II.    The  Comatulce. 

Ibid.     Vol.  XXVI.     Part  LX.     1888. 

A  Series  of  important  works,  chiefly  on  the  Morphology  of  the  Skeletal  System, 

ain'iearing  from   1870-1890    in   English  journals,  especially   Quart.    Journ.   of 
Microsc.  Science,  and  Annals  and  Magazine  of  Natural  History. 

G.  Cotteau.     Echinides.     Paleontologie  fran^aise.    Vols.  VII.,  IX.,  X.    Paris,  1862- 

1879. 

D.  C.  Danielssen  and  Joh.  Koren.     Hoi 'othurio idea.     The  Noncegian  North  Atlant;,- 

Expedition.     1876-1878.     VI.     Christiania,  1882. 

E.  Desor.     Synopsis  des  Echi aides  fossiles.     Paris  and  Wiesbaden,  1858. 

F.  Dujardin.     Eecherches  sur  la  Comatule  de  la  Mediterranee.      L'lnstitut.      Tome 

III.     1835. 

P.  M.  Duncan.  A  revision  of  the  genera  and  great  groups  of  the  Echinoidea. 
Journ.  Linn.  Soc.  London'.  Vol.  XXIII.  1889. 

P.  M.  Duncan  and  W.  Percy  Sladen.  A  memoir  on  the  Echinodermata  of  the  arctic 
sea  to  the  west  of  Greenland.  London,  1881. 

R.  Etheridge  jun.  and  P.  H.  Carpenter.  Catalogue  of  the  Blastoidea  in  the  Geo- 
logical Department  of  the  British  Museum  (Xatural  History] ,  ivith  an  account  of 
the  morphology  and  systematic  position  of  the  group  and  a  revision  of  the  genera 
and  species.  London,  1886. 

J.  W.  Fewkes.  On  the  serial  relationship  of  the  ambulacral  and  adambulacral  Cal- 
"iis  Plates  of  the  Starfishes.  Proceed.  Boston  Soc.  X.  H.  Vol.  XXIV. 
1890.  Primary  spines  of  Echinoderms.  Ibid. 

-  ' >ii  the  d^vi-lnpni'  nt  of  the  Calcareous  Plates  of  Asterias.     Bull.  Mus.  Harvard 
Coll     Vol.  XVII.     1888. 

—  On  the  development  of  the  Calcareous  Plates  of  Amphiura.     Bull.  Mus.  Comp. 
Zool.     Vol.  XIII.     No.  IV.     1887. 

Alex.    Foettinger.     Sur   la  structure  des  pedicellaires    gemmiformes    d'tfchinides. 

Arch,  de  BioL      Vol.  II.     1881. 
E.  Forbes.     A  history  of  British  Starfishes  and  other  animals  of  the  class  Echino- 

fhrmata.     London,  1841. 
E.  Fraas.     Die  Asterien  des  u-cissen  Jura  von  Schwaben  und  Franken,  rait  Unt<:r- 


554  COMPARATIVE  ANATOMY  CHAP. 

suchungen  iiber  die  Structur  der  Echinodermen  und  das  Kalkgeriist  der  Aslerien. 

Palaeontogr.     32  Bd.     1886. 
Wilhelm  Giesbrecht.     Derfeinere  Ban  der  Seeigelzdhne.     Morph.  Jahrbuch.     6  Bd. 

1880. 
J.  E.  Gray.     Synopsis  of  the  species  of  Starfishes  in  the  British  Museum.     London, 

1866. 

G.  Hambach.     Contributions  to  the  anatomy  of  the  genus  Pentremites,  with  descrip- 
tion of  new  species.     Trans,   of  the  Acad.  of  Science  of  St.  Louis.     Vol.   IV. 

No.  I.     1880. 
Clem.    Hartlaub.      Beitrag  zur  Kenntniss  der    Comatulidenfauna    dcs    Indischen 

Archipels.      Nova  Ada  Acad.   Cces.  Leop.-Carol.    Germ.   Nat.   Cur.      58    Bd. 

No.  1.     1891. 

C.  Heller.     Die  Zoophyten  und  Echinodermen  des  Adriatischen  Mceres.     Wien,  1868. 
0.  Jaekel.     Beitrage  zur  Kenntniss  der  Palceozoischen  Crinoiden  Deutschlands.     Pal. 

Abh.     New  Series.     III.     1895. 
K.   Lampert.     Die  Seewalzen.      Reisen  im  Archipel  der  Philippinen  von  Dr.   C. 

Semper.     2  Th.   Wiss.  Resultate.     4  Bd.     3  Abth.     Wiesbaden,  1885. 
A.  Ljungman.     Ophiuridea  viventia  hue  usque  cognita.     Stockholm,  1867. 
P.  de  Loriol.     Echinologie  helvetique.     L,  II.,  III.     Geneva,  1868-1875. 
—  Monographic  des  Crinoides  fossiles  de  la  Suisse.     Geneva,  1877-1879. 

Paleontologie  franqaise.      Terrain  jurassique.     Tome  XL      Crinoides.     Part  I. 

1882-1884.     Part  II.     1884-1889. 

S.   Loven.     fitudes  sur  les  fichinoidees.     K.    Svensk.   Vet. -Akad.  Handl.     11   Bd. 
(1873-75).     Stockholm,  1874. 

On  Pourtalesia,  a  genus  of  Echinoidca.     K.  Svensk.  Vet. -AJcad.  Handl.    19  Bd. 

1884. 

Echinologica.      Bihang  till  K.  Svensk.    Vet.-Akad.  Handl.    XVIII.     Afd.    4. 

1892. 

Chr.  Fr.  Liitken.     Additamenta  ad  historiam  Ophiuridarum.     Copenhagen,  1858- 

1869. 
Hubert  Ludwig.     Trichaster  clegans.     Zeitschr.  f.  wissensch.  Zool.     31  Bd.     1878. 

Zur  Kenntniss  der  Gatlung  Brisinga.     Ibid. 

— •  Das  Mundskelct  der  Asterien  und  Ophiuren.     Ibid.     32  Bd.     1879. 

Ueber  den  primaren  Steinkanal  der  Crinoideen  nebst  vergleichend-anatomischen 

Bemerkungen.     Ibid.     34  Bd.     1880. 

•  Zur  Entwickelungsgeschichte  des  Ophiurenskeletes.     Ibid.     36  Bd.     1882. 

•  Ophiopteron  elegans,  eine  neue,  wahrscJieinlich  schivimmende  Ophiuridenform. 

Ibid.     47  Bd.     1888. 

Ankyroderma  musculus  (Hiss.),  eine  Molpadiide  des  Mittelmeeres,  nebst  Bemer- 
kungen zur  Phylogenie  und  Systematik  der  Holothurien.     Ibid.     51  Bd.     1891. 

Holothuroidea  of  the  (< Albatross"  Expedition.     Mem.  Mus.     Harvard.     XArII. 

3.     1894. 

Th.   Lyman.      Ophiuridce  and  Astrophytidce.     Illustr.  Catalogue  of  the  Museum  of 

Comp.  Zool.  Harvard  College.     I.     Cambridge,  Mass.,  1865. 
Report  on  the   Ophiuridea.     Report  on   the  scientific  results  of  the   voyage   of 

H.M.S.  "CJiallenger."     Vol.  V.     Part  XIV.     London,  1882. 
Meyer.     Ueber  die  Laterne  des  Aristoteles.     Arch.  f.  Anat.  u.  Physiol.     1849. 
J.  S.  Miller.     A  natural  history  of  the  Crinoidea  or  lily -shaped  animals.     Bristol, 

1821. 
Job.  Miiller.     Ueber  den  Bau  des  Pentacrinus  caput  Medusce.     Abhandl.  d.  Akad.  d. 

Wissensch.     Berlin,  1841. 

Ueber  die  Gattung  Comatula  Lam.  und  Hire  Arten.     Abhandl.  d.  Akad.  d. 

Wissensch.     Berlin,  1847. 


vin  ECHINODERMATA— LITERATURE  555 

Job.  Miiller  and  Fr.  H.  Troschel.     System  der  Asteriden.     Braunschweig,  1842. 
Edm.  Perrier.      Observations  sur  les  relations  qui  existent  entre  les  dispositions  des 

pores  ambulacraires  a  TexUrieur  eta  Vintericur  du  test  des  JZchinides  reguliers. 

JVtwr.  Arch.  Museum.     Tome  V.     1869. 
Edm.  Perrier.     Recherches  sur  les  pedicellaires  et  les  ambulacres  des  Asteries  et  des 

Ov.rsins.     A/males  des  Sciences  natur.  (5).     Vols.  XII.  and  XIII.     1869-1870. 

Revision  de  la  collection  de  StelUrides  du  Museum  d'histoire  naturelle  de  Paris. 

Paris,  1875-1876. 

Memoire  sur  les  etoilesde  mej\  recueillies  dans  la  mer  des  Antilles  et  le  golfe  du 

M'.rique.     Paris,  1884. 

H.  Prouho.     Recherches  sur  le  Dorocidaris  papillata  et  quelques  autres  Bchinides  de 
la  mediterranee.     Arch,  de  Zool.  exptr.  (2).     Tome  V.     1887-1888. 

F.  A.  Quenstedt.     Petrefactenkunde  Deutschlands.      3   Bd.      Echiniden.     Leipzig, 

1872-1875. 

Ferd.  Rb'mer.     Monographic  der  fossilen  Crinoideenfamilie  der  Blastoideen.     Arch. 
f.  Xaturfjcschichte.     1851. 

G.  0.  Sars.     Memoire  pour  servir  a  la  connaissance  des  Crinoides  vivants.     Chris- 

tiania,  1868. 

—  Researches  on  the  structure,  etc.,  of  the  genus  Brisinga.     Christiania,  1875. 
M.  Sars.     Bidrag  til  Kundskaben  om  Middelhavets  Littoral- Fauna.      Christiania, 
1857. 

Oversiyt  of  Xorges  Echinodermer.     Christiania,  1861. 

E.  Selenka.     Bcitrdcje  zur  Anatomic  und  Systematik  der  Holothurien.     Zeitschr.  f. 

wissensch.  Zool.     17  and  18  Bd.     1867,  1868. 
C.  Semper.     Reisen  im  Archipel  der  PMlippinen.     1  Bd.     Holothuricn.     Leipzig, 

1868. 

W.  Percy  Sladen.     On  a  remarkable  form  of  Pedicellaria,  and  the  functions  per- 
formed thereby,  etc.     Annals  and  Mag.  of  Xat.  History  (5).     Vol.  VI.     1880. 

On  the  homologies  of  the  primary  larval  plates  in  the  test  of  Brachiatc  Echino- 

derms.      Quart.  Journ.  Microsc.  Science  (2).     Vol.  XXIV.     18"84. 

Report  upon   the  Asteroidea  collected  by  H.M.S.  "Challenger."      Vol.   XXX. 

1889. 

Hj.   Theel.       Report    on    the   Holothurioiclea  collected   during    the    voyage    of    the 
"  Challenger."     Parti.     Vol.  IV.     1882.     Part  II.     Vol.  XIV.     1886. 

On  the  formation  and  resorption  of  the  skeleton  in  the  Echinoderms.     Ofv.  Ak. 

Fork.     1894. 

Volborth.     Ueber  Achradocystitcs  und  C ystoblastus,  zwei  neu-e  Crinoiden-Gattungen. 

Mem.  Acad.  Imp.  Sc.  St.  Petersbourg.     1870.     Tome  XVI.     No.  II. 
C.  Viguier.     Anatomic  comparee  du  squdettc  des  Stclleridcs.     Arch.  Zool.  experim. 

Tome  VII.     1879. 
C.  Wachsmuth  and  F.  Springer.     Revision  of  the  Palceocrinoidca.     Proceed.  Acad. 

Xat.  Sc.  of  Philadelphia.     1879,  1881,  1885. 
—  (1)    Discovery  of  the  ventral  structure  of  Taxocrinu-s  and  Haplocrinus,  and 

consequent  modifications  in  the  classification  of  the  Crinoidea.     Proceed.  Acad. 

Xnt.  Science  Philadelphia.     1889. 

(2)  Crotalocrinus :  its  structure  and  zoological  position.     Ibid. 

-  The  perisomic  plates  of  the  Crinoids.     Pro.  Acad.  Nat.  Sc.  Philadelphia.     1890. 
T.  Wright.     Monograph  of  the  British  fossil  Echinodcrmata  of  the  bolithic  forma- 
tion.    London,  1857-1880. 

Monograph  of  the   British  fossil  Echinodermata  of  the  cretaceous  formation. 

London,  1864-1882. 
K.  A.  Zittel.     Handbv.ch  der  Paldontologie.     1  Bd.     1876-1880. 


556  COMPARATIVE  ANATOMY  CHAP. 

Anatomical  Monographs. 

A.  Agassiz.     Revision  of  the  Echini.     Mus.  Compar.  Anatomy  Harvard  Coll.     Vol. 

VII.     1872. 
North  American  Starfishes  (Mem.  of  the  Mus.  of  Comp.  Zool.     Vol.  V.     No. 

I.).     Cambridge,  Mass.,  1877. 

Report    on    the  Echinoidea    collected    by  H.M.S.    "Challenger"      Vol.    III. 

London,  1881.     (Also  contains  anatomical  details.) 

Nic.  Christo  Apostolides.     Anatomie  et  developpement  des  Ophiures.     Arch.  Zool. 

exper.  gtner.     Vol.  X.     1882. 
Alb.  Baur.     Beitrdge  zur  Naturgcschichte  der  Synapta  digitata.     Ada.  Acad.  Gees. 

Leop.-Carol.  Nat.  Curios.     1864. 
P.  H.  Carpenter.     Report  upon  the  Crinoidea  collected  during  the  voyage  of  H.M.S. 

"  Challenger"  during  the  years  1873-1876.     Part  1.     General  Morphology,  with 

descriptions  of  the  stalked  Crinoids.     Vol.  XI.     1884.     Part  2.     The  Comatulce. 

Ibid.     Vol.  XXVI.     1888.     (The  second  part  is  almost  exclusively  systematic 

and  descriptive.) 
William  B.  Carpenter.     Rcsearclies  on  the  structure,  physiology,  and  development  of 

Antedon  rosaceus.      Philos.    Transactions.      Vol.    CLVI.      1866.     Addendum. 

Pro.  R.  Soc.     Vol.  XXIV.     1876. 
L.    Cuenot.     Contribution  a  I'etude  anatomique  des  Asterides.     Arch.  Zool.  experim. 

(2).     Tome  V.     Supplementary  (1887-1890). 

Etudes  anatomiques  et  morphologiques  sur  les  Ophiures.     Arch.   Zool.  exper. 

(2).     Tome  VI.     1888. 

D.  C.  Danielssen  and  J.  Korea.  Holothurioidea.  The  Norwegian  North  Atlantic 
Expedition.  1876-1878.  Christiania,  1882. 

Fredericq.  Contributions  a  V etude  des  Echinides.  Arch,  de  Zool.  experim.  Tome 
V.  1876. 

0.  Hamann.  Beitrdge  zur  Histologie  der  Echinodermen.  1.  Heft.  Die  Holothurien. 
Jena,  1884.  2.  Heft.  Die  Asteriden.  Jena,  1885.  3.  Heft.  Anatomie  und 
Histologie  der  Echiniden  und  Spatangiden.  Jena,  1887.  4.  Heft.  Anatomie 
und  Histologie  der  Ophiuren  und  Crinoiden.  Jena,  1889.  (Reprints  from  the 
Jenaischen  Zeitschr.  f.  Naturwiss. ) 

Beitrdge  zur  Histologie  der  Echinodermen.     1.  Die  Holothurien  (Pedata)  und 

das  Nervensystem  der  Asteriden.     Zeitschr.  f.  wiss.  Zool.     39  Bd.     1883. 

6.  Herouard.     Recherches  sur  les   Holothuries  des  c6tes   de   France.     Arch.    Zool. 

exper.  (2).     Tome  VII.     1889. 
C.  F.  Heusinger.     Anatomische  Untersuchung  der  Comatula  mediterranea.     Zeitschr. 

f.  organische  Physik.     Tome  III.     1828. 
C.   K.   Hoffmann.     Zur  Anatomie  der  Echinen  und  Spatangcn.     Niederl.    Arch. 

Zool.     1  Bd.     1871. 

—  Zur  Anatomie  der  Asteriden.     Niederl.  Arch.  f.  Zool.     2  Bd.     1874. 
G.  F.  Jaeger.     De  Holothuriis.     Diss.  inaug.     Zurich,  1833. 
Et.    Jourdan.      Reclierches  sur  Vhistologie  des    Holothuries.      Ann.    Mus.    H.    N. 

Marseilles.     Tome  I.     1883. 
R.  Kbhler.     Reclicrches  sur  les  Ecliinidcs  des  cdtes  de  Provence.     Ann.  Mus.  H.  N. 

Marseilles.     Tome  I.     1883. 
W.   Lange.     Beitrag  zur  Anatomie  und  Histologie  der  Asterien  und   Ophiuren. 

Morph.  Jahrbuch.     2  Bd.     1876. 

Leydig.     Anatomische  Notizen  ilber  Synapta  digitata.     Mailer's  Arch.     1852. 
Loven.     EtudcssurlesEchinoid.es.     Svensk.  Vetensk.-Akad.     11  Bd.     1874. 
H.  Lvdwig.     Br.it rage  zur  Kenntniss  der  Holothurien.    Arbeit.  Zool.  Inst.  Wilrzburg. 

2  Bd.     1874. 


vm  ECHIXODERUATA—  LITERATURE  557 

H.  Ludwig.     Beitrage  zur  Anatomic  der   Crinoidccn.     Leipzig,   1877.     Zeitschr.  f. 

.  Zool.  26  Bd.     1877.     28  Bd.     1877. 

-  Zvr  An  atomic  des  Rhizocrinus  lofotensis.     Ibid.     29  Bd.     1877. 
Rltopalodina  lagcniformis.     Ibid.     1877. 


-  Beitra<i>  :  zur  Aimtuinie  der  Asteriden.     Ibid.     30  Bd.     1878. 

-  Ueber  Asthenosoma  rariv.m  Gnibe  und  iiber  ein  neues  Organ  bei  den  Cidariden. 
Ibid.     34  Bd.     1880.     Bericlitigung  im  Zool.  Am.     3  Jahrg. 

-  Ueber    den   primarcn    Stcinkanal    der    Crinoiden    nebst    vergl.-anatomischen 
Bemerkungen  iibcr  die  Echinodermen  iiberJiaupt.     Ibid.     34  Bd.     1880. 

--  X'-nf  Beit  rage  zur  Anatomie  der  Ophiuren.     Ibid.     34  Bd.     1880. 
—  Nochmals  die  Rhophalodina  Jageniformis.     Ibid.     48  Bd.     1889. 

-  Die  Seewalzen.     Bronns  Klassen  und  Ordnungen  des  Thicr-Reichs.     2  Bd.     3 
Abth.     Echinodermata.     Leipzig,  1889-1892. 

Hub.  Ludwig  and  Ph.  Barthels.     Beitrage  zur  Anatomie  der  Holothurien.    Zeitschr. 

f.  iff  iss.  Zool.     54  Bd.     1892. 
T.   Lyman.     Report   on   the  Ophiuridea.      Voyage   of  H.  M.S.    "Challenger"  Zool. 

Vol.  XV.     Part  14.     London,  1882. 

J.  Muller.  Uebcr  den  Ban  von  Pentacrinus  caput  Medusa'.  Abhandl.  d.  Akad.  d. 
Jrisscnsch.  Berlin,  1841. 

-  Ueber  Synapta  diyitata  und  iibcr  die  Erzeugung  von  Schnecken  in  Holothurien. 
Berlin,  1852. 

E.  Perrier.  Memoire  mr  I'  organisation  ct  le  devcloppement  de  la  Comatule  de  la 
Mtditerranee  (Antedon  rosacea  Linck}.  Xouv.  Archives  Mus.  Paris,  1886- 
1892. 

-  Recherches  sur  Tanatomie  et  la  regeneration  des  bras  de  la  Comatula  rosacea. 
Arch.  Zool.  exper.     Tome  II.     1872. 

H.  Prouho.     Recherches  sur  le  Dorocidaris  papillata  et  quelques  autres  £chinides  de 

Jo.  Medfarrante.     Arch,  de  Zool.  exper.  (2).     Tome  V.     1887. 
A.  de  Quatrefages.     Mtmoire  sur  le  Synapte  de  Duvernoy.     Ann.  Sc.  natur.     (2). 

Tome  XIV.     1842. 
C.   Semper.     Knr^  anatomische  Bemcrkungen  iiber  Comatula.     Arbeit.  Zool.  Inst. 

irurzburg.     1  Bd.     1874. 
C.  F.  and  P.  B.  Sarasin.      Uebcr  die  Anatomie  der  Echinothuriden  und  die  Phylo- 

genic  der  Echinodermen.     Ergcbnisse  Xat.  Forschungcn  Ceylon      1  Bd.     1888. 
G.   0.   Bars.     R<  searches  on  the  structure  etc.   of  the  genus  Brisinga.     Christiania, 

1875. 

-  Memoire  pour  servir  a  la  connaissance  des  Crinoides  vivants.    Christiania,  1868. 
Selenka.     Beitrage  zur  Anatomie  und  Systematik  der  Holothurien.     Zeitschr.  f. 

wiss.  Zool.     Two  papers  in  Vols.  17  and  18.     1867-1868. 
R.  Semon.     Beitrage  zur   Xaturgeschichte  der  Synaptiden  des  MUtelmeers.     Mitth. 

Zool  Stat.  in  Xcapd.     1  Bd.     1887. 

Von  Siebold.     Zur  Anatomie  der  Seesterne.     Muller'  s  Archiv.     1866. 
H.   Simroth.     Anatomie   und   Schizogonie  der   Ophiactis  virens.     Zeitschr.  f.   wiss. 

Zool.     Two  papers  in  Vols.  27  and  28.     1877. 
Reinhold  Teuscher.     Beitrage  zur  Ancdomie  der  Echinodermen.     I.   Comatula  medi- 

f>  -i-ranei't.  Jenaische  Zeitschr.     10  Bd.     1876. 
Friedr.     Tiedemann.      Anatomie    der    Rdhrcnholothurie,    des     'pomeranzenfarbigen 

Seestcrns  und  des  Stcin-Seeigcls.     Landshut,  1816. 
Hj.    Theel.      Import    on    the    Holothurioidea.       Voyage    of  H.M.S.    "Challenger." 

Zoology.     Part  I.     Vol.  VI.     Part  XIII.     1882.     Part  II.     Vol.  XIV.     Part 

XXXIX.     1885. 
J.  V.  Thompson.     Surle  Pcntacrinuseuropaeus,  Titat  de  jcunessc  du  genre  Comatula. 

L'Institut.     1835. 


558  COMPARATIVE  ANATOMY  CHAP. 

G.    Valentin.     Anatomie  du   genre    Echinus.      Monographies  d' Echinodermes,  par 

L.  Agassiz.     Neuchatel,  1841. 
Alex.  Weinberg.     Die  Morphologic  der  lebenden  Crinoiden  mit  Beziehung  auf  die 

form  Antedon  rosacea  LincTc.     Naturhistoriker.     5  Jahrg.  1883. 


Works  dealing  with  Single  Organs  or  Systems  of  Organs. 

H.  Avers.     On  the  structure  and  function  of  tlie  Sphceridia  of  the  Echinoidea.    Quart. 

Journ.  Microsc.  Science  (2).     Vol.  XXVI.     1886. 
E.  W.  M'Bride.     Tlie  development  of  the  genital  organs,  ovoid  gland,  axial  and  aboral 

minuses  in  Amphiura  squamata,  together  with  some  remarks  on  Ludwig's  Haemal 

System  in  this  Ophiurid.    Quart.  Journ.  Microsc.  Science.    Vol.  XXXIV.    1893. 
The  development  oftlie  dorsal  organs,  genital  rachis  and  genital  organs  in  Asterina 

gibbosa.     Zool.  Anz.     16  Jahrg.  1893. 
E.  Baudelot.     Contributions  a  I'histoire  du  systeme  nerveux  des  Echinodermes.    Arch. 

de  Zool.  cxpfr.     1872. 
W.  B.  Carpenter.     On  the  nervous  system  of  Crinoidea.     Proceed.  Roy.  Soc.  London. 

Vol.  XXXVII.     1884. 
L.  Cue'not.     Etudes  sur   le  sang,  son  role  et  sa  formation  dans  la  serie  animale. 

Part  II.     Invertebrts.     Arch.  Zool.  exptr.  (2).     Tome  IX.     1891. 

E.  Haeckel.     Ueber  die  Augen  und  Nerven  der  Seesterne.     Zeitschr.  f.  wiss.  Zool. 

10  Bd.     1860. 
Otto   Hamann.     Beitrdge  zur  Histologie  der  Echinodermen.      1.  Die  Holothurien 

(Pedata)  und  das  Nervensystem  der  Asteriden.     Zeitschr.  f.  wiss.  Zool.     39  Bd. 

1883.      2.  Mitth.    1.  Nervensystem  der  pedaten   Holothurien.      2.   Cuvier'sche 

Organe.     3.  Nervensystem  und  Sinnesorgane  der  Apoden.     Ibid. 
R.  Kohler.     Recherches  sur  Vappareil  circulatoire  des  Ophiures.     Ann.  Sc.  nat.  (7). 

Tome  II.     1887. 

F.  Leipoldt.    Das  angebliche  Excretionsorgan  der  Seeigel,  untersucht  an  Sphcerechinus 

granularis  und  Doroddaris  papillata.     Zeitschr.  f.  wiss.  Zool.     55  Bd.      1893. 
Theodore  Lyman.     The  stomach  and  genital  organs  of  Astrophytidce.     Bull.  Mus. 

Comp.  Zool.  Harvard  College.     Vol.  VIII.     No.  6.     Cambridge,  Mass.,  1891. 
A.    M.    Marshall.     On  the  nervous  system  of  Antedon  rosaceus.      Quart.    Journ. 

Microsc.  Science.     Vol.  XXIV.     1884. 
C.  Mettenheimer.      Ueber  die  Gesichtsorgane  des  violetten  Seesterns.     Mailer's  Arch., 

1872. 
E.  A.  Minchin.     Notes  on  the  Cuvierian  organs  of  Holothuria  nigra.     Ann.   and 

Mag.  Nat.  History  (6).     Vol.  X.     1892. 
J.  Niemiec.     Recherches  sur  les  ventouses  dans  le  regne  animal.     Recueil  Zool.  Suisse. 

Tome  II.     Encore  un  mot,  etc.     Ibid.     1885. 
Owsjanikoff.      Ueber  das  Nervensystem  der  Seesterne.     Bull.  Acad.  St.  Pttersbourg. 

1870.     Tome  XV. 
Ed.  Perrier.     Recherclies  sur  les  pedicellaires  et  les  ambulacres  des  Asteries  et  des 

Oursins.     Ann.  des  Sciences  natur.  (5).     12  and  13  Bd.     1869-1870. 
Recherches  sur    I'appareil  circulatoire  des    Oursins.      Arch,    de    Zool.    exper. 

Tome  IV.     1875. 

G.  J.  Romanes  and  J.  C.  Ewart.     Observations  on  the  locomotor  system  of  Echino- 

dermata.     Philos.  Transact.  London.     Part  III.     1881. 
A.  Russo.     Ricerche  citologiclie  sugli  elementi  seminali  delle    Ophiurece   (spermato- 

genesi-oogenesi]  Morfologia  delV  apparecchio  riprodutore.     Internal.  Monatschr. 

f.  Anat.  und.  Phys.     8  Bd.     8  Heft. 
C.    F.    and  P.    B.    Sarasin.     Die  Augen  und  das  Integument  der  Diadematiden. 


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Ergebnissc  natunciss.  Forschungen  auf  Ceylon  in  d.  Jahren  1884-1886.     1  Bd. 

1887. 
Rich.  Semon.     Das  Nervensystem  der  Holothurien.     Jenaische  Zeitschr.  f.  Natur- 

u-iss.     16  Bd.     1883. 
H.   S.  Wilson.      The  nervous  system  of  the  Asteridce.     Transact.  Linnean  Society. 

Vol.  XXIII.     1860. 

Ontogeny. 

A.  Agassiz.  Xorth  American  Starfishes,  1864.  Mem.  of  the  Museum  of  Comp.  Zool. 
Harvard  College.  Vol.  V.  1877. 

Revision  of  the  Echini.     Illustr.    Catalogue   of  the  Museum    of  Comp.   Zool. 

Harvard  College.     1872-1874. 

Nic.  Christo  Apostolides.     Anatomic,  et  developpement  des  Ophiures.     Arch.  Zool. 
•/•.     Tome  X.     1882. 

J.  Barrois.  Itecherches  sur  le  developpement  de  la  Comatule  (C.  mediterranea). 
Eccucil  Zool.  Suisse.  Tome  IV.  1888. 

E.  W.  M 'Bride.  The  development  of  the  genital  organs,  ovoid  gland,  axial  and  aboral 
sinuses  in  Amphiura  squamala,  together  with  some  remarks  on  Ludwig's  Hcemal 
System  in  this  Ophiurid.  Quart.  Journ.  Microsc.  Science.  Vol.  XXXIV. 
Part  II.  1893. 

—  The  development   of  the  dorsal    organ,  genital  rachis  and  genital  organs  in 
Astcrina  gibbosa.     Zool.  Anz.     16  Jahrg.     1893. 

H.  Bury.  The  early  stages  in  the  development  of  Antedon  rosacca.  Philos.  Transac- 
tions. Vol.  CLXXIX.  1888. 

Studies  in   the  embryology  of  the  Echinoderms.     Quart.  Journ.   Microsc.    Sc. 

Vol.  XXIX.     1889. 

-  Tlie  Metamorphosis  of  Echinoderms.     Ibid.      Vol.  XXXVIII.     1895. 
P.  H.  Carpenter.     On  some  points  in  the  anatomy  of  larval  Comatula.    Quart.  Journ. 
Mi.: rose.  Sc.  (2).     Vol.  XXIV.     1884. 

Xotcs  on  Echinoderm  morphology.     No.  11.     On  the  development  of  the  apical 

plates  iii   Amphiura,  squamata.     Quart.   Journ.    Microsc.   Sc.     Vol.    XXVIII. 
1888. 

William  B.   Carpenter.     Researches  on  the  structure,  physiology,  and  development  of 

Antedon  rosaccus.     Philos.   Transact.     Vol.    CLVI.     1866.      Addendum.  Pro. 

Roy.  Soc.     Vol.  XXIV.     1876. 
J.  W.  Fewkes.      On  the  development  of  calcareous  plates  of  Amphiura.    Bull,  of  the 

Museum  of  Comp.  Zool.  of  Harvard  College.     Vol.  XIII.     1887. 
A.  Fleiscnmann.    JJic  EnticickcJung  des  Eiesvon  Echinocardium  cordatum.    Zeitschr. 

f.  iciss.  Zool.     46  Bd.     1888. 
Geo.  W.  Field.     Tlie  larva  of  Asterias  vulgaris.     Quart.  Journ.  of  Microsc.  Sc.    Vol. 

XXXIV.     Part  2.     1892. 
Alexander   Goette.      Vergleichendc  EniicicMungsgeschicMe  der   Comatula  mediter- 

ranea.     Arch.  f.  mikr.  Anatomic.     Tome  XII.     1876. 
E.  Korshelt.     Zur  Bildung  des  mittleren  Keimblatts  bei  den  Echinodermen.     Zool. 

J.ihi-b.  AUh.  Morph.     3  Bd.     1889. 
Kowalevsky.      Beitrdge   zur    Eiiticick':hin<jS'jc»chicJtte    der    Holothurien.     M&m.    de 

/' Awl.  Imper.  de  St.  Petersbourg.     (7.)     Tome  XI.     1867. 
H.    Ludwig.     Zur   Entu-ickelungsgcschichte  des  Ophiurenskeletes.     Zeitschr.  f.  wiss. 

Zool.     36  Bd.     1881. 

Entwickelungsgeschichte  der  Asterina  gibbosa  Forbes.     Zeitschr.  f.  wiss.     Zool. 

37  Bd.     Also  in  Morph.  Studien  an  Echinodermen.     2  Bd.     1882. 

—  Zur    Entu'ickdungsgeschichte   der    Holothurien.      Sitzber.    K.    Preuss.    Acad. 
d.   Wiss.     1891.     X.     XXXII. 


560  COMPARATIVE  ANATOMY  CHAP,  vin 

E.  W.  M'Bride.      The  organogeny  of  Aster ina  gibbosa.     Pro.  11.  Soc.     L1V.     1894. 
E.  Metschnikoff.     Studien  uber  die  Entwickelung  der  Echinodermen  und  Nemertinen. 
Mem.  Acad.  St.  Petersburg.     Tome  XIV.  (No.  8).     1869. 

Entwickelung  von  Comatula.     Hull.  Acad.  St.  Petersbourg.     XV.     (Col.  508.) 

1871. 

Vergleichend-embryologische  Studien.     5.  Ueber  die  Sliding  der   Wandcrzellen 

bei  Asterien  und  Echiniden.     Zeitschr,  f.  wiss.  Zool.     42  Bd.     1885. 

Job.  Miiller.  Classical  Treatises  on  the  larval  forms  of  Echinoderms,  and  their  meta- 
morphoses. Abhandlungen  d.  Konigl.  Akad.  d.  Wissensch.  zu  Berlin.  1848, 
1849,  1850,  1852,  1853,  1855. 

Edm.  Perrier.  Memoire  sur  I" organisation  et  la  developpement  de  la  Comatule  de  la 
Me~diterran£e.  Nouv.  Arch,  du  Mus.  Hist.  nat.  Paris.  1886-1892. 

A.  Russo.     Several  treatises  in  Neapolitan  journals.     1891-1892. 

Oswald  Seeliger.  Studien  zur  Entwickelungsgeschichte  der  Crinoiden  (Antedon 
rosacea}.  Zool.  Jahrb.  v.  Spengel.  Abth.  f.  Anat.  u.  Out.  6  Bd.  Jena,  1892. 

Emil  Selenka.  Die  Keimbldtter  der  Echinodermen.  Studien  ub.  d.  Entwickel- 
ungsgesch.  d.  Thiere.  2  Heft.  1883. 

Zur  Entwickelung   der    Holothurien.      Ein    Beitrag  zur    Keimbldttertheorie. 

Zeitschr.  f.  wiss.  Zool.     27  Bd.     1876. 

R.  Semon.  Die  Entwickelung  der  Synapta  digitdta  und  die  Stammesgeschichte  der 
Echinodermen.  Jenaische  Zeitschr.  f.  Naturwiss.  22  Bd.  1888. 

Zur   Morphologic    der    bilateralen    Wimperschnure   der    Echinodermenlarven. 

Jena.     Zeitschr.  f.  Naturwiss.     25  Bd.     1891. 

C.  Wyville  Thomson.      On  the  embryology  of  the  Echinodermata.      Nat.  History 

Review.     1863-1864. 
—  On  the  embryogeny  of  Antedon  rosaceus.     Philosoph.  Transact,  of  the  Hoy.  Soc. 

London.     Vol.  CLIII.     1865. 
Hjalmar  Theel.      On  the  development   of  Echinocyamus  pusillus  (0.   F.    Mutter}. 

Nova  Add  Eeg.  Soc.  Sci.     Ser.  3.     A^ol.  XV.  (No.  6).     Upsala,  1892. 

Phylogeny. 

Besides  the  above-named  treatises  and  works  of  Al.  Agassiz,  P.  H.  Carpenter, 
Cuenot,  Haeckel,  Ludwig,  Neumayr,  Perrier.  Sarasin,  Seeliger,  Semon, 

compare  especially  0.  Biitschli,  Vcrsuch  der  Ableitung  des  Echinoderms  aus  einer 
bilateralen  Urform.     Zeitschr.  f.  iviss.  Zool.     53  Bd.     Suppl.  1892. 


CHAPTER   IX 
ENTEROPNEUSTA  * 

BILATERALLY  symmetrical,  long,  vermiform  animals  with  soft  skin. 
The  body  is  divided  into  (1)  a  preoral  proboscis,  (2)  a  short  "collar," 
(3)  a  long  trunk.  The  wide  mouth  lies  at  the  boundary  between  the 
proboscis  and  the  collar.  The  anus  is  terminal.  The  long  intestine 
may  be  divided  into  four  sections.  The  mouth  leads  direct  into  (1) 
the  buccal  cavity,  which  runs  through  the  collar,  and  sends  off  a 
diverticulum  anteriorly  into  the  proboscis.  The  buccal  cavity  leads 
into  (2)  a  branchial  intestine,  which  is  in  open  communication  with 
the  exterior  through  numerous  pairs  of  consecutive  gill-pouches.  The 
branchial  intestine  passes,  through  an  intermediate  portion,  into  (3) 
the  hepatic  intestine,  which  is  often  provided  with  two  longitudinal 
rows  of  caeca.  Lastly,  the  hepatic  intestine  runs  into  (4)  an  efferent 
intestine,  which  opens  outwards  through  the  anus. 

There  is  an  unpaired  proboscidal  coelom,  which  opens  outward  at 
the  base  of  the  proboscis  either  by  a  single  pore  on  the  left  side,  or  by 
two  symmetrical  pores.  The  coelom  of  the  collar  is  paired,  and  there 
are  two  pores  at  its  posterior  end.  The  coelom  of  the  trunk  is  also 
paired.  The  integumental  and  intestinal  musculature  are  derived 
from  the  ccelomic  walls.  The  nervous  system  consists  of  a  layer  of 
nerve  fibres  in  the  integument,  this  layer  being  thickened  to  form  a 
mediodorsal  nerve  cord  provided  with  ganglion  cells,  and  a  similar 
medioventral  cord,  both  extending  the  whole  length  of  the  trunk. 
The  dorsal  cord  sinks,  in  the  collar,  below  the  integument,  forming 
the  collar  cord.  A  capillary  network  is  found  within  all  the  limiting 
or  basal  membranes  of  the  body.  A  large  contractile  vessel  lies  in 
the  dorsal  middle  line  of  the  collar  and  trunk,  and  a  similar  vessel  in 
the  ventral  middle  line  of  the  trunk.  In  the  former,  the  blood  flows 

1  In  this  chapter  the  author,  relying  largely  upon  Spengel's  monograph  on 
Balanoglossus,  rejects  the  proposed  affinity  between  the  Enteropneusta  and  the  ancestors 
of  the  Vertebrata.  As  this  affinity  has  been  very  widely  accepted  in  England  and 
America,  the  student  should  consult  M'Bride's  "  Review  of  Spengel's  Monograph " 
(Quart.  Journ.  Micro.  Sci.,  vol.  xxxvi.,  1894),  which  is  written  from  this  latter  point  of 
view. — TR. 

VOL.  II  2  O 


562 


COMPARATIVE  ANATOMY 


CHAP. 


from  behind  forward,  in  the  latter,  from  before  backward.     A  pulsating 
heart  vesicle,  which  serves  to  propel  the  blood,  though  not  belonging 

to  the  vascular  system, 
is  found  in  the  proboscis, 
above  the  intestinal 
caecum. 

The  sexes  are  sepa- 
rate in  the  Entero- 
pneusta.  The  gonads  are 
tubes  or  sacs  which  lie 
in  longitudinal  rows  in 
the  anterior  region  of 
the  trunk  (in  and  behind 
the  posterior  branchial 
region)  and  open  through 
dorsal  genital  apertures. 
There  are  no  copulatory 
organs.  Reproduction 
is  sexual.  Development 
is  with  metamorphosis 
(in  which  case  the  larva 
is  known  as  theTornaria) 
or  with  abbreviated 
metamorphosis. 

Marine,  inhabiting 
sand  or  mud.  Four 
genera  :  Ptyehodera, 
Glandieeps,  Sehizoear- 
dium,  Balanoglossus. 


I.  Outer  Organisation 
(Fig.  455,  A,  B). 

In  the  longworm-like 
body,  three  principal 
regions  can  be  distin- 
guished, corresponding 
with  three  consecutive 
sections  of  the  coelom  : 
these  are  the  probos- 


Fio.  455.— A,  Ptyehodera  minuta,  from  the  dorsal  side ; 
B,  Balanoglossus  Kowalevskii.  After  drawings  by  Peters 
and  Minot,  in  Spengel's  monograph.  1,  Proboscis ;  "2,  collar ;  eldal  region,  the  COllar 
3,  branchiogenital  region ;  4,  hepatic  region ;  5,  genital  folds ;  poo-Jon  and  the  trunk 
6,  anus  ;  7,  branchial  pores. 

region.        Compared 
with  the  long  trunk,  the  first  two  regions  are  short. 

A.  The  proboscis  is  in  the  shape  of  an  acorn  (hence  its  German 
name  "Eichel").  It  can  be  distended  and  contracted,  and,  in  the 
limicolous  manner  of  life,  is  the  chief  organ  for  burrowing. 


ix  ENTEROPNEUSTA—BODY  EPITHELIUM  563 

B.  The  proboscis  is  joined  to  the  second  division — the  collar,  by 
a  short  stalk  or  neck. 

This  region  forms  a  somewhat  projecting  ridge  anteriorly  round 
the  neck  of  the  proboscis,  its  anterior  wall  surrounding  the  neck  like 
a  stand-up  collar.  The  neck  is  only  joined  to  the  collar  dorsally,  for 
between  the  neck  of  the  proboscis  and  the  ventral  wall  of  the  collar, 
gapes  the  wide  unarmed  mouth.  This  leads  into  the  buccal  cavity, 
which  runs  through  the  collar.  The  proboscis  is  thus  a  preoral 
division  of  the  body.  The  collar  is  marked  off  from  the  trunk  by  a 
circular  furrow  of  varying  depth,  over  which  it  sometimes  bulges  out 
posteriorly.  Further,  immediately  in  front  of  its  posterior  boundary, 
there  is  a  circular  furrow  round  the  collar  itself. 

C.  In  the  long  trunk,  in  the  genera  Ptychodera  and  Schizocardium, 
three    regions    can    be    distinguished :    the    branchio  -  genital,    the 
hepatic,  and  the  abdominal  regions. 

1.  The   anterior   part   of   the   branehio  -  genital   region,    which 
follows  the  collar,  is  distinguished   by  the  branchial  pores,  and  the 
posterior  part  by  the  gonadial  apertures.     The  gonads,  however,  may 
stretch  for  some  distance  anteriorly  into  the  branchial,  and  posteriorly 
into  the    hepatic  regions.     The    branchial   pores  are   found   on  the 
dorsal  side,  arranged    in  two  longitudinal  rows,  or,  when    they  are 
small   and   circular,  in  two  more  or  less  deep  longitudinal  furrows 
converging  posteriorly.     The  pores  may  take  the  form  of  transverse 
slits. 

The  genus  Ptycliodem  is  distinguished  by  a  longitudinal  fold  or 
ridge  on  each  side  for  the  reception  of  the  gonads.  These  two 
genital  folds  (Fig.  455,  A)  can,  when  well  developed,  bend  towards 
one  another  over  the  back,  and  so  form,  on  the  dorsal  side  of  the 
branchial  region,  a  branchial  vestibule. 

2.  The  hepatic  region  is  distinct  only  in  Ptychodera  ''Fig.  455,  A) 
and    SchfajwriJium.     In     these    genera    it    is    distinguished    by    two 
longitudinal  rows  of  projecting  brown  or  green  liver-caeca.     Even  in 
cases    in    which    these  caeca    do  not    appear    to  be  arranged   in  two 
longitudinal  rows,  it  can  be  shown  that  their  insertions  on  the  body 
form  two  such  rows,  but,  there   not  being  sufficient  room  for  their 
swollen  ends  one   behind   the   other,  many  of  them  are  pushed  out 
irregularly  to  the  right  or  left.     A  mediodorsal  strip  of  the  hepatic 
region  always  remains  uncovered. 

3.  The  cylindrical,  delicate  walled  abdominal  region  tapers  off 
posteriorly,  as  a  rule  gradually,  to  the  terminal  anus. 


II.  The  Body  Epithelium. 

The  body  is  everywhere  covered  by  a  thick  ciliated  epithelium, 
in  which,  apart  from  the  nerve-elements,  undifferentiated  epithelial 
cells  and  gland  cells  can  be  distinguished.  The  latter  are  always 


564  COMPARATIVE  ANATOMY  CHAP. 

epithelial  in  position.     The  finer  structure  of  the  epithelium  cannot  be 
dealt  with  here. 


III.  The  Nervous  System  (Figs.  456  and  457). 

The  facts  of  fundamental  importance  will  be  treated  of  first. 

The  whole  nervous  system,  with  the  single  exception  of  a  part 
situated  in  the  collar,  lies  in  the  body  epithelium. 

Below  the  surface  of  the  whole  of  the  body  epithelium,  there  is 
an  uninterrupted  layer  of  nerve  fibres,  a  close  continuous  nerve 
plexus. 

The  principal  or  main  portions  of  the  nervous  system  are 
merely  local  thickenings  of  this  network  ;  and  are  the  two  longi- 
tudinal nerve  cords,  one  mediodorsal  and  the  other  medioventral, 
which  run  throughout  the  whole  length  of  the  trunk. 

At  the  boundary  between  the  collar  and  the  trunk,  the 
network  of  nerve  fibres  thickens  into  a  nerve  ring,  which  forms  a 
more  specialised  connection  between  the  dorsal  and  ventral  nerve 
cords. 

The  dorsal  cord  is  produced  anteriorly  as  far  as  to  the  base  of 
the  proboscis,  where  it  divides  into  two  diverging  branches,  which 
encircle  that  base.  This  circular  thickening  of  the  epithelial  nerve 
plexus,  however,  is  not  sharply  circumscribed. 

The  collar  portion  of  the  dorsal  cord  leaves  its  epithelial  position, 
and  runs  longitudinally  through  the  coelom  of  the  collar  above  the 
buccal  cavity. 

Special. — Besides  smaller  ganglion  cells,  others  of  remarkable  size,  so-called 
giant  ganglion  cells,  sometimes  occur  in  the  thickenings  of  the  nerve  plexus,  especi- 
ally in  the  collar  region.  Near  the  nerve  cords  and  fibres  the  body  epithelium  has 
few  or  no  glands,  and  above  the  cords  it  is  thickened.  Further,  all  along  .these 
cords,  i.e.  in  the  dorsal  and  ventral  middle  lines,  and  especially  in  the  latter,  the 
body  wall  appears  to  have  sunk  in.  , 

The  cord  in  the  collar,  which  may  best  be  regarded  as  the  central  portion  of  the 
nervous  system,  forms  the  so-called  dorsal  nerve  cord  of  the  collar.  This  lies  in 
the  median  line  in  the  coelom  of  the  collar  above  the  pharynx,  and  consists  of  several 
parts.  There  are  two  perihsemal  tubes,  which  clasp  between  them  the  collar 
portion  of  the  dorsal  blood  vessel ;  the  nerve  substance  itself  lies  upon  these,  or 
in  a  channel  formed  by  them.  The  nerve  cord  of  the  collar  is  a  thick,  almost 
cylindrical  band,  the  dorsal  part  of  which  consists  of  cells,  which,  however,  are  not 
nerve  cells  but  may  be  glandular  ;  the  ventral  part,  that  turned  to  the  intestine, 
consists  of  nerve  tissue,  and  is  a  direct  prolongation  of  the  dorsal  nerve  cord  of  the 
trunk.  Anteriorly,  i.e.  at  the  anterior  end  of  the  collar,  this  band  divides,  one 
portion  running  into  the  nerve  tissue  of  the  base  of  the  proboscis,  and  the  other 
into  the  epithelial  nerve  tissue  of  the  circular  collar  ridge  which  surrounds  this  base. 

In  the  genus  Ptychodera,  the  nerve  cord  of  the  collar  is  connected  with  the 
epithelium  of  the  dorsal  body  wall  in  the  median  plane  by  means  of  a  varying 
nnniber  of  epithelial  tubes,  the  so-called  roots  of  the  nerve  cord.  Of  these,  only 


ix  ENTEROPKEUSTA—  ALIMENTARY  CANAL  565 


one  or  more  of  the  most  anterior  are  really  hollow,  containing  an  axial  canal. 
The  posterior  roots  are  solid  strands  of  epithelium.  No  outer  aperture  of  the  axial 
canal  has  been  observed,  although  the  root  tissue  passes  directly  into  the  body 
epithelium,  and  the  limiting  membrane,  elsewhere  found  below  the  epithelium,  is 
interrupted  at  the  point  where  they  join.  On  the  peripheral,  outer  side  of  the 
roots,  the  plexus  of  nerve  fibres  of  the  integument  is  continued  into  the  nerve  tissue 
of  the  collar  cord.  The  roots  themselves  are  connected  only  with  the  dorsal  layer 
of  cells  of  that  cord. 

The  collar  cord  contains  cavities :  these  are  either  numerous  small  medullary 
cavities,  arranged  more  or  less  exactly  in  two  longitudinal  rows,  or  they  form  one 
continuous  central  cavity,  an  axial  canal  (Ptychodera)  which  either  (in  one  species) 
opens  outward  at  the  anterior  and  posterior  boundaries  of  the  collar  region,  or  (in  all 
other  species  in  which  this  point  has  been  investigated)  ends  blindly  at  these  points. 

The  axial  canals  of  the  roots  of  the  collar  cord  (which  run  in  the  dorsal  mesentery) 
are  in  communication  either  with  the  axial  canal  of  that  cord  or  with  its  medullary 
cavities. 

IV.  Sensory  Organs. 

Even  the  most  recent  careful  investigations  have  not  been  able  to 
demonstrate  with  certainty  the  existence  of  specific  sensory  organs. 
Undifferentiated  sensory  cells  may  be  scattered  over  the  whole  of  the 
integument.  At  the  posterior,  and  especially  at  the  postero-ventral 
part  of  the  proboscis,  and,  further,  at  the  anterior  edge  of  the  collar, 
the  constitution  of  the  body  epithelium  is  such  as  to  make  it  highly 
probable  that  it  is  a  sensory  epithelium.  In  one  species  alone, 
Balanoglossus  canadensis,  in  the  postero-ventral  wall  of  the  proboscis, 
there  is  a  deep  epidermal  pit,  which  is  the  only  structure  that  can, 
with  any  probability,  be  claimed  as  a  localised  sensory  organ. 

On  the  sensory  organs  of  the  free-swimming  larva,  cf.  the  section 
on  Ontogeny,  p.  586. 


V.  The  Alimentary  Canal. 

The  alimentary  canal  runs  as  a  large  and  usually  straight  epi- 
thelial tube  through  the  body,  from  the  wide  oral  aperture,  at  the 
anterior  and  ventral  end  of  the  collar,  to  the  terminal  anus.  It  is,  as  a 
rule,  attached  to  the  body  wall  by  both  a  dorsal  and  a  ventral  mesentery 
traversing  the  body  cavity.  It  is  developed  in  ways  peculiar  to  the 
different  regions  of  the  body.  Especially  noteworthy  is  the  fact  that, 
in  the  branchial  region,  it  forms  a  branchial  intestine,  communicating 
by  means  of  two  longitudinal  rows  of  branchial  canals  (gill-slits)  with 
the  exterior. 

A.  The  mouth  is  followed  by  the  spacious  buecal  cavity,  which 
traverses  the  collar  region,  and  is  provided  with  a  thick  epithelial 
Avail. 

B.  The  roof  of  the  buecal  cavity  grows  out  to  form  a  diverticulum 
directed  anteriorly  ;    this  runs  through  the  neck  of  the   proboscis, 


566 


COMPARATIVE  ANATOMY 


CHAP. 


reaching  as  far  as  the  base  of  that  organ.  This  is  the  proboseidal 
divertieulum  of  the  buccal  cavity  (Fig.  456),  and  is  preoral.  Its 
epithelial  wall  is  a  continuation  of  the  epithelial  wall  of  that  cavity. 

Special. — In  the  proboseidal  divertieulum.  a  narrower  posterior  neck  and  an 
anterior  head  or  body  can  usually  be  distinguished.     In  section,  the  neck  appears 


FIG.  456.— Ptychodera  minuta.  The  proboseidal,  collar,  and  anterior  branchial  regions,  cut 
through  the  middle  line,  and  seen  from  the  cut  surface,  diagrammatic  (after  Spengel).  1,  Probos- 
eidal divertieulum  of  the  buccal  cavity  ;  2,  ventral  septum  of  the  proboscis ;  3,  skeleton  of  the 
proboscis  ;  4,  buccal  cavity ;  5,  ventral  vessel  of  the  collar ;  6,  ventral  nerve  cord  of  the  trunk  ;  7, 
oesophagus  ;  8,  ridge  forming  the  boundary  between  the  oesophagus  and  the  branchial  intestine  ;  1), 
ventral  blood  vessel  of  the  trunk ;  10,  dorsal  blood  vessel  of  the  trunk  ;  11,  branchial  intestine, 
with  the  gill-slits ;  12,  dorsal  nerve  cord  of  the  trunk ;  13,  dorsal  blood  vessel  of  the  collar ;  14, 
roots  of  the  collar  cord ;  15,  collar  cord  ;  16,  proboscis  pore ;  17,  crelom  of  the  collar  traversed 
by  muscle  fibres ;  18,  anterior  wall  of  the  collar ;  19,  nerve  layer  at  the  base  of  the  proboscis  ; 
20,  central  blood  vascular  cavity  of  the  proboscis  ;  21,  heart  vesicle  ;  22,  proboseidal  glomerulus  ; 
23,  proboseidal  epithelium  ;  24,  a  part  of  the  longitudinal  musculature  traversing  the  proboseidal 
cavity. 

semilunar,  with  the  concavity  directed  downwards.  In  Schizocardium  and  Glandi- 
ceps,  the  head  is  continued  anteriorly  into  a  narrow  blind  canal,  the  vermiform 
process,  which  runs  through  the  proboseidal  cavity  almost  axially. 

In  Balanoglossus  canadensis,  the  neck  is  wanting,  the  head  of  the  divertieulum 
consequently  forming  a  constricted  vesicle.  In  other  species,  the  continuity  of  the 
lunun  may  be  interrupted  here  and  there. 


IX 


EXTEROPNEUSTA—  ALIMEXTARY  CANAL 


567 


Ill  certain  sections,  the  tissue  of  the  proboscidal  diverticulum,  especially  that  of 
its  head,  has  a  vesicular  appearance  which  recalls  that  of  the  noto-chord  of  Verte- 
brates. The  proboscidal  diverticulum  of  the  Enteropneusta  has  even  been  called  the 
chorda,  and  has  been  directly  homologised  with  this  structure  in  Vertebrates.  More 
recent  researches  have,  however,  shown  that  the  tissue  of  the  diverticulum  is  an  epi- 
thelium in  direct  continuation  with  the  intestinal  epithelium  of  the  buccal  cavity. 
This  epithelium  seems  to  consist  of  thread-like  cells,  which  at  one  point  swell  up  to 
form  vesicles  or  vacuoles  containing  a  fluid  clear  as  water.  The  vacuoles  of  adjacent 
cells  have  not  room  enough  to  lie  in  the  same  level  of  the  epithelium.  They  have 
to  make  room  for  one  another,  and  thus  come  to  lie  in  very  different  levels  of  the 
thickened  epithelium.  This  makes  the  proboscidal  diverticulum  appear  to  have  a 
vesicular  structure,  especially  in  tangential  sections. 

C.  The  branchial  intestine. — At  the  posterior  end  of  the  collar, 
the  buccal  cavity  passes  on  into  the  branchial  intestine,  which  lies  in  the 
anterior  portion  of  the  branchio- 
genital  region  of  the  body. 

Here,  as  already  mentioned, 
the  intestine  is  in  communication 
with  the  exterior  through  two 
rows  of  pouch-like  canals,  which 
will  be  described  more  in  detail 
later. 

The  numerous  pouches  of  each 
longitudinal  row  follow  one  an- 
other closely,  and  mutually  flatten 
one  another,  so  that  their  lumina 
become  slit -like,  and  lie  trans- 
versely in  the  body  at  right  angles 
to  its  longitudinal  axis  (cf.  Figs. 
457,  458,  459). 

From  each  of  these  flat, 
vertical,  and  transverse  gill- 
pouches,  an  inner  aperture  the 

*  process  ;  6,  branchial  septum  ;  7,  lowest  tip  of  the 

gill-Slit,  leads  into  the  alimentary    branchial  tongue  ;  S,  wall  of  the  alimentary  canal ; 
Canal,  and   an    OUter  aperture,  the    9>  ventral  vessel ;  10,  dorsal  mesentery ;  11,  ventral 

branchial    pore,    leads    to    the  ™^m. 12f gi 
exterior. 

The  inner  aperture,  the  gill -slit,  is  as  long  as  the  gill -pouch 
itself,  and  would  have  the  shape  of  a  very  long  0  were  it  not  com- 
plicated by  the  formation  of  the  tongue.  The  intestinal  wall  projects 
from  the  upper  end  of  the  gill-slit  in  the  form  of  a  hollow  process 
down  into  the  slit,  changing  the  0  into  a  very  long  vertical  TJ 
(Fig.  457,  12).  This  hollow  process  is  the  tongue.  Its  cavity  is  in 
open  communication  with  the  coelom  of  the  trunk.  It  either  hangs 
freely  down  into  the  gill-slit  (Balanoglossus,  Glandiceps),  or  is  attached 
to  the  wall  of  the  gill-pouch  by  means  of  rods  or  buds,  the  so-called 
synaptieulse  which  run  across  the  limbs  of  the  U-shaped  gill-slit 
transversely,  making  the  latter  fenestrated. 


9    8    11  2 

FIG.  457.— Portion  of  the  branchial  region  of 
an  Enteropneustan,  cut  down  through  the 
middle  line,  and  seen  from  the  cut  surface, 
diagrammatic.  1,  Dorsal  vessel ;  2,  body  wall  ; 
3,  cavity  of  the  trunk  ;  4,  branchial  pore ;  5,  con- 


568 


COMPARATIVE  ANATOMY 


CHAP. 


The  partitions  between  the  consecutive  gill -pouches  are  called 
septa,  and  the  edges  of  these  which  are  turned  to  the  intestine  are 
the  septal  edges.  If  a  lateral  wall  of  the  branchial  intestine  be 
viewed  from  the  intestinal  cavity,  the  septal  edges  and  branchial 


JO  11 


FIG.  458. — Ptychodera  minuta,  transverse  section  through  the  branchial  region,  somewhat 
diagrammatic  (after  Spengel).  1,  Dorsal  nerve  cord  ;  2,  dorsal  blood  vessel ;  3,  branchial  furrow  ; 
4,  body  epithelium ;  5,  gonad ;  6,  longitudinal  muscle  layer  of  the  integument ;  7,  ventral  blood 
vessel ;  8,  ventral  nerve  cord ;  9,  ccelom  of  the  trunk  ;  10,  genital  pore ;  11,  branchial  pore ;  12, 
branchial  tongue  ;  13,  dividing  ridges ;  14,  branchial  septum  ;  15,  cavity  of  the  branchial  intestine  ; 
16,  oesophagus. 


tongues  are  seen  regularly  alternating.  The  septa,  like  the  tongues, 
are  hollow,  their  cavities  communicating  with  the  coelom  of  the  trunk. 
But  whereas  the  septal  edges  are  continued  both  dorsally  and  ventrally 
into  the  wall  of  the  intestine,  the  edges  of  the  tongues  turned  towards 
the  intestine,  the  so-called  backs  of  the  tongues,  are,  of  course,  only 
in  connection  with  the  intestinal  wall  dorsally. 


ENTEROPNEUSTA— ALIMENTARY  CANAL 


The  epithelial  walls  of  the  gill -pouches  and  of  the  tongues  are 
ciliated. 

The  depth  (measured  dorsoventrally)  of  the  area  occupied  by  the 
gill-slits  on  the  lateral  wall  of  the  branchial  intestine  varies  greatly. 

In  all  cases  the  gill-slits  leave  only  a  narrow  strip  of  the  intestinal 
wall  in  the  dorsal  median  line ;  this  strip  is  the  epibranehial  streak. 
Ventrally  they  never  extend  so  far  towards  the  median  line.  They 
either  leave  a  narrow  strip  of  the  intestinal  wall,  the  hypo-branchial 
streak,  which  is  at  any  rate  wider  than  the  epibranehial  streak  (Schizo- 
cardium),  or  they  only  extend  a  very  short  way  on  to  the  ventral  wall 
(Glandiceps),  or  again  they  only  reach  about  half  way  down  the  lateral 
wall  (Balonoglossiis).  In  the  last  case  the  hypobranchial  streak 


FIG.  459.— Vertical  longitudinal  section 
through  the  anterior  part  of  a  row  of 
gills,  and  through  a  collar  pore  of  Schizo- 
cardium  brasiliense  (after  Spengel).  cpj, 
Anterior  aperture  of  the  collar  canal  (into 
the  coeloin  of  the  collar) ;  cpo,  posterior  aper- 
ture (collar-pore)  of  the  same  (into  the  first 
gill-pouch) ;  bpi-bpQ,  first  to  sixth  branchial 
pores  (outer  apertures  of  the  gills) ;  bsi-bs^, 
first  to  fifth  gill-pouches  ;  6/4,  bf5,  fourth  and 
fifth  gill-slits  (apertures  of  the  gill-pouches 
into  the  branchial  intestine) ;  dvm,  dorsoven- 
tral  musculaturp ;  cc,  cceloin  of  the  collar ; 
re,  ccelom  of  the  trunk  ;  v,  blood  vessels ; 
Ic,  continuation  of  the  ccelom  of  the  trunk 
into  the  branchial  tongues;  I,  branchial 
tongues ;  s,  branchial  septa  ;  szi,  first  anterior 
septal  bar  or  prong ;  lz,  tongue  bars  or 
prongs ;  crs,  septum  dividing  the  trunk 
from  the  collar. 


occupies  the  ventral  or  nutritive  half  of  the  branchial  intestine,  which 
is  thus  more  or  less  distinct  from  the  dorsal  or  respiratory  half,  into 
which  the  gill-slits  open.  The  distinction  between  these  two  halves  is 
still  more  marked  in  Ptychodem  (Fig.  458,  15,  16),  inasmuch  as  they  are 
here  separated  by  longitudinal  ridge-like  projections  of  the  intestinal 
wall,  which  run  on  each  side  along  the  boundary  between  the  two 
(13).  The  two  ridges  growing  towards  one  another  may  even  touch, 
in  which  case  open  communication  between  the  branchial  intestine 
above  and  the  oesophagus  below  ceases. 

The  form  of  the  outer  apertures  of  the  gill-pouches,  the  branchial  pores,  has 
been  described  above.  The  furrows  in  which  they  lie  correspond,  in  Balanoglossus, 
Glandiceps  and  Schizocardium,  with  the  submedian  line,  which  is  indicated  by  the 


570  COMPARATIVE  ANATOMY  CHAP. 

interruption  of  the  longitudinal  musculature.  In  Ptychodera,  the  branchial  pores 
lie  mediad  of  this  line. 

The  gills  are,  as  a  rule,  paired,  but  in  species  of  Ptychodera,  those  belonging  to 
one  side  may  be  shifted  in  front  of  those  of  the  other  side  by  as  much  as  half  the 
breadth  of  a  gill. 

At  the  posterior  end  of  the  branchial  region,  even  in  adult  animals,  new  gills  are 
continually  being  formed. 

In  Ptychodera  clavigera,  each  gill -pouch  has  a  long  ventrally  directed  diver- 
ticulum. 

The  cavity  of  the  branchial  tongue  is  lined  with  endothelium,  and  traversed  in 
various  directions  b}r  fibres,  some  of  which,  no  doubt,  are  muscular. 

The  efferent  section  of  the  gill-pouches  is  provided  with  a  musculature,  which 
cannot  here  be  described.  The  pores  also  may  be  provided  with  an  encircling  sphincter 
musculature  of  their  own. 

In  Balanoglossus  Kowalevskii,  the  posterior  edge  of  the  collar  is  continued  back- 
ward as  two  outgrowths,  which  cover  the  most  anterior  branchial  pores.  These 
outgrowths  have  been  called  the  opercula,  and  the  small  space  they  enclose,  the 
atrium. 

For  the  blood  vessels  and  the  skeleton  of  the  branchial  intestine,  see  below, 
pp.  584  and  580. 

D.  The  afferent  intestine,  which  follows  the  branchial  intestine, 
runs  through  the  posterior  gill-less  part  of  the  branchio-genital  region, 
and  at  its  posterior  end  passes  over  into  the  hepatic  or  stomach  intestine. 
In  some  forms  the  afferent  intestine  is  distinguished  by  the  fact  that 
it  sends  off  dorsally  to  right  and  left  short  canals,  which  open  outward 
on  the  dorsal  surface.    These  efferent  canals  are  known  as  the  unpaired 
intestinal  pores,  because  they  are  for  the  most  part  unpaired. 

Special. — These  unpaired  intestinal  pores  are  found  in  Schizocardium  brasiliense, 
Glandiceps  Hacksii,  and  GL  talaboti.  In  Schi.  brasiliense  the  openings  are  irregular, 
either  paired  or  unpaired.  Twenty-nine  in  all  have  been  observed,  thirteen  on  the 
left  and  sixteen  on  the  right  side,  and  among  them  seven  pairs.  The  afferent  section 
of  the  alimentary  canal  is,  in  this  species,  distinguished  by  a  strong  circular 
musculature.  In  GL  Hacksii,  nine  unpaired  pores  were  observed  in  the  young 
animals  examined,  the  most  anterior  being  on  the  right,  and  the  rest  on  the  left. 
In  GL  talaboti,  all  the  pores  in  this  region  are  unpaired  ;  in  the  animals  examined 
they  are  arranged  in  nine  groups  at  irregular  distances  from  one  another.  The 
efferent  canals  of  each  group  probably  open  into  a  common  ampulla,  which,  on  its 
part,  opens  outward  through  a  single  aperture. 

E.  The  hepatic  or  stomachal  region  of  the  intestine  is,  in  all 
Enteropneusta,  distinguished  by  the  fact  that  its  epithelium  is  ciliated 
and  contains  numerous  globules  of  a  secretion,  usually  green  in  colour. 
This  section  of  the  intestine  seems  to  have  a  musculature  of  its  own 
only  in  Schizocardium  brasiliense,  in  which  it  is  developed  as  a  fine 
layer  of  longitudinal  fibres.     The  hepatic  intestine  is  no  doubt  the 
part  of  the  alimentary  canal  of  the  greatest  importance  for  digestion ; 
the  network  of  vascular  capillaries,  which  will  be  described  later,  is 
specially  strongly  developed  in  its  walls. 

The    hepatic    intestine    appears    as   a   specialised    section    of  the 


ENTEROPXEUSTA— CCELOMIC  SACS  571 

digestive  tract  only  in  those  species  of  Pfychodera  and  Schizocardium 
in  which  it  gives  off  on  each  side  dorsally  a  row  of  finger-  or  wedge- 
shaped  outgrowths,  which  push  out  the  body  wall  in  such  a  way 
as  to  form  the  above-mentioned  liver-caeca.  The  aperture  of  each 
caecum  into  the  alimentary  canal  is  a  narrow  transverse  slit.  Food 
never  passes  into  the  liver-caeca.  The  capillary  network  is  exceedingly 
close  in  their  walls,  and  the  intestinal  epithelium  of  the  caeca  is,  as  a 
rule,  much  folded. 

In  Glandiceps  Hacksii,  an  accessory  intestine  occurs  in  the  hepatic 
region  :  this  is  a  straight  canal,  ca.  6  mm.  long,  which  branches  off 
from  the  median  dorsal  surface  of  the  intestine  proper  about  the 
middle  of  the  region,  and  again  enters  it  at  the  posterior  end  of  the 
same  region. 

In  Schizocardium  brasiliense,  Glandiceps  Hacksii,  Balanoglossus  Kowa- 
levskii,  and  B.  Merschkovskii  (but  not  in  Ptycliodera  and  not  in  B.  Kupfferi, 
and  B.  canadensis)  paired  intestinal  pores,  leading  outward  dorsally,  are 
found  in  the  most  anterior  hepatic  region,  or  in  the  region  immediately 
in  front  of  it,  intercalated  between  it  and  the  afferent  intestine.  Schi. 
brasiticnse  has  one  pair,  Gl.  Hackm  three  pairs,  and  Balanoglossus  Kowa- 
l''c*kii  four  to  six  pairs  of  such  pores.  They  emerge  mediad  of  the 
submedian  line,  and  may  be  provided  with  cilia  and  with  sphincter 
muscles. 

F.  The  hepatic  intestine  is  followed  by  the  efferent  section, 
which  gradually  passes  into  the  narrower  rectum,  this  in  its  turn 
opening  outward  through  the  anus.  Where,  in  this  section,  a  proper 
musculature  is  found,  it  is  very  weakly  developed. 


VI.  The  Coelomie  Sacs  and  the  Body  Musculature. 

We  here  use  the  expression  coelomic  sacs  rather  than  eoelomic 
cavities,  the  former  implying  that  they  have  walls  of  their  own. 

Five  eoelomie  sacs  occur  in  the  body  of  an  Enteropneustan,  these 
being  divided  among  the  principal  regions  of  the  body  as  follows  : — 

The  proboscis  contains  one  unpaired  eoelomie  sac. 
The  collar  contains  two  paired  ecelomie  sacs. 
The  trunk  contains  two  paired  eoelomie  sacs. 

The  coelomic  sacs  fill  up  almost  the  whole  of  the  space  between 
the  intestinal  epithelium  and  the  body  epithelium,  i.e.  the  seg- 
mentation cavity  or  blastoeoel  of  the  larva,  with  the  exception  of 
a  system  of  spaces,  serving  as  the  blood  vascular  system,  which  will 
be  described  later. 

In  each  coelomic  sac  there  can  be  distinguished,  at  the  least,  a 
visceral  wall  in  contact  externally  with  the  intestinal  epithelium,  and 
a  parietal  wall,  in  contact  internally  with  the  body  epithelium. 

Where   the   ccelomic   sacs   are   paired,  i.e.   in  the  collar   and   the 


572  COMPARATIVE  ANATOMY  CHAP. 

trunk,  the  two  lateral  sacs  come  in  contact  with  one  another  above 
the  intestine  to  form  a  bilaminar  dorsal  mesentery,  and  below  the 
intestine  to  form  a  bilaminar  ventral  mesentery. 

In  the  adult  animal  these  mesenteries  are  nowhere  retained  in 
their  full  extent. 

Each  coelomic  sac  has  an  anterior  and  a  posterior  wall.  The 
posterior  wall  of  the  collar  sac  becomes  applied  to  the  anterior  wall 
of  the  trunk  sac  and  thus  forms  a  bilaminar  transverse  and  vertical 
septum,  separating  the  ecelom  of  the  collar  from  that  of  the 
trunk. 

The  walls  of  the  ccelomic  sacs,  in  the  larva,  are  epithelial. 
Throughout  the  greater  part  of  these  sacs,  however,  the  epithelial 
cells  become  transformed  into  muscle  fibres  to  form  the  musculature  of 
the  body  and  of  the  intestine.  And  this  takes  place  to  such  an 
extent  that  over  large  areas  no  endothelial  lining  to  the  body  cavity 
is  any  longer  demonstrable. 

Connective  tissue  is  also  produced  by  the  walls  of  the  coelomic 
sacs. 

The  musculature  of  the  Enteropneusta  consists  exclusively  of 
smooth  fibres. 

Lymph  cells  (probably  amoeboid)  float  in  the  fluid  of  the  body 
cavities :  these  are  presumably  produced  by  the  peritoneal  endo- 
thelium. 

A.  The  Coelom  of  the  Proboscis. 

The  proboscis  coelom  is,  as  above  mentioned,  unpaired.  The 
parietal  wall  lies  under  the  proboscidal  epithelium,  the  visceral  wall 
envelops  not  only  the  proboscidal  diverticulum  of  the  buccal  cavity, 
but  a  complex  of  other  organs  as  well,  which  lie  posteriorly  in  the 
base  of  the  proboscis ;  these  basal  organs,  to  a  certain  extent,  bulge 
out  the  coelomic  wall,  like  the  finger  of  a  glove,  from  behind  forward, 
into  the  proboscis  cavity. 

This  cavity  has  three  outgrowths  directed  backward  towards  the 
neck,  one  ventral  and  two,  a  right  and  a  left,  dorsal.  The  left  out- 
growth is  produced  backward  into  a  canal  lined  with  ciliated  epithe- 
lium, which  opens  outwards  through  the  proboscis  pore.  This  pore 
lies  dorsally  to,  and  on  the  left  side  of,  the  neck,  at  a  greater  or 
less  distance  from  the  median  line. 

In  a  few  forms  (constantly  in  Balanoglossus  Kupfferi  and  B.  cana- 
densis,  and  occasionally  in  Ptyclwdem  minuta  and  B.  KowalevsJcii)  a 
second  proboscis  pore  occurs,  through  which  the  right  dorsal  outgrowth 
of  the  proboscis  coelom  opens  outward.  This  secondary  proboscis 
pore  rises  much  later  ontogenetically  than  the  primary. 

It  has  been  conjectured  that  water  is  taken  in  through  these  pores 
for  the  purpose  of  swelling  the  proboscis.  There  is  no  justification 
for  ascribing  to  them  any  excretory  function. 

The  visceral  wall  of  the  proboscidal  coelomic  sac  and,  in  general. 


ix  EXTEROPXEUSTA—CCELOMIC  SACS  573 

the  walls  of  the  posterior  outgrowths,  retain  an  epithelial  character, 
while  the  parietal  wall  develops  muscle  and  connective  tissue.  This 
parietal  wall  consists  of  the  following  parts  : — 

1.  Close  under  the  basal   or   limiting   membrane   of   the  body 
epithelium,  there  is  an  outermost  layer  of  circular  muscle  fibres. 

2.  This  latter  is  followed   by  a  massive  layer  of   longitudinal 
muscles,  filling  up   the   greater    part  of   the  proboscis.      The  very 
complicated  course  of  the  longitudinal  muscle  fibres  cannot  here  be 
described    in    detail.      They    are    stretched    like    the    strings  *of  an 
instrument  between  two  points  of  the  proboscidal  wall,  one  behind  the 
other,  so  that  they  cross  one  another  in  every  direction. 

3.  Dorsoventral    muscle   fibres    form   a   dorsoventral   muscle 
septum    exactly   in   the    median    plane    of  the    proboscis.     This    is, 
however,   not    developed    through   the    whole  length  of  the   cavity, 
but  reaches  only  as  far  forward  as  the  proboscidal  diverticulum  of 
the  intestine  or  its  vermiform  process.     This  muscle  septum  thus  has  a 
free  anterior  edge.     The  fibres  of  the  septum,  which  descend  from  the 
median  line,  when  they  reach  the  basal  organs,  diverge  to  right  and 
left,  clasping  these  organs  between  them,  then  again  uniting  beneath 
them,  form  the  ventral  portion  of  the  septum.     This  ventral  portion 
is  distinctly  a  double  muscle  lamella.     The  two  constituent  lamellae 
are  separated  by  a  structureless  limiting  lamella,  which  is  a  continua- 
tion of  the  limiting  membrane  of  the  ventral  proboscidal  epithelium. 
The  circular  muscle  layer  of  the  proboscis  passes  through  the  limiting 
membrane  of  the  ventral  septum  in  bundles. 

The  ventral  septum  is  interrupted  at  its  most  posterior  part,  so 
that  it  has  a  free  posterior  edge  as  well  as  a  free  anterior  edge. 

That  portion  of  the  proboscidal  cavity  which  is  free  from  muscle 
fibres,  is  to  a  great  extent  filled  with  connective  tissue,  in  which 
irregular  spaces  are  found  as  remains  of  the  cavity.  A  space  free 
from  connective  tissue  and  varying  in  size  is  retained  round  the  basal 
organs. 

B.  The  Coelomie  Sacs  and  the  Musculature  of  the  Collar. 

The  collar  region  of  the  body  contains  not  only  its  own  two 
coelomic  sacs,  but  outgrowths  or  processes  of  the  trunk  ccelom  as 
well ;  these  latter  have  been  called  peripharyngeal  or  perihaamal 
cavities,  and  will  be  described  with  the  trunk  ccelom.  The  two 
lateral  ccelomic  sacs  of  the  collar  are,  in  adults,  nowhere  completely 
separated  from  one  another  by  mesenteries.  The  median  ventral 
mesentery  is  retained  for  a  short  distance  in  the  posterior  region  of  the 
collar.  The  dorsal  mesentery  extends  further  forward,  but  never  as 
far  as  to  the  anterior  end  of  the  collar.  In  Balanoglossus  Kupfferi,  both 
the  mesenteries  are  altogether  wanting. 

The  divisions  of  the  collar  ccelom  are  complicated  by  the  appear- 
ance of  folds  in  the  inner  or  visceral  wall.  The  two  lamellae  of 


574  COMPARATIVE  ANATOMY  CHAP. 

these  folds  lie  close  to  one  another,  being  only  separated  by  a  limiting 
membrane  containing  vessels.  According  to  the  courses  of  these 
vascular  folds  which  project  into  the  collar  ccelom,  the  Enteropneusta 
can  be  divided  into  two  groups. 

GroUp  i— Balanoglossus,  Glandiceps,  Schizocardium. —A  fold  commences  at  the 
posterior  end  of  the  ccelom  on  each  side,  near  the  ventral  median  line,  and  ascends 
diagonally  in  a  curve  anteriorly  to  the  neck  of  the  proboscis. 

Group  2 — Ptychodera. — A  medio-ventral  vascular  fold  runs  anteriorly  from  the 
posterior  end  of  the  collar  region,  dividing  at  a  short  distance  from  the  anterior 
end  of  that  region  into  two  folds,  which  ascend  perpendicularly  and  encircle  the 
buccal  cavity. 

The  walls  of  the  collar  coelom  are  for  the  most  part  developed  as 
muscles. 

1.  The  parietal  wall  consists  first  of  an  outer  layer  of  longitudinal  muscle 
fibres.     These  commence,  it  is  true,  posteriorly  in  the  visceral  wall,  then  run  slant- 
ingly forwards  and  outwards  towards  the  integument,  traversing  the  coelom.     Only 
in  the  anterior  collar  region  do  they  run  close  under  the  integument  to  the  anterior 
end  of  the  collar.     Only  in  this  anterior  region  of  the   collar  also  is  a  circular 
muscle  layer  developed  on  the  inner  side  of  the  longitudinal  musculature. 

2.  The  visceral  wall  contains  first  an  inner  longitudinal  musculature,  according 
to  the  arrangement  of  which  the  Enteropneusta  may  be  divided  into  two  groups. 

Group  1— Schizocardium,  Glandiceps,  Balanoglossus. — A  bundle  of  longitudinal 
fibres  rises  on  each  side  anteriorly,  from  the  proboscidal  skeleton,  which  will  be 
described  later.  These  muscles  spread  out  fan-like  towards  the  septum  between  the 
collar  and  the  trunk.  The  fibres  composing  this  fan  slope  more  and  more  the 
nearer  the  ventral  middle  line  they  become  attached  to  that  septum.  Anteriorly,  the 
two  bundles  of  fibres  surround  the  efferent  vessel  of  the  collar. 

Group  2  —  Ptychodera.  —  The  numerous  bundles  of  longitudinal  fibres  run 
parallel  to  one  another.  Only  a  few  of  them,  viz.  those  lying  nearest  the  median 
line,  run  as  far  as  the  neck,  enclosing  the  efferent  collar  vessel  and  becoming 
attached  to  the  skeleton  of  the  proboscis.  None  of  the  rest  reach  the  anterior 
end  of  the  collar  region,  but  become  attached  to  the  posterior  wall  of  the  vascular 
fold  mentioned  above  as  encircling  the  buccal  cavity. 

The  visceral  wall  of  the  collar  coelom  further  consists  of  a  transverse  musculature. 
This  also  is  differently  arranged  in  the  groups  mentioned  above,  its  distribution 
being  determined  by  the  courses  of  the  vascular  folds. 

In  group  1  (Schizocardium,  Gflandiccps,  and  Balanoglossus)  the  transverse  fibres 
run  out  laterally  from  the  dorsal  point  of  attachment  of  the  ventral  mesentery  on 
each  side  and  then  upwards,  to  become  attached  to  the  vascular  fold  above  described 
which  runs  from  behind  and  below,  anteriorly  and  upward. 

Where  the  ventral  mesentery  is  wanting,  the  transverse  fibres  run  without 
interruption  from  the  right  to  the  left  vascular  fold,  surrounding  the  buccal  cavity 
ventrally.  The  transverse  fibres  are  quite  short  posteriorly  and  limited  to  the 
ventral  side,  for  here  the  vascular  folds  are  but  commencing  to  diverge.  Anteriorly, 
as  the  folds  gradually  meet  over  the  buccal  cavity,  the  fibres  become  longer  and 
longer  ;  they  finally  form,  near  the  insertion  of  the  neck,  circular  loops  almost 
completely  surrounding  the  buccal  cavity,  these  loops  being  interrupted  for  only  a 
short  distance  dorsally. 

In    group    2    (Ptychodera}  this   transverse  musculature   is   altogether   wanting 


ix  EXTEROPXEUSTA— CCELOMIC  SACS  575 

throughout  the  larger  part  of  the  collar,  viz.  in  all  that  part  which  lies  behind  the 
circular  vascular  fold.  In  front  of  this  fold,  however,  it  is  as  highly  developed  as 
in  the  same  area  in  group  1.  The  muscles  arise  dorsally  at  each  side  of  the  pro- 
boscidal  skeleton,  and  form  loops  encircling  the  buccal  cavity. 

3.  The  anterior  wall  of  the  collar  ccelom. — A  strong  bundle  of  muscle  fibres 
arises  to  right  and  left  of  the  neck  of  the  proboscis  ;  these  radiate  from  the  anterior 
wall  of  the  collar  ccelom  to  the  edge  of  the  collar.  Both  ventrally  and  dorsally 
the  marginal  fibres  of  these  two  radiating  bundles  usually  pass  beyond  the  median 
line,  cross  one  another,  and  intermingle  at  these  points. 

Besides  the  musculature  hitherto  described  of  the  parietal  visceral  and  anterior* 
walls  of  the  collar  ccelom,  there  are  further  isolated  radial  muscle  fibres,  which] 
connect  the  outer  with  the  inner  wall,  and  also  with  the  anterior  wall.  These\ 
fibres  together  form  a  system  of  crossing  fibres,  some  running  slantingly  from  the] 
outer  wall  inward  and  forward,  and  others  inward  and  backward. 

The  cavity  of  the  collar  is  filled  with  connective  tissue,  which  penetrates  every- 
where between  the  muscles,  leaving  only  certain  spaces  free.  Such  a  free  space  is 
as  a  rule  found  to  the  right  and  left  in  the  posterior  part  of  the  collar.  The  collar 
ccelom  here  is  continued  on  each  side  into  a  canal  lined  with  a  ciliated  cylin- 
drical epithelium  ;  each  canal  opens  through  a  pore  (collar-pore),  not  on  the 
external  surface  of  the  body,  but  on  the  anterior  wall  of  the  first  gill-pouch,  near 
the  branchial  pore.  The  canal  projects  from  this  pore  freely  forward  into  the 
collar  ccelom,  and,  on  its  outer  surface,  that  turned  to  this  ccelom,  is  covered  with 
plate  epithelium. 

As  to  the  function  of  these  two  collar  pores,  we  can  say  no  more  than  what 
was  said  above  about  that  of  the  proboscis  pore. 

In  Balanoglossus  Kupfferi  a  cushion-like  thickening  of  the  epithelium  is  found 
on  each  side  of  the  body  on  the  septum  dividing  the  collar  from  the  trunk.  This 
thickening  occurs  both  on  the  anterior  surface,  that  turned  to  the  collar  ccelom, 
and  on  the  posterior  surface,  that  turned  to  the  trunk  ccelom.  These  thickenings 
probably  function  as  lymph  glands. 


C.  The  Coelomic  Sacs  and  the  Musculature  of  the  Trunk. 

The  Perihsemal  and  Peripharyngeal  Cavities  of  the  Collar  Region. 

The  trunk  ccelom  is  uninterrupted  throughout  its  whole  length. 
Its  composition  out  of  two  lateral  ccelomic  sacs  can  still  be  recognised 
in  the  adult  animal,  the  ventral  mesentery  being  entirely,  the  dorsal 
partially,  retained. 

The  ccelomic  sacs  of  the  trunk  send  outgrowths  anteriorly  into 
the  cavity  of  the  collar,  which  push  before  them  the  wall  of  that 
cavity ;  these  are  the  perihsemal  and  peripharyng-eal  cavities 
(Fig/460). 

The  perihsemal  cavities  are  two  dorsal  prolongations  of  the  trunk 
ccelom,  which  traverse  the  collar  region  and  the  neck  of  the 
proboscis  as  far  as  the  proboscidal  skeleton.  They  run  below  the 
collar  cord  and  above  the  buccal  cavity.  In  the  median  line  the  two 
cavities  are  separated  by  a  structureless  partition,  a  limiting  mem- 
brane, in  which  the  dorsal  vessel  runs.  The  perihaemal  spaces  are 
almost  entirely  filled  by  longitudinal  muscle  fibres  formed  by  their 


576 


COMPARATIVE  ANATOMY 


CHAP. 


dorsal  walls.  These  are  the  immediate  anterior  continuations  of  the 
dorsal  longitudinal  musculature  of  the  trunk.  In  Ptychodera,  a  single 
weak  layer  of  longitudinal  muscle  also  develops  on  the  ventral  walls 
of  the  perihsemal  cavities.  In  Schizocardium  and  Glandiceps,  on  the 
contrary,  a  transverse  musculature  is  here  developed.  In  the  genus 
Balanoglossus,  both  longitudinal  and  transverse  muscles  are  wanting 
in  the  ventral  wall. 

Besides  the  muscles  which  have  been  mentioned,  there  are  fibres 
which  traverse  the  perihaemal  cavity  transversely,  chiefly  in  a  dorso- 
ventral  direction. 

The  peripharyngeal  cavities  are  also  anterior  continuations  of 


FIG.  460.— Ptychodera  minuta,  diagram  of  the  collar 
coelom  and  of  the  anterior  region  of  the  trunk,  in  an 
almost  median  longitudinal  section  (after  Spengel),  some- 
what modified.  1,  Anterior  wall  of  the  collar ;  2,  collar 
coelom ;  3,  peripharyngeal  cavity ;  4,  perihsemal  canal ; 
5,  buccal  cavity  ;  6,  septum  dividing  the  collar  from  the 
trunk  ;  V,  trunk  coelom  ;  8,  oesophagus. 


FIG.  461.— Ptychodera  minuta, 
transverse  section  of  the  body  through 
the  genital  region,  diagram  to  illustrate 
the  arrangement  of  the  coelom.  d,  Dor- 
sal ;  v,  ventral ;  1,  dorsal  mesentery ; 
2,  dorsal  accessory  chambers  of  the 
trunk  coelom  ;  3,  lateral  mesenteries  ; 
4,  body  epithelium ;  5,  parietal  wall  ; 

6,  visceral  wall  of  the  trunk  coelom ; 

7,  intestinal  epithelium  ;  8,  intestinal 
cavity ;    9,  principal  chamber  of   the 
trunk  coelom  ;  10,  ventral  mesentery. 


the  trunk  coelom,  which  push  in  between  the  buccal  cavity  (pharynx) 
on  the  one  side  and  the  collar  ccelom  on  the  other,  and,  in  Ptychodera, 
surround  the  buccal  cavity.  Anteriorly,  they  end  dorsally  at  the 
point  where  the  proboscidal  diverticulum  of  the  intestine  arises,  and 
laterally,  at  the  points  of  attachment  of  the  vascular  folds.  The 
inner  wall  of  the  peripharyngeal  cavities  consists  of  a  layer  of  circular 
muscle  fibres  which  surrounds  the  buccal  cavity,  closely  applied  to 
its  epithelium,  and  only  separated  from  it  by  a  limiting  membrane. 

In  Schizocardium,  the  two  peripharyngeal  cavities  are  less  exten- 
sive, they  lie  at  the  sides  of  the  buccal  cavity,  without  surrounding 
it  either  ventrally  or  dorsally.  Each  peripharyngeal  cavity  forms  a 


ix  EXTEROPNEUSTA—CCELOMIC  SACS  577 

triangle,  whose  sides  are  constituted  as  follows.  The  first  (posterior) 
side  corresponds  with  its  origin  out  of  the  trunk  ccelom ;  the  second 
(dorsal)  with  the  lateral  edge  of  the  perihaemal  cavity;  the  third 
(anterior  and  lower)  with  the  line  of  insertion  of  the  vascular  fold. 
There  is  a  corresponding  limitation  and  circumscription  of  the  trans- 
verse muscle  fibres  on  the  lateral  walls  of  the  buccal  cavity.  Never- 
theless a  closed  muscular  envelope  is  formed:  (1)  dorsally,  by  means 
of  the  transverse  musculature  on  the  lower  walls  of  the  perihaemal 
cavities ;  (2)  ventrally,  by  the  transverse  musculature  of  the  collar 
coelom. 

Peripharyngeal  cavities  are  found,  not  only  in  Ptychodera  and  Schizocardium, 
but  in  Balanoglossui  Kowalevskii,  in  the  same  form  as  in  Schizocardium,  but  pro- 
vided with  longitudinal  instead  of  transverse  muscles,  not  belonging,  however,  to 
the  inner  or  visceral  wall,  but  to  the  outer  parietal  wall  which  is  in  contact  with 
the  collar  ccelom. 

In  Ptychodera,  a  further  complication  occurs  in  the  divisions  of  the  trunk 
ccelom.  In  the  anterior  hepatic  and  the  branchiogenital  regions,  an  accessory  or 
lateral  mesentery  (Fig.  461)  occurs  on  each  side  dorsally  in  the  submedian  line,  in 
addition  to  the  two  principal  (median)  mesenteries.  This  accessory  mesentery  runs 
from  the  intestine  to  the  integument,  dividing  the  ccelom  at  this  point  into  four 
chambers,  two  large,  ventral,  principal  chambers,  and  two  small  dorsal  accessory 
chambers.  The  accessory  chambers  open  posteriorly  into  the  principal  chambers, 
the  accessory  mesentery  disappearing  ;  anteriorly,  they  narrow  and  end  in  the 
branchial  region.  They  are  here  no  longer  in  contact  with  the  intestine,  but  only 
with  the  integument,  the  accessory  mesenteries  here  shifting  their  visceral  edges 
of  attachment  on  to  the  integument,  in  a  manner  which  cannot  here  be  described 
more  in  detail. 

By  far  the  greater  part  of  the  walls  of  the  trunk  coelom  are 
taken  up  in  the  formation  of  musculature.  The  parietal  wall  most 
especially  becomes  differentiated  into  a  powerful  dermo-museular  tube 
which  gradually  diminishes  in  strength  posteriorly. 

The  most  important  and  constant  part  of  this  dermo-muscular 
tube  is  the  longitudinal  musculature. 

The  longitudinal  musculature,  which  is  specially  strongly  developed  on  the 
ventral  side  of  the  body,  in  the  genital  folds  (where  these  are  developed),  and  on 
the  dorsal  side  in  the  branchial  region,  is  interrupted  in  the  dorsal  and  ventral 
median  lines  by  the  median  mesenteries.  A  similar  interruption  takes  place  in 
the  branchiogenital  region  in  the  submedian  lines,  in  which  the  gonads,  and,  in  the 
genera  Balanoglossus,  Gflandiceps,  and  Schizocardium  the  gills  also,  open. 

By  these  four  lines  of  interruption,  the  longitudinal  musculature  is  divided 
into  two  dorsal  and  two  ventrolateral  areas.  (B.  canadensis  has  two  streaks  on 
each  side  free  from  muscle,  and  gonads  open  in  both.) 

Each  longitudinal  fibre  runs  in  a  curve  between  two  points,  one  behind  the 
other,  on  the  limiting  membrane  of  the  body  epithelium.  Each  fibre  thus  crosses 
numberless  others. 

In  Ptychodera,  in  addition  to  these,  an  outer  circular  muscle 
layer  is  also  differentiated  from  the  parietal  wall  of  the  trunk  coelom  ; 
the  fibres  of  this  layer  pass  through  the  mesenteries. 

VOL.  II  2  P 


578  COMPARATIVE  ANATOMY  CHAP. 

A  true  circular  muscle  layer  is  nowhere  else  developed.  Such  a 
layer  is,  however,  functionally  replaced  by  pseudo-circular  muscle 
fibres  which  run  on  the  inner  side  of  the  longitudinal  musculature, 
but  which  in  reality  do  not  form  a  closed  ring. 

In  Schizocardium,  the  bundles  of  these  pseudo-circular  muscle  fibres  run  on 
each  side  from  the  dorsal  edge  of  the  mediodorsal  mesentery  to  the  dorsal  edge  of 
the  ventral  mesentery.  Similar  bundles  arise  near  the  ventral  edge  of  the  ventral 
mesentery,  and  break  up  into  fibres  on  the  lateral  walls  of  the  body,  ascending 
along  the  inner  side  of  the  longitudinal  musculature,  traversing  it,  and  becoming 
attached  to  the  limiting  membrane  of  the  body.  In  Glandiceps,  this  latter  system 
is  repeated  (but  of  course  reversed)  on  the  dorsal  side. 

Balanoglossus  has  neither  the  outer  true,  nor  the  inner  pseudo-,  circular  mus- 
culature. 

Radial  muscle  fibres  connect  the  limiting  or  basal  membrane  of 
the  body  epithelium  with  the  limiting  membrane  of  the  intestine 
throughout  the  whole  coelom.  In  the  genital  folds,  these  fibres  are 
stretched  between  opposite  points  of  the  integument.  In  the  region 
of  the  lateral  mesenteries,  similar  fibres  stretch  between  these  and  the 
integument. 


VII.  The  "Heart  Vesicle"  (Figs.  456,  21,  p.  566  ;  464,  11,  p.  583). 

This  is  one  of  the  names l  suggested  for  a  small  closed  sac  which 
lies  upon  the  proboseidal  divertieulum  of  the  intestine  in  the  basal 
part  of  the  proboscis.  Its  ventral  wall  bends  down  somewhat  over 
the  divertieulum  to  right  and  left,  and  it  is  separated  from  the  latter 
by  a  small  blood  sinus.  Posteriorly,  tOAvards  the  neck  of  the  pro- 
boscis, the  "  heart  vesicle "  is  drawn  out  to  a  small  tip,  which  is 
traversed  by  fibres,  most  probably  muscular,  chiefly  in  a  transverse 
direction,  while  the  rest  of  the  vesicle  contains  a  fluid  as  clear  as 
water.  The  median  part  of  the  posterior  and  dorsal  wall  is  in  con- 
tact with  the  body  epithelium  of  the  neck  of  the  proboscis. 

The  ventral  wall  is  formed  of  a  single  layer  of  transverse  muscle 
fibres  and  pear-shaped  cells,  while  the  rest  of  the  wall  is  represented 
by  a  plate  epithelium.  The  existence  of  a  closed  circular  muscula- 
ture has  not  yet  been  demonstrated. 

The  "  heart  vesicle  "  in  Schizocardium  (and  to  a  lesser  degree  in  Glandiceps  also) 
is  produced  anteriorly  into  two  large  symmetrically  arranged  tips,  the  auricles. 
From  the  posterior  tip  of  the  "heart  vesicle  "  two  bundles  of  muscle  fibres  arise  which 
pass  anteriorly  into  these  auricles,  each  one  giving  off  fibres,  one  after  the  other,  to 
the  wall  of  the  auricle  it  enters. 

It  must  be  emphasised  that  the  "heart  vesicle"  does  not  belong  to  the  blood 
vascular  system,  and  does  not  communicate  with  it,  but  is  merely  in  contact  with 
part  of  that  system.  If,  therefore,  the  vesicle  propels  the  blood,  this  can  only 

1  Morgan  suggests  "Proboscis  vesicle. "— TR. 


EXTEROPXEUSTA— LIMITING  MEMBRANES  579 

occur  in  a  way  similar  to  that  seen  in  the  lower  Crustacea,  where  the  contractions 
of  the  intestines  are  able  to  set  the  body  fluid  in  motion. 

The  "  heart  vesicle  "  appears,  according  to  recent  researches,  to  be  of  ectodermal 
origin,  and  thus  cannot  be  considered  as  a  ccelomic  vesicle. 


VIII.  The  Limiting  Membranes,  the  Proboseidal  Skeleton,  and 
the  Branchial  Skeleton. 

Throughout  the  whole  body  of  the  Enteropneusta,  the  walls  of 
organs  which  are  in  contact  are  separated  from  one  another  by 
structureless  limiting  membranes,  which  are  to  be  regarded  as 
secretions  of  these  walls.  These  limiting  membranes  must  be  thought 
of  as  composed  for  the  most  part  of  two  adhering  laminae.  The  blood 
vessels  lie  within  the  limiting  membranes ;  they  represent  a  system 
of  spaces  between  the  two  laminae. 

In  secreting  the  limiting  membranes,  the  histological  character  of 
the  secreting  walls  is  of  no  consequence.  A  muscle  wall  can  secrete 
a  limiting  membrane  just  as  well  as  an  epithelial  wall. 

After  the  foregoing  description  the  reader  will  be  able  to  under- 
stand without  further  assistance  the  occurrence  and  arrangement  of 
limiting  membranes  in  the  body.  He  will,  for  example,  know  that  a 
limiting  membrane  exists  everywhere  below  the  body  epithelium, 
secreted  by  that  epithelium  on  the  one  hand,  and  by  the  parietal 
wall  of  the  coelom  on  the  other. 

A  similar  limiting  membrane  must  also  occur  between  the  visceral 
wall  of  the  coelomic  sacs  and  the  intestinal  epithelium,  as  also  between 
the  anterior  and  posterior  walls  composing  the  septa,  which  separate 
collar  from  trunk,  peripharyngeal  cavities  from  collar  ccelom,  etc.  etc. 
At  certain  points,  especially  in  the  proboscis  and  on  the  branchial 
intestine,  the  limiting  membrane  becomes  thickened,  and  forms  the 
proboseidal  and  branchial  skeletons. 

A.  The  proboseidal  skeleton  consists  of  a  median  body  and  two 
limbs  diverging  backward.  The  body  of  the  proboseidal  skeleton  lies 
in  the  neck  of  the  proboscis,  between  the  neck  of  the  proboseidal 
diverticulum  of  the  buccal  cavity  above,  and  the  ventral  body 
epithelium  of  the  neck  of  the  proboscis  below.  The  limbs  diverge  to 
right  and  left  into  the  collar  region,  clasping  from  above  the  entrance 
to  the  buccal  cavity,  and  in  close  contact  with  its  epithelium. 

The  proboseidal  skeleton  is  further  strengthened  laterally  by  the 
ehondroid  tissue  which  becomes  attached  to  it.  The  ground  sub- 
stance of  this  tissue  is  identical  with  the  substance  of  the  proboseidal 
skeleton,  and  of  the  limiting  membranes  generally.  It  is  secreted  by 
the  anterior  wall  of  the  collar  ccelom,  and  by  the  posterior  wall  of 
the  proboseidal  ccelom,  or  by  the  latter  alone ;  but  cell  processes  of 
these  walls  remain  in  the  secreted  ground  substance,  and  these  processes 
may  break  up  into  cell  groups  or  nests,  which  give  sections  of  the 


580 


COMPARATIVE  ANATOMY 


CHAP. 


chondroid  tissue  a  certain   similarity  to  cartilage.     This  chondroid 
tissue    is  most  developed,  forming  a   mass    thicker    than  the   pro- 


S-Ji 


FIG.  462.—  Gill  slits  and  branchial  skeleton  of  an  Enteropneustan.  The  six  hindermost 
gills  seen  from  the  intestine,  the  three  posterior  in  the  act  of  forming,  diagrammatic.  The  black 
parts  represent  the  U-shaped  gill  slits  ;  the  dotted  parts,  the  skeletal  forks.  1,  Branchial  tongue  ; 
2,  branchial  septum  ;  3,  anterior  prong  ;  4,  median  or  septal  prong  ;  5,  posterior  lingual  prong  of 
a  three-pronged  skeletal  fork. 

boscidal  skeleton,  which  always  remains  at  its  centre,  in  the  genera 

Schizocardium  and  Glandiceps. 

B.  The  Branchial  skeleton  (Figs.  462  and  463).     (Cf.  here  pp. 

567  and  568  on  the  gill  slits,  the  branchial  septa,  and  the  branchial 

tongues.) 

The  branchial  skeleton  here,  again,  consists  of  local  thickenings  of 

the  limiting  membrane,  which  separates 
the  epithelium  of  the  branchial  intes- 
tine from  the  visceral  wall  of  the  trunk 
coalom  of  the  branchio-genital  region. 
These  thickenings  are  in  the  form  of 
upright  three  -pronged  skeletal  forks, 
which  are  arranged  on  each  side,  in  a 
single  longitudinal  row,  throughout  the 
whole  length  of  the  branchial  region. 
The  number  of  forks  corresponds  with 
that  of  the  gills.  The  free  ends  of  the 
prong  are  turned  downwards,  and  the 
connecting  piece  upwards.  The  three 
prongs  of  a  fork  are  arranged  as 
follows.  The  middle  prong  lies  in  a 
branchial  septum,  under  the  surface 

FIG.  463.—  The  three  anterior  forks  of  Of    the   septal   edge,    which   is   turned 

l™^  ^  ™ity  <*  the  branchial  in- 

tCStme.       This    Septal     prong     f  Orks    at 

Jtg   free   lower   Clld,    giving    off   a   short 
.  , 

anterior  and  a  posterior  branch. 

The  anterior  prong  of  a  fork  lies 
on  the  posterior  wall  of  the  branchial  tongue,  immediately  in  front  of 
the  septum  ;  the  posterior  prong  in  the  anterior  wall  of  the  branchial 


4 
A 

i 

/—- 

n 

I 

"Y 

E 

P 
A^ 

I 

^% 
II 

-2 

—z 

f 

3  — 

J          ' 

V 

J 

V 

/ 

I 

anterior  (I)  has  only  two  prongs.  1,  A  pos- 
terior  lingual  prong  ;  2,  a  septal  prong  (in 
its  origin  double);  3,  an  anterior  lingual 

prong 


ix  ENTEBOPNEUSTA— BLOOD  VASCULAR  SYSTEM          581 


tongue,  immediately  behind  the  septum.  Each  fork  thus  has  a 
median  septal  prong,  and  an  anterior  and  a  posterior  lingual  prong. 
Each  tongue  has  two  prongs,  one  anterior  and  the  other  posterior, 
but  these  belong  to  two  different  forks.  Each  septum  has  only  one 
prong.  A  very  minute  examination  shows,  however,  that  each  septal 
prong  consists  of  two  fused  prongs.  Two-pronged  forks  must,  there- 
fore, be  the  ultimate  elements  of  the  branchial  skeleton.  Each  fork 
would  lie  with  one  prong  in  a  tongue  and  the  other  in  a  septum. 
The  two  septal  prongs  belonging  to  two  consecutive  two-pronged 
forks,  are,  however,  in  every  case  fused  together. 

The  most  anterior  skeletal  fork,  and  it  alone,  has  two  prongs. 

In  the  formation  of  the  lingual  prongs,  the  branchial  epithelium 
(belonging  to  the  intestine)  and  the  mesodermal,  inner  wall  of  the 
lingual  cavity  (belonging  to  the  visceral  wall  of  the  trunk  ccelom) 
take  part,  but  the  septal  prongs  are  secreted  exclusively  by  the 
branchial  epithelium  of  the  septal  edge. 


IX.  The  Blood  Vascular  System. 

The  blood  vascular  system  consists  of  spaces  in  the  limiting  mem- 
branes of  the  body.  The  two  lamellae  of  the  limiting  membranes 
simply  remain  apart  at  certain  points,  thus  forming  the  walls  of 
the  vessels.  An  endothelium-like  covering  of  the  inner  side  of  the 
separating  lamellae  has  only  been  found  in  Ptychodera,  and  in  isolated 
parts  in  SchizocanUum  and  Glandiceps.  Nothing  of  the  sort  has  been 
observed  in  Balanoglossus.  In  Ptychodera,  isolated  blood  cells  float  in 
the  colourless  blood  fluid. 

The  lacunar  blood  vessels  of  the  Enteropneusta  do  not  arise  by 
the  separation  of  the  formerly  contiguous  lamellae  of  a  limiting 
membrane.  They  are,  rather,  persistent  portions  of  the  larval 
segmentation  cavity  or  blastoeoel.  The  organs  of  the  larva  lie  in 
a  spacious  blastoeoel,  which  narrows  and  disappears  in  proportion  as 
the  organs  (especially  the  ccelomic  sacs)  increase  in  size,  these 
swelling  up  in  such  a  way  that  their  walls  come  in  contact  with  one 
another  and  with  the  body  and  intestinal  epithelium.  Certain 
cavities,  however,  persist,  which  afterwards  form  the  blood  vascular 
system.  The  blood  cells  and  endothelial  cells  of  the  vascular  system 
are,  in  all  cases,  of  mesenchymatous  origin. 

The  arrangement  of  the  vascular  system  may  be  roughly  described 
as  follows. 

There  is  a  capillary  network  in  all  the  limiting  membranes  of 
the  body,  especially  in  that  of  the  integument  and  of  the  intestine. 
This  network  is  in  connection  with  larger  vessels,  i.e.  (1)  with  a  dorsal 
vessel  which,  in  the  dorsal  mesentery,  runs  through  the  trunk  and 
collar  and  communicates  with  the  blood  vessels  of  the  proboscis,  and 
(2)  with  a  ventral  vessel  which,  running  in  the  ventral  mesentery  of 


582  COMPARATIVE  ANATOMY  CHAP. 

the  trunk,  receives  blood  from  the  proboscis  through  two  lateral  vessels 
or  vascular  plexuses  within  the  two  vascular  folds  of  the  collar,  these 
vessels  or  plexuses  usually  uniting  in  the  ventral  median  line  at  the 
posterior  end  of  the  collar  region.  The  dorsal  and  the  ventral 
vessels  of  the  trunk  have  muscular  walls,  which,  however,  do  not 
properly  belong  to  them,  but  are  borrowed  from  the  apposed  walls  of 
the  mesenterial  portions  of  the  coelomic  sacs.  In  the  proboscis,  the 
blood  vascular  system,  by  increase  of  its  surface  towards  the  probosci- 
dal  ccelom,  to  right  and  left  of  the  basal  complex  of  organs,  gives 
rise  to  the  so-called  proboseidal  gill  or  glomerulus. 

Special. — The  finer  details  cannot  be  entered  upon. 

1.  Vessels  of  the  trunk. — While   the   dorsal  mesentery   is  retained   in  the 
abdominal  part  of  the  body  (as  already  noted,   the  ventral   mesentery  persists 
throughout  the  whole  trunk)  it  may  disappear  in  the  anterior  trunk  region  with  the 
exception  of  the  part  which  contains  the  dorsal  longitudinal  vessel.     The  muscles 
of  the  vascular  trunks  are  transverse  or  circular  muscles,  and,  in  part  at  least, 
continuations  on  to  the  mesenteries  of  the  circular  musculature  of  the  body.     The 
ventral  vascular  trunk  of  B.  Kowalevskii  is  provided,  not  with  a  transverse,  but 
with  a  longitudinal   musculature.     The  musculature   (which   is   yielded   by   the 
mesenterial  endothelium)    always    lies   on   the   side   of    the   limiting    membrane 
away  from  the  lumen  of  the  vessel,  and  facing  the  body  cavity. 

2.  The  dorsal  vessel  of  the  collar  is  the  direct  continuation  of  the  dorsal  vessel 
of  the  trunk.     It  runs  between  the  two  perihfemal  cavities,  from  whose  walls  it 
borrows  its  musculature.     Passing  out  again  from  between  these  cavities,  it  loses 
its  musculature  and  opens,  in  the  proboscis,  into  a  blood  sinus,  the  basal  sinus  of 
the  proboscis.     This  is  a  space  left  between  various  heterogeneous  organs, — the 
proboscis  pore,  the  diverticulum  of  the  intestine,  the  posterior  tip  of  the  ' '  heart 
vesicle,"  the  epithelium  of  the  neck  of  the  proboscis. 

This  basal  sinus  communicates,  on  the  one  side,  with  the  capillary  network  in 
the  wall  of  the  proboscis,  and  on  the  other,  through  a  narrow  slit,  with  the  central 
blood  sinus  of  the  proboscis  which  lies  in  front  of  it. 

3.  The  central  blood  sinus  of  the  proboscis  (Fig.  464,  9)  is  a  space  in  that  limit- 
ing membrane  which  separates  the  "heart  vesicle"  (dorsally)  from  the  proboseidal 
diverticulum  of  the  buccal  cavity  (ventrally).     It  has  no  musculature  of  its  own. 
This,  however,  is  supplied  by  the  ventral  transverse  musculature  of  the   ' '  heart 
vesicle  "  which  lies  above  it.     In  Schizocardium,  the  central  blood  sinus  is  con- 
tinued in  a  peculiar  manner,  which  cannot  here  be  further  described,  on  to  the 
two  "auricles"  of  the  "heart  vesicle." 

4.  The  proboseidal  glomerulus  (Fig.  464,  10)  consists  of  two  lateral  principal 
portions  and  a  dorsal  connecting  piece.     Each  of  the  principal  portions  has  the 
form  of  a  unilaniinar  honeycomb,  with  deep  cells.     The  base  of  the  comb  is  formed 
by  the  right  or  left  lateral  walls  of  the  basal  complex  of  proboseidal  organs,  i.e.  of 
the  "heart  vesicle  "  and  the  proboseidal  diverticulum  of  the  intestine.    The  apertures 
of  the  single  "cells,"  however,  are  turned  towards  the  proboseidal  ccelom.     The 
walls  of  the  cells  are  formed  by  folds  of  the  visceral  endothelium  of  the  proboseidal 
ccelom.     They  are  hollow,  and  the  cavities  are  blood  sinuses,  which  open  into  a 
common  cleft-like  sinus  in  the  base  of  the  comb.     This  latter,  again,  communicates, 
by  means  of  a  slit-like  transverse  aperture,  between  the  "heart  vesicle"  and  the 
proboseidal  diverticulum,  with  the  central   sinus  of  the  proboscis  (Fig.  464,   9). 
Posteriorly,  each  lateral  principal  part  of  the   glomerulus  becomes  simpler  and 


. 


ENTEROPNEUSTA— BLOOD  VASCULAR  SYSTEM 


583 


simpler,  and  its  sinus,  which  anteriorly  is  so  complicated,  becomes  on  each  side  a 
simple  vessel  in  the  limiting  membrane  between  the  proboscidal  intestine  and  the 
visceral  coelomic  endothelium. 

These  two  vessels  are  the  efferent  proboscidal  vessels. 

Blood  reaches  the  central  sinus  of  the  proboscis  in  the  following  ways  :  (1) 
from  the  dorsal  vessel  of  the  trunk  and  collar  through  the  basal  blood  sinus, 
(2)  out  of  the  integumental  vascular  network  of  the  proboscis  through  vessels  or  a 
vascular  plexus,  which  ascends  in  the  limiting  membrane  of  the  ventral  septum, 
and  lastly  (3)  out  of  this  vascular  network  through  a  vessel,  which  descends  along 
the  free  edge  of  the  "heart  vesicle." 

The  "  heart  vesicle  "  propels  the  blood  by  means  of  its  ventral  wall,  which  lies 
upon  the  central  sinus.  Its  function  is  considered  to  be  that  of  driving  the  blood 


Fn;.  4<U.— Diagrammatic  transverse  section  through  the  proboscis  of  an  Enteropneustan. 
1,  Dorsal  proboscidal  septum  ;  2,  proboscidal  epithelium ;  3,  blood  lacunae  of  the  integument ;  4, 
circular  musculature  ;  5,  longitudinal  musculature  ;  6,  ventral  proboscidal  septum ;  7,  proboscidal 
diverticulmn  of  the  buccal  cavity  ;  8,  proboscidal  coelom  ;  9,  central  blood  sinus  of  the  proboscis  ; 
10,  proboscidal  glomerulus  ;  11,  "heart  vesicle/' 

through  the  narrow  passages  of  the  glomerulus,  and  through  the  efferent  proboscidal 
vessels  finally  into  the  ventral  vessel  of  the  trunk. 

If  the  proboscis  pore  takes  in  water  for  swelling  the  proboscis,  it  appears 
pretty  certain  that  the  glomerulus  (formerly  called  the  proboscidal  gill)  must, 
irrespective  of  other  unknown  functions,  also  serve  for  respiration. 

.">.  The  efferent  proboscidal,  and  collar,  vessels.— From  their  origin  out  of  the 
two  posterior  tips  of  the  lateral  portions  of  the  glomerulus,  these  vessels  turn 
v<  nt rally,  and,  running  very  close  to  one  another,  traverse  the  chondroid  tissue  of 
the  neck.  In  the  anterior  and  upper  area  of  the  collar  region,  they  enter  the  two 
vascular  folds,  in  whose  limiting  membranes  they  run,  breaking  up  into  more  or 
less  rich  plexuses.  Their  courses  then,  naturally,  correspond  with  those  of  the 
vascular  folds,  which  take  their  name  from  the  vessels  within  them.  In  Schizo- 
cardiinii,  Balanoylossus,  and  Glandiceps,  where  the  two  vascular  folds  descend 
slantingly  to  the  ventral  median  line  of  the  posterior  end  of  the  collar,  they  neces- 


584  COMPARATIVE  ANATOMY  CHAP. 

sarily  also  have  slanting  systems  of  blood  vessels,  which  open  into  the  anterior 
end  of  the  ventral  vessel  of  the  trunk. 

In  Ptychodera,  on  the  contrary,  where  the  vascular  folds  descend  perpendicularly 
in  the  anterior  part  of  the  collar  region,  and  form  a  ring  around  the  buccal  cavity, 
which  is  then  continued  as  a  medioventral  fold  to  the  posterior  end  of  the  collar, 
the  vessels  which  run  in  these  folds  naturally  also  form  a  similar  ring,  which 
passes  into  a  medioventral  collar  vessel  continuous  with  the  ventral  vessel  of  the 
trunk. 

The  vessels  of  the  collar  are  distinguished  from  the  principal  vessels  of  the 
trunk  by  the  fact  that  they  possess  no  musculature. 

A  circular  space,  running  in  the  limiting  membrane  of  the  septum  separating 
the  'collar  from  the  trunk,  is  in  open  communication  with  the  ventral  vascular 
trunk. 

6.  The  vascular  capillary  networks  of  the  integument  and  of  the  intestine  are 
everywhere  in  communication  with  the  two  principal  vessels.     In   the  collar,   a 
connection  is  formed  between  the  integumental  and  the  intestinal  plexuses  by 
plexuses  running  in  the  mesenteries.     Where  peripharyngeal  cavities  are  found 
(Ptychodera,  Schizocardium}  the  integumental  plexus  lies  in  the  peripheral  walls  of 
the  cavities,  viz.  those  turned  to  the  ccelom  of  the  collar.     Of  all  the  sections  of 
the  intestine,  the  hepatic  is  most  distinguished  by  the  closeness  of  its  capillary 
plexus  and  its  rich  supply  of  blood. 

In  some  species  of  Ptychodera,  dendriform,  blindly-ending,  vascular  cseca  project 
from  the  dorsal  side  of  the  collar  cord  and  sometimes  also  from  the  dorsal  septum. 

7.  Lateral  vessels— Ptychodera. — Two  lateral  vessels,  provided  with  muscular 
walls,    run  through  the   branchiogenital  and    hepatic   regions.     They   originate 
anteriorly  out  of  the  vascular  network  of  the  integument,  run  backward  in  the 
submedian  line,  and  enter  the  lateral  mesenteries,  in  whose  limiting  membranes 
they  run.     At  the  posterior  ends  of  these  mesenteries,  at  the  boundary  between 
the  branchiogenital  and  the  hepatic  regions,  they  pass  over  on  to  the  intestine, 
being  continued  in  two  vessels  running  along  close  below  the  liver-caeca  of  the 
intestine,   and  finally  open  into  the  intestinal  capillary  network.     The  anterior 
portions  of  the  lateral  vessels,   which  might   be  called  the   genital  vessels,  are 
connected   with   the   capillary   network    of  the   gonadial   walls.     Similar   lateral 
vessels  also  occur  in  Schizocardium. 

In  Balanoylossus  and  Glandiceps  there  are,  usually  in  the  hepatic  region,  two 
lateral  vascular  trunks  of  the  intestine,  which  open  anteriorly  and  posteriorly  into 
its  capillary  network.  Their  musculature  consists,  in  Glandiceps,  of  circular 
fibres  ;  in  Balanoglossus,  of  longitudinal  fibres.  These  perhaps  correspond  with 
the  posterior  or  intestinal  portion  of  the  lateral  vessels  of  Ptychodera. 

8.  The  branchial  vessels. — These  vessels  have  been  best  investigated  in  Ptycho- 
dera.    A  branchial  capillary  network  is  found  in  the  limiting  membranes  both  in 
the  branchial  tongues  and  the  branchial  septa,  i.e.   in  the  limiting  membrane 
which  separates  the  epithelium  of  the  branchial  intestine  from  the  visceral  layer  of 
the  mesoderm  which  lies  outside  it.     Into  this  plexus,  vessels  having  a  definite  con- 
stant course  and  of  large  size  enter  :  (1)  a  vessel  along  the  back  of  each  tongue,  (2) 
a  vessel  along  the  inner  side  of  each  lingual  prong,  i.e.  on  the  side  turned  to  the 
lingual  cavity,  (3)  a  vessel  along  that  edge  of  each  septal  prong  which  is  turned  to 
the  body  wall.    These  last-named  vessels  run  ventrally  into  the  capillary  network 
of  the  lower,  nutritive  part  of  the  branchial  intestine  (i.e.  of  the  oesophagus), 
and  must  be  considered  as  efferent  vessels  of  the  branchial  septa. 

The  branchial  capillary  network  receives  its  blood  from  afferent  branchial 
vessels,  which  originate  out  of  the  dorsal  vessel  and  (in  Ptychodera  clavigera]  have 
the  following  arrangement :  Each  afferent  branchial  vessel,  soon  after  its  origin 


ENTEROPXEUSTA— GONADS  585 

out  of  the  dorsal  vessel,  divides  into  two,  one  running  into  a  tongue  and  the  other 
into  the  branchial  septum  next  in  front.  The  lingual  vessel  divides  again  into 
two  branches,  which  are  continued  into  the  two  above-mentioned  vessels  of  the 
lingual  prongs  ("tongue-bars"). 

It  is  not  known  in  what  way  the  blood  is  again  carried  out  of  the  branchial 
tongues. 

X.  The  Gonads. 

The  sexes  are  separate  in  the  Enteropneusta.  The  gonads  are 
simple  or  branched  sacs  of  various  shapes  which  project  into  the 
body  cavity  of  the  trunk,  towards  which,  however,  they  are  completely 
closed.  They  form  on  each  side  a  conspicuous  longitudinal  row  in 
the  genital  region  of  the  trunk,  which,  however,  is  not  sharply  de- 
marcated from  the  branchial  region  in  front  of  it  nor  from  the 
hepatic  region  behind  it.  At  the  posterior  end  of  each  row  of  gonads, 
a  constant  formation  of  new  gonads  takes  place. 

The  gonadial  sacs  open  outward  through  simple  efferent  ducts 
and  genital  pores,  which  always  lie  dorsally  in  the  submedian  line 
close  to,  but  on  the  outer  side  of,  the  branchial  pores  (Fig.  458). 

These  gonads,  which  open  laterally  to  the  branchial  pores,  form 
the  row  of  principal  gonads,  and  their  pores  are  the  primary 
principal  pores. 

A  certain  agreement  in  the  number  of  the  gonadial  pores  with 
that  of  the  branchial  pores  is  sometimes  found. 

The  arrangement  of  the  gonads  may  become  complicated. 

A.  One  and  the  same  gonadial  sac  may  open  outward  through 
accessory    pores,   which    lie    either    medianly,  or    laterally,  to    the 
principal  pore. 

Such  accessory  pores  are  found  in  Schizocardium  toasiliense  and 
Glandiceps  talaboti,  in  the  latter  in  great  numbers. 

B.  Accessory  gonads   may   occur  in  addition  to  the   principal 
gonads,  opening  outward  through  secondary  genital  pores. 

In  Balanoglossus  Kupfferi,  such  accessory  gonads  form  a  complete 
row  running  parallel  with  the  principal  row,  along  its  median  side. 
The  same  is  the  case  in  Glandiceps  talaboti,  although  here  the  accessory 
row  is  not  quite  complete.  In  Balanoglossus  canadensis,  both  principal 
and  accessory  gonads  occur,  there  being  several  rows  of  each.  The 
pores  of  all  the  gonads  lie  in  the  submedian  lines,  which  are  free 
from  muscle,  and  are  in  this  case  widened  into  broad  streaks. 

When  accessory  gonads  occur  in  species  of  PtycTiodera  (e.g.  Pt. 
aurantiaca,  bahamensis,  erytlima)  their  pores  always  lie  laterally  to  the 
principal  pores. 

Structure  of  the  gonads. — The  gonads  consist  (1)  of  a  Avail  turned  to  the 
coelomic  cavity  and  belonging  to  it,  constructed  of  a  tesselated  epithelium  and 
fine  muscle  fibres,  and  (2)  of  a  massive  inner  germinal  layer,  consisting  of  germinal 
cells  and  covering  or  follicle  cells  ;  this  layer  is  continued  into  the  epithelium  of 
the  efferent  ducts. 


586 


COMPARATIVE  ANATOMY 


CHAP. 


Between  these  two  layers  lies  (3)  a  limiting  membrane,  in  which  a  rich  capillary 
network  may  be  developed,  or  else  the  membrane  is  divided  into  its  two  lamellte 
by  a  continuous  slit-like  blood  sinus. 

The  origin  of  the  gonads  is  not  yet  certainly  known.  They  were  formerly  held  to 
be  derived  from  the  ectoderm,  but  the  most  recent  researches  seem  to  show  that 
they  arise  as  local  accumulations  of  the  mesenchyme  cells  which  occupy  the 
blastocoel.  In  any  case  the  connection  of  the  gonads  with  the  body  epithelium 
by  means  of  the  ducts  is  secondary.  They  originally  lie  isolated  between  this 
epithelium  and  the  parietal  layer  of  the  ccelom. 


XI.  Ontogeny. 

The  development  of  the  Enteropneusta  is  sometimes  connected  with  metamor- 
phosis, a  pelagic  larva,  the  Tornaria  larva,  being  developed.  This  larva  in  many 
respects  recalls  the  Bipinnaria  larva  of  the  Asteroids,  and 
was  at  first  considered  to  be  an  Echinoderm  larva.  In  other 
cases  development  is  abbreviated,  and  is  indeed  almost  direct, 
for  though  a  free  larva  develops  from  the  fertilised  egg,  it 
lives  at  the  bottom  of  the  sea,  and  shows  no  signs  of  many  of 
the  most  important  characters  of  Tornaria. 

A.  Structure  and  Metamorphosis  of  the  Tornaria  larva. 


FIG.  465.— Very  young 
specimen  of  Tornaria 
Krohni,  from  the  side 
(after  Spengel).  1,  Ap- 
ical plate  with  eyes  ; 
2,  preoral  area;  3,  pre- 


The  egg  segmentation  and  gastrula  are  unknown. 
1.  Outer  organisation.  —  The  youngest  larval  stages 
observed  are  almost  egg-shaped  (Fig.  465).  At  the  anterior 
pole  there  is  a  pair  of  brown  eye-spots,  at  the  posterior  the 
anal  aperture,  and  in  the  middle  of  the  ventral  side  the 
long  transverse  mouth.  The  thin  transparent  integument  is 
oral  ciliated  ring ;  thickened  only  in  the  region  of  two  ciliated  rings,  which 
4,  oesophagus;  5,  mouth;  border  in  a  manner  soon  to  be  described,  a  somewhat 
(3,  stomach  intestine ;  ,  ,  ,  ,  ,  ,  ...  „, 

7  anus-  s  hind-gut;  aeePened  °ral  area,  at  whose  centre  the  mouth  lies.  The 
9]  pastoral  ciliated  ring ';  ciliated  rings  are  strictly  bilaterally  symmetrical.  A  preoral 
10,  postoral  area ;  ii,  ciliated  ring  runs  from  the  anterior  ventral  edge  of  the  oral 
proboscis  pore ;  12,  pro-  area  forwards  and  upwards  on  each  side  to  the  frontal  region, 

nZSlapSl  Plate?'     ^here  the  ^  lie>  and  marks  °ff  a  Pre°ral  ^      A  SeCOnd 
ciliated  ring  runs  back  on  each  side,  almost  longitudinally, 

from  the  frontal  region,  then  bends  round  on  to  the  ventral  side,  and  here, 
behind  the  mouth,  passes  into  the  corresponding  ciliated  band  of  the  other  side 
of  the  body. 

This  postoral  ciliated  ring  forms  the  dorsal  and  posterior  boundary  to  the  oral 
area,  and  marks  off  a  postoral  area,  which  comprises  the  dorsal  and  posterior  (anal) 
regions  of  the  larval  body.  The  preoral  and  postoral  ciliated  rings  unite  for  a  very 
short  distance  at  the  apical  pole.  The  oral  area  enclosed  within  these  two  rings  has 
the  form  of  a  transverse  ventral  saddle,  drawn  out  on  each  side  towards  the  apex. 

The  next  remarkable  change  which  is  externally  visible  is  the  appearance  of  a 
ciliated  ring  at  right  angles  to  the  principal  axis.  This  surrounds  the  posterior 
part  of  the  postoral  area,  and  is  the  principal  ciliated  ring  (Fig.  466,  9).  The 
postoral  area  is  by  it  divided  into  an  anterior  and  a  posterior  region.  The  posterior 
region  is  the  anal  area,  with  the  anus  at  its  centre.  In  the  anterior  region,  a 
dorsal  area  can  be  distinguished  from  a  ventral  zone.  Behind  the  principal  ciliated 
ring,  a  second  weaker  ciliated  anal  ring  may  appear  (Fig.  466,  8). 


IX 


ENTEROPNEUSTA— ONTOGENY 


587 


During  the  further  transformations  which  take  place  in  the  larva,  the  anal  area 
which  is  bordered  by  the  greatly  developed  principal  ciliated  ring  remains  almost 
unaltered,  while  the  oral  area,  pushing  out  before  it  the  preoral  and  the  postoral 
rings,  sends  symmetrical  extensions  (Fig.  466)  into  the  pre-  and  postoral  regions, 
as  follows  : — 

From  the  anterior  and  lateral  tips  of  the  oral  region  which  stretch  to  near  the 
apical  pole,  two  extensions,  one  on  each  side,  run  posteriorly  and  ventrally  into  the 
preoral  area  (13),  two  more  posteriorly  and  dorsally  into  that  region  of  the  postoral 
area  which  was  above  distinguished  as  the  dorsal  area  (2). 

In  this  way  the  larva,  seen  from  the  apical  pole,  has  temporarily  a  four-rayed 
appearance. 

From  the  lateral  and  dorsal  regions  of  the  oral  area,  two  extensions  invade  a 
postoral  area  dorsally  (4).  From  the  posterior  lateral  edges,  two  inconspicuous 


~^~!6 


2    °  —i  UT 

Fm.  4»56.— A,  B,  C,  Tornaria  Miilleri  (?).  A,  From  the  ventral  side  ;  B,  from  the  dorsal  side ;  C, 
in  profile  (after  Spengel).  1,  Apical  plate,  with  the  eyes  and  apical  tuft ;  2,  anterior  dorsal  lobes  of 
the  oral  area ;  3,  "  heart  vesicle  " ;  4,  posterior  torsal  lobe  of  the  oral  area ;  5,  collar  coelora  ;  6,  trunk 
crelom  ;  7,  anus  ;  8,  secondary  anal  ciliated  ring ;  9,  principal  anal  ciliated  ring ;  10,  postoral  area  ; 
11,  proboscis  pore  ;  12,  proboscidal  ccelom  (water  sac) ;  13,  anterior  ventral  lobe  of  the  oral  area. 
14,  oral  area;  15,  ventral  "saddle";  16,  ventral  zone  of  the  postoral  area;  17,  anal  area;  IS, 
cesophagus ;  19,  stomach-intestine. 

extensions  may  spread  posteriorly.  The  ventral  zone,  however,  bulges  forwards 
ventrally  towards  the  oral  area  (15). 

These  changes  bring  about  the  peculiar  indented  course  of  the  preoral  and 
postoral  ciliated  rings,  shown  in  the  figures. 

The  ciliated  rings  may  even  become  still  more  folded.  Such  folding  reaches 
the  highest  degree  in  Tornaria  Grenachcri,  hardly  1  cm.  long,  in  which  the 
anterior  ventral  and  the  anterior  dorsal  extensions  of  the  oral  area  bulge  out  at 
the  cilia-carrying  edge  to  form  numerous  long,  narrow,  freely  projecting  accessory 
lobes,  resembling  tentacles. 

In  the  frontal  region,  on  the  apical  eye-bearing  plate,  which  here  becomes 
differentiated,  a  tuft  of  delicate  immobile  cilia  develops  early. 

The  larva  swims  in  such  a  way  that  the  anterior  or  apical  pole  is  directed 
upward  and  the  anal  pole  downward. 

The  metamorphosis  of  the  Tornaria  larva  into  the  young  Enteropneustan  is 
accompanied  by  the  following  external  processes  : — 


588  COMPARATIVE  ANATOMY  CHAP. 

The  body  lengthens  and  its  preoral  section  becomes  produced  into  the  proboscis, 
at  whose  tip  the  eyes  are  still  long  visible,  until  they  degenerate  together  with  the 
apical  plate  and  tuft. 

The  preoral  and  postoral  ciliated  rings  degenerate,  but  the  whole  body  becomes 
covered  with  cilia. 

The  principal  ciliated  ring  (Fig.  467)  persists  for  some  time,  eventually,  i.e. 
when  the  anal  area  has  increased  in  length,  surrounding  the  body  about  half 
way  between  the  mouth  and  anus.  A  circular  furrow  between  it  and  the  base 
of  the  proboscis  is  the  first  indication  of  the  posterior  boundary  of  the  collar 
region. 

The  whole  ectoderm  of  the  oral  area  degenerates  during  metamorphosis,  the 
body  epithelium  proceeding  exclusively  from  the  ectoderm  of  the  preoral,  postoral, 
and  anal  areas,  which  increases  in  thickness.  This  phenomenon,  together  with  the 
lengthening  of  the  larva,  causes  a  very  marked  diminution  in  the  transverse  section 
of  the  body,  an  essential  accompaniment  of  which  is  the  approximation  of  the 
larval  integument  to  the  parietal  walls  of  the  ccelomic  vesicles. 

2.  Anatomical. — The  apical  plate  of  the  Tornaria,  larva,  which  completely 
degenerates  later,  consists  of  a  dorsal  and  a  ventral  half.  Below  the  surface  of  the 
specially  thick  dorsal  half  there  is  a  layer  of  nerve  fibres. 

The  centre  of  the  apical  plate  is  formed  by  a  small  group  of  long  narrow 
sensory  cells  carrying  delicate  immobile  cilia  (sensory  hairs). 

The  two  eyes  which  are  embedded  in  the  apical  plate  are  optic  pits,  whose 
floors  are  formed  by  cells  which  are  pigmented  at  their  bases  (retinal  cells  ?).  The 
optic  pit  is  filled  with  a  clear  substance,  which  is  a  continuation  of  the  cuticle 
covering  the  apical  plate,  and  is  called  the  lens.  The  apertures  of  the  two  pits 
diverge  anteriorly  and  laterally.  Round  them  in  the  apical  plate  the:e  is  a 
deeper  layer  of  elements,  which  are  considered  to  be  ganglion  cells. 

The  alimentary  canal. — Even  the  youngest  larvae  observed  showed  the  division 
of  the  alimentary  canal  into  oesophagus,  mid-gut  or  stomach,  and  hind-gut,  which 
is  characteristic  of  all  the  larval  stages. 

The  oesophagus  ascends  vertically.  It  is  a  flattened  tube  provided  with  a 
circular  musculature,  its  thickened  dorsal  and  ventral  walls  being  ciliated. 

The  stomach  is  a  large  egg-shaped  sac,  whose  axis  lies  horizontally,  and  into 
whose  anterior  pole  the  oesophagus  enters.  The  epithelial  cells  of  the  sac  are  at 
first  low,  but  they  lengthen  at  a  later  stage.  The  originally  thin-skinned  stomach 
develops  in  this  way  a  thick  wall,  which  is  probably  non-ciliated  except  at  two 
points.  A  ciliated  cushion  is  found  ventrally,  at  the  entrance  to  the  stomach,  and 
the  efferent  aperture  is  surrounded  by  long  hairs,  which  perhaps  act  as  a  fish-trap 
apparatus. 

The  hind-gut,  in  the  youngest  larvae,  is  an  almost  cylindrical  tube  with  thin 
walls.  At  a  later  stage  its  anterior  part  becomes  swelled  up,  so  that  the  hind-gut 
as  a  whole  becomes  funnel-shaped  or  conical.  As,  however,  the  aperture  com- 
municating with  the  stomach  remains  small,  the  wall  of  the  hind-gut  is  applied 
over  a  considerable  area  to  the  posterior  wall  of  the  stomach.  The  aperture  lies  at 
the  centre  of  this  area.  Immediately  in  front  of  the  anal  aperture  there  is  a  circle 
of  cells  provided  with  cilia. 

Formation  of  the  gills. — The  formation  of  the  first  pair  of  gills  takes  place 
shortly  before  or  after  "metamorphosis.  Two  lateral  cseca  arise  at  the  posterior  end 
of  the  oesophagus,  grow  out  towards  the  integument,  and  break  out  laterally  and 
dorsally  through  the  external  branchial  pores.  The  cesophageal  apertures  of  these 
branchial  diverticula  are  at  first  round,  later  they  become  U-shaped,  by  the  lateral 
dorsal  intestinal  wall  growing  out  into  the  diverticulum.  This  outgrowth  is  the 
rudiment  of  the  branchial  tongue. 


ix  EXTERUPXEUSTA— ONTOGENY  589 

In  Tornaria  Ayassizi,  it  was  observed  that  the  pores  of  the  second  pair  of  gills 
arise  earlier  than  those  of  the  first. 

In  other  cases,  the  new  pairs  of  gills  always  form  behind  the  last  produced. 
During  the  process,  the  growing  cesophagus,  the  posterior  part  of  which  is  now  the 
branchial  intestine,  passes  from  a  vertical  to  a  horizontal  position. 

Even  in  the  adult  animal,  new  gill-pouches  are  constantly  being  formed  at  the 
posterior  end  of  the  branchial  region.  The  course  of  development  is  always  the 
same  as  that  of  the  first  gills  in  the  larva. 

The  collar  canals,  with  their  pores,  develop  shortly  before  the  rudiments  of  the 
second  pair  of  gills  arise,  most  probably  as  outgrowths  of  the  anterior  wall  of  the 
first  gill-pouch,  near  its  external  pore.  These  outgrowths  run  forwards  towards  the 
collar  ccelom. 

The  first  rudiment  of  the  proboscidal  diverticulum  of  the  buccal  cavity  has 
been  observed  in  Tornaria  Agassizi,  as  a  small  bulging  of  the  anterior  wall  of  the 
larval  oesophagus  directed  anteriorly,  and  lying  immediately  above  the  mouth. 

The  proboscidal  ccelom. — The  rudiment  of  the  proboscidal  ccelom  has  not  as 
yet  been  observed  with  as  much  certainty  as  could  be  desired.  According  to  one 
observer,  it  arises  as  an  outgrowth  of  the  intestine  at  the  boundary  between  the 
cesophagus  and  the  stomach. 

In  the  youngest  stages  which  have  been  closely  observed,  the  proboscidal  ccelom 
(the  so-called  water  sac  of  the  larva)  is  an  almost  cylindrical  tube  lined  with 
tesselated  epithelium,  the  inner  half  of  which  is  slightly  widened.  This  tube 
becomes  attached  by  its  inner  end  to  the  anterior  wall  of  the  oesophagus,  near 
the  point  at  which  the  latter  opens  into  the  stomach.  Here  it  sends  two  processes 
(the  reins)  to  right  and  left  on  to  the  lateral  walls  of  the  cesophagus,  on  which 
it  appears  to  ride.  From  the  cesophagus,  the  tube,  traversing  the  blastocoel, 
ascends  almost  vertically.  Shortly  before  it  reaches  the  dorsal  ectodermal  wall,  it 
is  continued  into  a  short  internally  ciliated  tube,  the  epithelium  of  which  suddenly 
changes  its  character.  This  short  tube,  lined  with  cylindrical  epithelium,  is  the 
rudiment  of  the  proboscidal  canal,  and  the  pore  to  which  it  leads  is  the  proboscis 
pore. 

The  proboscidal  ccelom  is  further  attached  to  the  apical  plate  by  means  of  a 
strand  in  the  manner  illustrated  in  Fig.  466  ;  the  strand  consists  of  contractile 
fibres  surrounded  by  a  nucleated  envelope,  a  continuation  of  the  wall  of  the  water 
sac.  The  contractile  fibres  of  this  strand  are  continued  on  to  the  tips  or  reins  of 
the  water  sac,  which  grasp  the  ossophagus  between  them. 

The  further  fate  of  the  water  sac,  briefly  stated,  is  as  follows.  It  swells  up, 
and  soon  changes  from  a  tube  into  a  vesicle,  the  greater  part  of  the  wall  of  which, 
during  metamorphosis,  becomes  applied,  as  parietal  wall  of  the  proboscidal  ccelom. 
to  the  body  epithelium  of  the  anterior  or  proboscidal  section  of  the  body,  the 
muscular  apical  strand  becoming  shorter  and  shorter,  and  finally  altogether 
disappearing. 

The  manner  in  which  the  epithelial  walls  of  the  water  sac  become  differentiated 
into  the  proboscidal  musculature  cannot  here  be  described. 

"  Heart  vesicle." — Authorities  ditfer  as  to  the  first  rudiments  of  this  organ.  In 
the  youngest  stage  investigated  by  the  most  recent  observers,  it  is  a  small  cellular 
structure  with  an  internal  cavity  lying  so  close  to  the  ectoderm  that  it  may  be 
either  ectodermal  or  mesenchymatous.  It  appears  to  the  right,  in  front  of  and 
near  the  proboscis  pore.  The  body  becomes  a  hollow  vesicle,  leaves  the  ecto- 
derm, sinks  below  the  surface,  and  becomes  applied  to  the  right  side  of  the  water 
sac.  Transverse  muscle  fibres  develop  on  its  ventral  wall.  The  water  sac  then  forms 
posterior  outgrowths,  which  grow  round  the  "heart  vesicle'1  on  its  right  dorsal 
and  ventral  sides.  The  two  dorsal  posterior  and  the  ventral  posterior  sections 


590 


COMPARATIVE  ANATOMY 


CHAP. 


of  the  proboscidal  ccelom  thus  come  into  existence.  The  "heart  vesicle"  on  its 
lower  side,  however,  always  remains  separated  from  the  dorsal  wall  of  the  ventral 
posterior  outgrowth  of  the  water  sac  by  a  space,  into  which  the  proboscidal 
diverticulum  of  the  buccal  cavity  grows  from  behind,  but  in  such  a  way  that 
between  it  (the  diverticulum)  and  the  superimposed  "heart  vesicle"  a  space 
remains,  which  appears  to  become  filled  with  blood  at  an  early  stage.  This  is  the 
central  blood  sinus  of  the  proboscis. 

The  coelomic  sacs  of  the  collar  and  trunk. — These  two  pairs  of  coelomic  sacs 
appear,  in  Tornaria,  to  have  a  common  rudiment  in  the  following  way.  The 
edge  formed  by  the  anterior  wall  of  the  hind-gut,  bending  round  backwards  into  its 
lateral  walls,  is  produced  anteriorly  to  right  and  left  as  hollow  sacs,  or  in  other  cases 

as  a  pair  of  solid  bilaminar  plates.  These  become 
applied  to  the  stomach,  but  are  on  the  other 
hand  separated  by  a  large  space  from  the  ecto- 
derm. These  two  sacs  or  plates  become  con- 
stricted from  their  matrix,  the  hind -gut,  and 
grow  round  the  stomach  dorsally  and  ventrally. 
In  each,  apparently,  an  anterior  portion  becomes 
constricted  off.  This  anterior  pair  of  sacs  or 
plates  is  the  rudiment  of  the  collar  coelomic 
sacs,  the  posterior,  which  only  secondarily 
extend  backwards  along  the  sides  of  the  hind 
gut,  is  the  rudiment  of  the  trunk  coelomic  sacs 
(Fig.  467).  These  two  coeloms  are  therefore 
enteroccels.  Where  the  first  rudiments  of  the 
cceloms  are  solid  bilaminar  plates,  a  space  soon 
arises  in  them  by  the  separation  of  the  two 
lamiiise.  These  small  spaces,  whether  present 
from  the  first,  or  formed  later,  begin  to  increase 
in  size  at  the  end  of  the  larval  period.  The 
two  pairs  of  coelomic  rudiments  become  vesicular. 
The  outer  wall  becomes  applied  to  the  body 
epithelium  as  the  transverse  section  of  the 


coelom;    5,  septum  between  the  collar 
and  the  trunk  ;  6,  trunk  coelom  ;  7,  ven- 


FIG.  467. — Collar  and  trunk  of  an 
Enteropneustan    (Tornaria    Krohni) 
immediately  after  metamorphosis,  from 
the  ventral  side  (after  Spengel).    1,  Pro-    growing  larva  decreases  during  metamorphosis 
boscis;  2,  collar;  3,  trunk;  4,  collar    in  the  way  already  described,  and   forms  the 

dermomuscular  tube.     The  inner  wall  lies  upon 

tral  mesentery;  8,  principal  ciliated  the  intestine,  and  represents  the  visceral  layer 
ring ;  9,  wall  of  the  mid-gut ;  10,  wall  of  the  ccelomic  sac.  The  dorsal  and  ventral 
of  the  hind-gut ;  11,  anus.  mesenteries  are  formed  where  the  right  and 

left  trunk  ccelomic  sacs  and  the  right  and  left 

collar  ccelomic  sacs,  in  surrounding  the  intestine,  come  in  contact  dorsally  and 
ventrally  in  the  median  plane. 

These  processes,  of  course,  go  hand  in  hand  with  a  progressive  reduction  of  the 
blastoccel,  which  contains  a  number  (small  at  first,  but  increasing  later)  of 
mesenchyme  cells  of  unknown  origin.  The  remains  of  the  segmentation  cavity 
represent  the  blood  vascular  system. 

Nervous  system. — Shortly  before  the  conclusion  of  metamorphosis,  the  two 
longitudinal  nerve  trunks  arise  as  local  differentations  of  the  body  epithelium, 
below  the  surface  of  which  a  layer  of  nerve  fibres  forms.  The  collar  cord,  also,  at 
first  lies  superficially  in  the  integument,  and  is  nothing  more  than  the  collar 
portion  of  the  dorsal  epithelial  longitudinal  cord.  This  part  only  sinks  below  the 
surface  at  a  later  stage.  According  to  recent  observers,  the  process  recalls  the 
sinking  in  and  constriction  of  the  neural  tube  in  Vertebrates. 


ix  EXTEROPNEUSTA—PHYLOGEXY  591 

Gonads.  —  The  development  of  the  gonads  has  already  been  sufficiently  described 
above,  p.  586. 

B.  The  almost  Direct  Development  of  the  Balanoglossus  Kowalevskii. 

"\Ve  can  only  select  a  few  of  the  principal  points  in  this  development  for 
description.  Segmentation  is  total  and  equal,  and  leads  to  the  formation  of  a 
cceloblastula,  out  of  which,  by  invagination,  a  ccelogastmla  is  produced.  This 
latter  becomes  covered  with  cilia,  and  a  ciliated  ring  forms  round  the  blastopore, 
which  diminishes  in  size  and  finally  closes  ;  this  ring  corresponds  with  the  principal 
ciliated  ring  of  the  Tomaria  larva.  At  this  stage  the  larva  leaves  the  egg  to  live 
at  the  bottom  of  the  sea,  without  showing  any  trace  of  the  form,  or  of  the  charac- 
teristic ciliated  rings,  of  Tornaria. 

The  differentiations  which  take  place  in  the  archenteron  are  important.  Its 
anterior  part  becomes  constricted  off  as  a  semilunar  vesicle  lying  transversely.  This 
takes  up  the  whole  of  the  most  anterior  part  of  the  blastoccel  and  becomes  the 
proboscidal  ccelom,  which  thus,  according  to  these  observations,  is  an  enteroccel. 
Two  pairs  of  lateral  outgrowths  become  constricted  off  from  the  rest  of  the  archen- 
teron, the  anterior  being  the  rudiment  of  the  collar  ccelom,  and  the  posterior  that 
of  the  trunk  ccelom. 

The  blastoccel  is  small  from  the  first. 

The  mouth  is  said  to  arise  by  the  simple  breaking  through  of  the  intestine  out- 
wards, and  the  anus  in  a  similar  way,  in  the  place  of  the  original  blastopore.  Thus 
the  whole  of  the  intestinal  wall  is  of  an  endodermal  origin. 

The  developmental  processes  in  B.  Kowalevskii  cannot  here  be  further  described  : 
we  refer  the  reader  to  the  account  of  the  formation  of  the  organs  in  the  Tornaria 
larva,  given  above. 

XII.  Phylogeny. 

The  systematic  position  of  the  Enteropneustan  must  still,  or  rather  again,  be 
considered  as  altogether  uncertain.  In  any  case,  the  Enteropneusta  are  not  closely 
related  to  any  single  large  division  of  the  animal  kingdom.  Special  affinities  with 
the  Chordata,  the  Echinodermata,  and  the  Xemertines  have  been  long  suggested. 
and  in  quite  recent  times  also  with  Cephalodiscus  and  Rhabdopleura. 

A.  The  relation  of  the  Enteropneusta  to  the  Chordata  has  been  maintained 
on  the  following  grounds  :  — 

1.  The  Chordata  and  the  Enteropneusta  show  a  very  far-reaching  and  extra- 
ordinary agreement  in  their  gills.      This  agreement  holds  good  even  in  details 
(branchial  tongues,  branchial  skeleton,  synapticulae)  if  the  gills  of  An^hioxus  are 
taken  for  comparison. 

2.  The  proboscidal  di  verticulum  of  the  Enteropneusta  is,  in  structure  and  origin, 
comparable  with  the  chorda  of  the  Chordata. 

3.  The  proboscidal  skeleton  of  the  Enteropneusta  corresponds  with  the  sheath 
of  the  chorda. 

4.  The  body  cavities  in  the  two  groups  are  of  enteroccelomic  origin  ;  the  pro- 
boscidal   ccelom   corresponds   with    the    anterior    unpaired   mesoderm   vesicle   of 


5.  The  collar  cord  of  Balanoglossus  corresponds  with  the  dorsal  cord  of  the 
Chordata,  and  arises  in  the  same  way  as  the  neural  tube  of  Vertebrates,  by  sinking 
in  and  covering  over. 

The  most  recent  researches  have,  however,  yielded  results  unfavourable  to  this 
assumed  homology. 


592  COMPARATIVE  ANATOMY  CHAP. 

1.  The  great  similarity  of  the  Enteropneustan  gills  to  those  of  Amphioxus  in 
finer  structure  remains,  but  the  detailed  comparison  of  point  with  point  makes  a 
real  homology  doubtful,  and  seems  to  oblige  us  to  consider  the  resemblance  as  at 
the  most  a  very  remarkable  case  of  convergence. 

Further,  the  following  considerations  must  also  be  taken  into  account.  The 
gills  of  Amphioxits  arise  ontogenetically  as  segmental  structures,  while  those  of 
the  Enteropneusta,  although  standing  in  a  longitudinal  row,  belong  to  the  unseg- 
mented  trunk. 

The  gills  of  the  Chordata  receive  their  blood  from  the  ventral  vascular  trunk, 
those  of  the  Enteropneusta  from  the  dorsal  trunk. 

Should  it  be  proved  that  the  larval  oesophagus  of  the  Enteropneusta  proceeds 
from  an  ectodermal  invagination,  and  is  a  stomodaeum,  then  the  gills  of  the 
Enteropneusta  would  lie  in  an  ectodermal  intestinal  region,  in  contradistinction 
to  those  of  the  Vertebrata,  which  belong  to  an  endodermal  intestinal  region. 

2.  The  proboscidal  diverticulum  is  a  preoral  outgrowth  of  the  wall  of  the  buccal 
cavity,  and  is  lined  with  epithelium.     It  does  not  thus  agree  with  the  tissue  of 
the    noto-chord.      It   is    further   very   questionable   whether   the   buccal   cavity, 
and,  with  it,   the  diverticulum   are   endodermal    formations.      The    proboscidal 
diverticulum  lies  below  the  continuation  of  the  dorsal  blood  vascular  trunk  (below 
the  central  blood  sinus  of  the  proboscis)  ;  the  chorda  of  Vertebrates,  on  the  con- 
trary, lies  above  the  dorsal  blood  vascular  trunk  (aorta).     No  homology  between 
the  two  is  possible. 

3.  The  proboscidal  skeleton,  as  a  thickened  limiting  membrane,  could  at  the 
most  be  compared  only  with  the  inner  cuticular  sheath  of  the  chorda. 

4.  The  body  cavity  is  an  enterocoel,  in  many  different  divisions  of  the  animal 
kingdom  (either  constantly  or  exceptionally)  other  than  the  Enteropneusta  and  the 
Chordata.     The  ccelomic  vesicles  (mesoderm  vesicles,  primitive  vertebrae)  of  the 
Vertebrata  show  a  segmental  arrangement,  corresponding  with  the  metamerism  of 
the  other  organs,  while  in  the  Enteropneusta  no  such  arrangement  exists. 

5.  The  collar  cord  of  the  Enteropneusta  is  only  the  anterior  continuation  of  the 
dorsal  nerve  cord  of  the  trunk.     It  does  not  sink  below  the  surface  until  all  its 
parts  are  formed.     The  corresponding  ventral  nerve  cord  of  the  Enteropneusta  does 
not  exist  anywhere  in  the  Chordata. 

The  following  further  points  must  be  emphasised. 

The  gonads  of  Amphioxus  arise  segmentally  from  the  endothelium  of  the  body 
cavity,  while  the  rows  of  gonads  in  the  Enteropneusta  lie  in  an  unsegmented 
region.  The  first  origin  of  the  gonads  of  the  Enteropneusta  is,  indeed,  unknown, 
but  their  rudiments  are  found  in  the  blastoccel  very  early.  The  manner  in  which 
the  genital  products  are  ejected  in  the  two  groups  is  altogether  different. 

In  the  Chordata,  the  blood  in  the  dorsal  vessel  flows  from  before  backward,  in 
the  ventral  from  behind  forward  ;  the  reverse  is  the  case  in  the  Enteropneusta. 

A  comparison  of  the  two  collar  pores  in  the  Enteropneusta  with  the  most 
anterior  pair  of  nephridia  in  Amphioxus  could  only  be  of  value  were  the  develop- 
ment of  these  organs  known.  In  all  probability  the  former  are  of  ectodermal  and 
the  latter  of  mesodermal  origin. 

There  is  nothing  we  know  of  in  the  Chordata  comparable  with  the  Tornaria 
larva. 

These  considerations  render  any  relationship  between  the  Enteropneusta  and 
the  Chordata,  at  least  at  present,  highly  improbable. 

B.  The  relationship  between  the  Enteropneusta  and  the  Nemertina  is  so  very 
problematical  that  it  cannot  here  be  discussed. 

C.  Relation  of  the  Enteropneusta  with  the  Annelida.— The  attempt  to  bring 
the  Euteropneusta  and  the  Annelida  into  even  a  distant  genetic  relationship  is 


ix  ENTEROPNEUSTA— PHYLOGENY  593 

supported  chiefly  upon  a  comparison  of  larval  forms.  The  following  characteristics 
of  the  Trochophoran  and  the  Tornarian  larvae  have  been  pointed  out. 

The  neural  plate,  the  apical  sensory  organs,  and  the  muscles  which  become 
attached  to  the  neural  plate  correspond  in  the  two.  The  divisions  of  the  intestine 
also  agree,  that  is,  if  the  fore-gut  of  the  Tornaria  is  an  ectodermal  stomodaeum, 
and  the  hind-gut  a  proctodseurn.  The  two  pairs  of  coelomic  sacs  are  comparable 
with  the  two  anterior  pairs  of  mesodenn  vesicles. 

The  comparison  of  the  ciliated  bands  presents  difficulties.  Three  ciliated  rings, 
a  preoral  (surrounding  the  apical  area  with  the  neural  plate),  a  postoral,  and  a 
preanal,  are  typically  ascribed  to  Trochophora,  the  last  of  which  is  supposed  to 
correspond  with  the  principal  ciliated  ring  of  Toraaria,  while  the  preoral  and 
the  postoral  rings  of  Trochophora  are  wanting  in  Tornaria.  And  it  is  argued 
that  the  absence  of  these  rings  in  Tornaria  has  led  to  the  specialisation  of  the 
preanal  as  the  principal  ciliated  ring. 

On  the  other  hand  the  preoral  ciliated  ring  of  Tornaria  cannot  be  compared 
with  the  preoral  ring  of  Trochophora,  because  it  does  not  surround  the  apical 
plate.  This  latter  lies,  on  the  contrary,  outside  the  frontal  area,  at  the  opposite 
end  of  the  oral  region  to  the  mouth  ;  the  preoral  ring  passes  in  front  of  it. 

Compared  with  the  principal  ciliated  ring,  the  pre-  and  postoral  rings  are, 
perhaps,  of  small  morphological  significance,  since  they  are  wanting  in  the  non- 
pelagic  larva  of  Balanoylossus  Kowalevskii,  whereas  the  principal  ring  occurs 
in  it. 

In  a  comparison  of  the  Tornaria  Avith  the  Trochophora  larva,  the  great  import- 
ance of  the  following  differences  must  not  be  overlooked. 

1.  Trochophora  possesses  typically  one  pair  of  primitive  kidneys,  which  are 
wanting  in  Tornaria. 

2.  Tornaria  has  a  preoral  coelom,  which  is  absent  in  the  Trochophora  larva. 

If  now  we  turn  to  the  organisation  of  the  adult  animals  for  light  as  to  this 
question  of  the  relationship  between  the  Annelida  and  the  Enteropneusta,  we  find 
immediately  that  insurmountable  difficulties  stand  in  the  way  of  any  close  com- 
parison. Only  in  the  blood  vascular  system  is  any  fundamental  agreement  found. 
The  blood  in  the  dorsal  and  ventral  longitudinal  vessels  has  the  same  course  in 
both  groups.  But,  on  the  other  hand,  a  comparison  of  the  nervous  system  of  the 
Enteropneusta  with  that  of  the  Annelida  encounters  difficulties  similar  to  those 
found  in  comparing  it  with  that  of  the  Vertebrata.  The  gills  in  the  two  groups 
are  altogether  heterogeneous.  The  typical  Annelidan  kidneys  are  wanting  in  the 
Enteropneusta,  for  the  collar  pores,  which  are  probably  of  ectoblastic  origin,  can 
hardly  be  regarded  as  a  pair  of  nephridia. 

Thus  the  relationship  of  the  Enteropneusta  to  the  Annelida  appears  at  the 
best  to  be  extremely  distant. 

D.  The  relation  of  the  Enteropneusta  to  the  Echinodermata. — This  relation- 
ship is  claimed  on  the  ground  of  the  agreement  existing  between  the  larval  forms. 
The  similarity  of  the  Tornaria,  especially  with  the  Bipinnaria  larva  of  the 
Astcroidea,  is,  indeed,  so  striking  that  the  first  observer  of  Tornaria  expected 
for  certain  that  it  would  develop  into  an  Echinoderm. 

A  closer  comparison  yields  the  following  results  : 

1.  If  we  place  the  Tornaria  and  the  Bipinnaria  similarly  with  regard  to  the 
position  of  the  mouth  and  the  anus,  a  striking  agreement  in  the  conformation 
of  the  regions  of  the  body  and  in  the  ciliated  rings  bordering  them  is  observed 
(Fig.  468).  In  both  we  find  a  separate  preoral  ciliated  ring  in  the  same  position, 
bordering  a  preoral  area.  In  both  we  can  distinguish,  lying  behind  this,  a 
deepened  oral  area  with  the  mouth  in  its  ventral  centre.  The  postoral  ciliated 
ring  of  the  Tornaria  corresponds  with  the  large  circumoral  ring  of  the  Bipinnaria. 
VOL.  II  2  Q 


594  COMPARATIVE  ANATOMY  CHAP. 

The  region  of  the  body  lying  behind  this  occupies  in  both  a  large  part  of  the  dorsal 
posterior  side  of  the  body  ;  in  it  lies  the  anus. 

2.  Whereas,  in  Tornaria,  in  the  postoral  area  of  the  body,  a  large  ciliated 
ring  bounds  an  anal  area,  such  a  ring  is  wanting  in  Eipinnaria. 

3.  The  apical  plate  with  the  two  eyes  and  the  tuft,  which  is  so  sharply  marked 
in  Tornaria,  is  wanting  in  the  developed  Hipinnaria.     Too  great  a  significance 
must  not  now,  however,  be  attributed  to  this  fact,  since  something  like  a  neural 
plate  (an  ectodermal  thickening  with  long  cilia)  has  been  observed  in  the  quite 
young  larvae  of  Asteroids  and  Echinoids,  and  a  neural  plate  with  a  layer  of  nerve 
fibres,  ganglion  cells,  and  ciliated  tuft,  although  without  eyes,  has  been  demon- 
strated in  the  apical  region  of  the  Antedon  larva. 

4.  The  intestine  of  Tornaria  shows  the  same  divisions  as  are  found  in  that 
of  the  Echinoderm  larvse,  viz.  :    oesophagus,   stomach -intestine,    and  hind -gut. 
Whether,  however,  these  three  sections  correspond  with  one  another  in  the  two 


FIG.  468.— A,  B,  C,  Aurlcularia,  Bipinnaria,  and  Tornaria  (Enteropneustan  larva),  from  the 
right  side,  diagrammatic.  1,  Preoral  area  ;  2,  oral  area  ;  3,  postoral  area  ;  4,  anal  area  ;  I,  preoral ; 
II,  circumoral ;  III,  anal  or  principal  ciliated  ring  ;  5,  neural  plate  ;  os,  mouth  ;  an,  anus. 

groups  must  remain  uncertain  so  long  as  the  origin  of  the  oesophagus  and  hind- 
gut  is  not  definitely  ascertained.  In  this  matter  the  Echinoderms  are  in  the  same 
position  as  the  Enteropneusta. 

The  oesophagus,  in  the  Echinoderms,  is  sometimes  described  as  an  ectodermal 
stomodseum,  sometimes  as  a  section  of  the  archenteron.  The  latter,  according  to 
the  most  recent  investigations,  is  the  case,  e.g.,  in  the  Holothurioidea,  and  the 
former  in  Antedon  and  others.  In  this  case  the  oesophagus,  even  within  the 
Echinodermata,  is  not  an  homologous  structure  !  In  the  case  of  the  Enteropneusta, 
in  the  interest  of  other  views,  doubt  has  been  thrown  upon  the  statement  that  the 
larval  oesophagus  is  a  part  of  the  archenterou. 

The  hind-gut  in  the  Echinodermata  is,  by  all  authorities,  considered  to  be 
endodermal.  The  same  was  affirmed  of  the  hind-gut  of  the  Enteropneusta,  but 
this  has  recently  been  strongly  doubted. 

5.  The  condition  of  the  coelom  in  the  two  larval  forms  would  show  great  agree- 
ment if  two  pairs  of  ccelomic  sacs  can  really  be  attributed  typically  to  the  Echino- 
derm larva,  a  point  which  recent  research  makes  more  and  more  probable,  and  if 
also  the  endodermal  origin  of  the  ccelomic  vesicles  of  the  Enteropneusta  could  be 
proved. 

It  would  then  be  evident  that  the  two  anterior  ccelomic  sacs  of  the  Echinoderm 
j  (the  left  of  which  is  the  hydroccel)  correspond  with  the  two  ccelomic  sacs  of 


ix  ENTEROPNEUSTA—  LITERATURE  595 

the  collar  of  the  Enteropneusta,  and  the  two  posterior  sacs  of  the  former  with  the 
two  trunk  ccelomic  sacs  of  the  latter.  The  two  anterior  sacs  are  in  communication 
with  the  exterior  through  the  collar  pores  ;  this  communication  (hydropore,  water 
pore)  in  the  Echinodermata  is  usually  limited  to  the  left  anterior  sac,  i.e.  to  the 
left  hydroccel  vesicle,  but  occasionally  in  Asteroids — a  matter  of  great  importance 
— appears  on  the  right  side  as  well. 

From  these  considerations,  it  seems  that  the  prospect  of  establishing  a  funda- 
mental agreement  in  structure  between  the  Enteropneustan  larva  and  that  of  the 
Echinodermata  is  very  hopeful.  This  relationship  between  the  Enteropneusta  and 
the  Echinodermata  seems  to  rest  upon  more  solid  ground  than  do  any  of  the  others 
which  have  been  attributed  to  either  of  these  two  groups. 

At  the  same  time  any  attempt  to  compare  adult  Echinoderms  with  adult 
Enteropneusta  is  at  present  completely  futile.  The  Echinoderms  and  Entero- 
pneusta could,  as  far  as  we  can  see,  only  be  genetically  connected  through  some 
common  racial  form  far  back  in  their  phylogeny — a  form  which  corresponded  with 
the  Tornarian  and  the  Dipleurulan  larva?. 

Further,  before  we  can  feel  any  certainty  on  these  questions  of  affinity,  new  and 
more  exact  ontogenetic  researches  must  be  made.  The  origin  of  the  proboscidal 
crelomic  vesicle  of  the  Enteropneusta  has  to  be  established,  as  has  also  that  of  the 
"heart  or  proboscis  vesicle."  Attention  must  be  directed  to  the  question  as  to 
whether  a  preoral  section  of  the  body  corresponding  with  the  proboscis  of  the 
Enteropneusta  is  present  (if  only  as  a  rudiment)  in  the  Echinoderm  larvae,  as  for 
instance  in  the  preoral  section  of  the  body  in  the  Antedon  larva  (?),  .or  in  the  larval 
organs  of  Astcrina  and  other  Asteroids  (?).  With  reference  to  the  "  heart  vesicle  " 
we  are  reminded  of  the  statement  that  a  "pulsating  vesicle"  occurs,  apparently 
not  of  enteroccelomic  origin,  in  Echinoderni  larvae.  This  has  to  be  confirmed. 

The  Relationship  of  the  Enteropneusta  to  Cephalodiscus  and  Rhabdopleura 
will  be  considered  in  the  Appendage  to  this  chapter. 


Literature. 

Alex.  Agassiz.      The  history  of  Balanoglossus  and  Tornaria.     Mem.  Amer.  Acad. 

of  Arts  and  Sc.     Vol.  IX.     1873. 
W.  Bateson.      The  early  stages  in  the  development  of  Balanoglossus.     Quart.  Journ. 

Mlt-rosc.  Sc.  (X.S.).     Vol.  XXIV.     1884. 

The  later  stages  in  the  development  of  Balanoglossus  Kou:alevsJcii,  with  a 

suggestion  on  the  affinities  of  the  Enteropneusta.     Quart.  Joum.  Microsc.  Sc. 
(JV.tf.).     Vol.  XXV.,  Suppl.     1885. 

Continued  account  of  the  later  stages  in  the  development  of  Balanoglossus 

Koicalevskii  and   on  the   morphology  of  the  Enteropneusta.      Quart.  Journ. 
Microsc.  Sc.  (N.S.).     Vol.  XXVI.     1886. 

The  ancestry   of  the    Chordata.      Quart.  Journ.  Microsc.  Sc.  (X.S.}.     Vol. 

XXVI.     1886. 

Gilbert  C.  Bourne.      On  a   Tornaria  found  in  British  Seas.      Journ.  Mar.  Biol. 

Assoc.  (2).     Vol.  I.     1889. 
E.   Kb'hler.      Rcehc  relies  anatomiques  sur    unc    nouvelle   espece  de  Balanoglossus. 

Bull.  Soc.  Sc.  Nancy  (2).     Tome  VIII.     1886.     An  almost  identical  work  in 

Intcrnat.  Monatsschr.  Anat.  Hist.     3  Bd.     1886. 
A.  Kowalevsky.     Anatomie   des  Balanoglossus  delle  Chiajc.     Mem.  Acad.   Imp. 

Sc.  St.  Petersbourg  (7).     Tome  X.     1867. 
A.    Krohn.     Bcobachtungen   iibcr  Echinodermenlarven.     Arch.  f.  Anat.,  PhysioL 

u.  wissciisch.  Mtd.     1854. 


596 


COMPARATIVE  ANATOMY 


CHAP. 


A.  F.  Marion.     ^Etudes  zoologiques  sur  deux  especes  d1  Elite"  ropncustcs.     Arch.  Zool. 

gtner.  et  exper.  (2).     Tome  IV.     1886. 
E.    Metschnikoff.      Untersuchungen  ilber  die  Metamorphose  einiger  Seethiere.     1. 

Ueber  Tornaria.     Zeitschr.  f.  wiss.  Zool.     20  Bd.     1870. 
T.H.Morgan.    Growth  and  metamorphosis  of  Tornaria.    Journ.  Morph.    V.    1892. 

The  Development  of  Balanoglossus.     Journ.  Morph.     Vol.  IX.     1894. 

Job.  Miiller.      Ueber  die  Larven  und  die  Metamorphose  der  Echinodermen.     Part  2. 

Akad.  d.  Wissensch.     1848.     Berlin,  1850. 
Wladimir  Schimkewitsch.     The  Fauna  of  the  White  Sea :  Balanoglossus  Meresch- 

kovskii  Wagner.     St.  Petersburg,  1889.     (In  Russian. ) 
J.  W.  Spengel.     Die  Enteropneusten.     Fauna  and  Flora  des  Golfes  von  Neapel. 

18  Monographic.     Berlin,  1893.     The  most  important  recent  work. 
W.  F.  R.  Weldon.     Preliminary  note  on  a  Balanoglossus  larva  from  the  Bahamas. 

Proceed.  Roy.  Soc.  London.     Vol.  XLII.     1887. 
R.  v.  Willemoes-Suhm.     Biologische  Beobachtungen  uber  niedere  Meeresthiere.     4. 

Ueber  Balanoglossus  Kupfferi  aus  dem  Oeresund.     Zeitschr.  f.  wissensch.  Zool. 

21  Bd.     1871. 
A.  Willey.     Amphioxus  and  the  ancestry  of  the  Vertebrates.     1894. 


Appendage  to  the  Enteropneusta. 

Cephalodiseus  and  Rhabdopleura. 
I.    Cephalodiseus  (Figs.  469-471). 

The  body  is  about   1   mm.  long,  almost  bean-shaped,  bilaterally 

symmetrical;  it  is  rounded 
posteriorly  and  anteriorly 
flat,  with  a  slight  backward 
slope.  The  most  important 
organs  which  can  be  distin- 
guished externally  are  found 
in  this  anterior  sloping  sur- 
face, while  in  the  whole  of 
2  the  rest  of  the  body  only 
one  organ  appears,  viz.  a 
cylindrical  stalk  or  pedicle, 
which  rises  from  the  ventral 
side  of  the  rounded  posterior 
end  of  the  body. 

In  that  part  of  the  body 
which  projects  most  anteri- 
orly, i.e.  the  anterior  end 
of  the  dorsal  side,  lies  the 

anus,   while    somewhat    be- 

FIG.  469.— Cephalodiseus  dodecaiophus,  from  the  hind    the    anterior    end    of 

ventral  side  (after  M'IntOSh).     1,  Tentacles;   2,  buccal    4-},^  vpr,tral  ~\A~    4->,p  nirmtfc 
shield  =  proboscis ;  3,  mouth ;  4,  buds ;  5,  pedicle ;  6,  trunk      tn°  V6ntral  Slde>  tne  mOUth, 

and    between    the  two  the 
slope  mentioned   above,  the  inter -stomatal  region.      The  median 


IX 


CEPHALODISCUS 


597 


line  between  the  mouth   and   anus   is   the   inter -stomatal   middle 
line. 

In  the  inter-stomatal  region  or  in  its  immediate  vicinity  lie  the 
following  parts  of  the  body  :  (1)  the  preoral  buccal  shield  with  its 
two  pores ;  (2),  the  central  nervous  system ;  (3),  the  twelve  feathered 
tentacles ;  (4),  the  apertures  of  the  two  oviducts ;  (5),  the  postoral 


\ 


15 


FIG.  470.— Median  section  through  Cephalodiscus  dodecalophus  somewhat  near  the  median 
plane  (after  Harmer).  2,  Nervous  system  ;  3,  anterior  paired  coelom  (collar  coelom);  5,  fold  of  the 
anterior  trunk  region  ;  8,  paired  trunk  coelom;  9,  pharynx  ;  10,  o?sophagus  ;  11,  stomach-intestine ;  12, 
hind-gut ;  13,  buccal  ca vity  ;  14,  pedicle  ;  15,  ovary  ;  16,  anus  ;  17,  oviduct ;  18,  buccal  shield  ;  19, 
coelom  of  the  same  =  proboscidal  ccelom  ;  20,  one  of  the  proboscis  pores  ;  21,  anterior  diverticulum 
(proboscidal  diverticulum)  of  the  pharynx. 

lamella ;  (6),  the  two  postoral  pores  of  the  body  cavity ;  (7),  the 
two  gill-slits  (Figs.  470,  471). 

The  buccal  shield,  which  is  comparable  with  the  proboscis  of  the 
Enteropneusta,  is  a  plate,  which  projects  downwards  from  the  inter- 
stomatal  region  immediately  in  front  of  the  mouth  by  means  of  a 
rather  short  stalk,  in  such  a  way  that  its  free  surface,  the  epithelium 
of  which  is  enormously  thickened,  faces  ventrally  (Fig.  470,  18). 

The  central  nervous  system  lies  in  the  hypodermis,  almost  at 


598 


COMPARATIVE  ANATOMY 


CHAP. 


the  centre  of  the  inter-stomatal  region.  If  we  describe  the  mouth 
as  lying  posteriorly  and  ventrally  to  the  stalk  of  the  buccal  shield, 
the  central  nervous  system  lies  exactly  opposite,  that  is,  posteriorly 
and  dorsally  to  the  same  structure. 

Twelve  tentacles  rise  dorsally  at  the  base  of  the  stalk  of  the 
buccal  shield,  six  to  the  right  and  six  to  the  left  of  the  central  nervous 

system.  The  nervous  system 
extends  into  the  dorsal  hypo- 
dermis  of  these  tentacles  (Fig. 
471).  The  tentacles  are  large, 
are  feathered  on  both  sides  like 
the  feathers  of  birds,  and  are 
knobbed  at  their  free  ends. 

Two  pores,  placed  symmet- 
rically to  the  inter-stomatal 
median  line,  break  through 
the  most  anterior  part  of  the 
central  nervous  system.  There 
is  thus  open  communication 
between  the  body  cavity  of  the 
buccal  shield  (the  proboscis)  and 
the  exterior  (proboscis  pores). 
Between  the  central  nervous 
system  and  the  anus  there  is  on 
each  side  an  aperture.  These 
two  apertures  belong  to  the 
oviducts. 

Outside  of  the  inter- 
stomatal  region,  but  in  its 
immediate  proximity,  the 
following  parts  are  found. 

Immediately     behind     the 
mouth,    covered    by    the    oral 
disc,  a  thin  lamella  hangs  ven- 
trally and  laterally  down  from 
FIG.  47i.-Horizontai  section  through  cephaio-  the  body  like  an  apron ;  this 

discus  (after  Harmer).     1,  Tentacles;   2,  nervous    •      ,1  i    i  n       /!?• 

system;  3,  anterior  paired  body  cavity  (collar  coelom);    1S    the    POStOral    lamella    (1  Ig. 
4,  collar  pore  ;  5,  folds  of  the  anterior  tnmk  region,  the    471,      5).         In      the      posterior 

r^fd1tamta;tbTh^lp0res;7'mesentery;  anSle   formed  V  this  Amelia 

8,  paired  trunk  ccelom ;  9,  pharynx ;  10,  oesophagus ;         -~L    .1       T       i  •      c  i 

11,  stomach ;  12,  hind  gut.  Wltn  the  body,  a  pore  is  found 

on    each    side    (collar    pore). 

These  pores  lead  into  the  paired  middle  body  cavity,  of  which  we  shall 
speak  later.  Immediately  behind  these  pores,  and  like  them  over- 
arched by  the  lateral  folds  of  the  postoral  lamella,  two  branchial  pores 
lead  from  without  into  the  pharyngeal  section  of  the  intestinal  canal. 
Musculature. — From  near  the  mouth,  longitudinal  muscle  fibres 
run  back  along  the  ventral  side,  and  enter  the  pedicle.  Muscles  are 


ix  CEPHALODISCUS  599 


also  found  in  the  stalk  of  the  buccal  shield,  which  radiate  out  from 
the  stalk  into  the  shield. 

Body  cavity. — In  a  young  bud,  the  body  appears  to  be  divided 
into  three  sections  (anterior,  middle,  and  posterior)  by  two  circular 
furrows.  Each  of  these  three  sections  possesses  a  separate  body 
cavity.  The  most  anterior  section,  out  of  which  the  buccal  shield 
proceeds,  has  an  unpaired  body  cavity,  into  which  a  short  intestinal 
diverticulum  enters  from  the  middle  section.  The  body  cavity  in 
both  the  middle  and  posterior  sections  is  paired,  the  two  lateral  halves 
being  separated  by  mesenteries.  The  boundary  between  the  middle 
and  posterior  sections,  the  latter  of  which  becomes  the  greater  part  of 
the  body  cavity  in  the  adult,  becomes  more  or  less  indistinct.  The 
body  cavity  of  the  middle  section  is  retained  in  the  adult  in  the 
postoral  lamella  and  in  the  region  of  the  central  nervous  system  and 
of  the  feathered  tentacles,  into  which  it  is  continued.  The  body 
cavity  of  the  posterior  section  in  the  adult  contains  the  whole  of  the 
alimentary  canal  and  the  ovaries,  these  organs  almost  entirely  filling  it. 
It  is  continued  into  the  pedicle. 

The  alimentary  canal  forms  a  loop  in  the  body,  with  a  ventral 
section  which  runs  backwards  and  into  which  the  mouth  leads,  and  a 
dorsal  section  running  forwards  and  opening  anteriorly  through  the 
anus.  The  mouth  leads  first  into  the  "pharynx,"  which  com- 
municates with  the  exterior  by  means  of  the  two  gill-slits  mentioned 
above.  A  thin  divertieulum  runs  out  anteriorly  from  the  pharynx 
below  the  nervous  system  into  the  stalk  of  the  oral  disc.  The 
pharynx  is  followed  first  by  an  oesophagus  and  then  by  a  very 
spacious  stomach  or  stomach  intestine,  which  occupies  by  far  the 
greater  part  of  the  body  cavity.  At  the  point  where  the  pedicle  joins 
the  body,  the  stomach  passes  over  into  a  narrower  section  of  the 
intestine,  which,  immediately  behind  the  stomach,  ascends,  and  then, 
bending  forward,  runs  along  the  dorsal  side  as  the  hind-gut  to  the 
anus. 

Genital  organs. — Male  genital  organs  have  not  been  observed. 
The  female  organs  consist  of  two  ovaries,  lying  in  the  anterior 
part  of  the  body ;  they  are  continued  into  two  strongly  pigmented 
oviducts,  which  open  outward  through  the  apertures  already  men- 
tioned (Fig.  470,  17). 

Reproduction. — Besides  multiplying  sexually  by  means  of  eggs, 
Cephalodiscus  also  reproduces  itself  asexually  by  gemmation.  The 
buds  always  form  on  the  pedicle,  near  to  its  free  end.  Almost  all 
adult  individuals  have  from  1  to  3  buds. 

Many  individuals  of  Cephalodiscus  live  together  in  a  ramifying 
and  anastomosing  system  of  tubes  secreted  by  themselves,  these  tubes 
having  occasional  apertures.  The  animals,  throughout  life,  when 
not  disturbed,  remain  in  the  immediate  vicinity  of  these  apertures, 
through  which  they  protrude  their  unfolded  crowns  of  tentacles. 


600  COMPARATIVE  ANATOMY  CHAP. 

C.  dodecalophus,  the  only  known  representative  of  the  genus,  was 
found  in  the  Magellan  Straits  at  a  depth  of  245  fathoms. 

Systematic  position. — Cephalodiscus  shows  in  the  following 
points  a  remarkable  agreement  with  the  Enteropneusta. 

1.  The  body  falls  into  three  sections  (distinct  even  in  the  young 
bud),  one  preoral  and   two  postoral.     The  preoral    section,  the  so- 
called  buccal  shield,  corresponds  with  the  proboscis,  the  middle  section 
with  the  collar,  and  the  larger  posterior  section  and  the  pedicle  with 
the  trunk  of  the  Enteropneusta. 

2.  These  sections  correspond  with  special  sections  of  the  ccelom, 
an  unpaired  ccelom  in  the  buccal  shield,  and  two  pairs  of  cceloms  in 
the  body  proper.     We  recognise  here  the  unpaired  proboscidal  coelom 
and  the  paired  collar  and  trunk  cceloms  of  the  Enteropneusta. 

3.  The  pores  of  the  ccelom  of  the  buccal  shield  correspond  with 
the  proboscis  pores  of  the  Enteropneusta  proper,  which  are  also  often 
two  in  number. 

4.  The  pores  of  the  pair  of  cceloms  in  the  anterior  body  correspond 
with  the  collar-pores. 

5.  Cephalodiscus  and  the  Enteropneusta  have  gill-slits,  the  former 
having  one  and  the  latter  many  pairs. 

6.  The  anterior  diverticulum   of  the  buccal  cavity  corresponds 
with  the  proboscidal  diverticulum  of  the  Enteropneusta. 

7.  The  central  nervous  system  corresponds  with  the  collar  cord 
(which,  however,   in  this  case  is   not  sunk  below  the  skin)  of  the 
Enteropneusta  and  with  its  immediate  continuation  on  to  the  base  of 
the  proboscis. 

The  differences  existing  between  Cephalodiscus  and  the  Entero- 
pneusta may  well  be  attributed,  at  least  in  part,  to  the  tubicolous,  half- 
sedentary  manner  of  life  of  the  former.  These  are  :  (1)  the  anterior 
position  of  the  anus,  and  the  consequent  looped  course  of  the 
alimentary  canal ;  (2)  the  general  crowding  together  of  the  most 
important  external  organs  (apart  from  the  pedicle  or  stalk)  at  the 
most  anterior  part  of  the  body ;  (3)  the  presence  of  a  tentacular 
crown,  consisting  of  twelve  feathered  tentacles ;  (4)  the  presence  of 
the  pedicle  or  stalk;1  (5)  the  occurrence  of  asexual  reproduction  by 
means  of  gemmation ;  (6)  the  small  number  of  gill-slits  and  genital 
organs ;  (7)  the  form  of  the  body  in  general  and  especially  that  of  the 
proboscis  \  (8)  the  absence  of  a  blood  vascular  system. 


II.  Rhabdopleura. 

This  form,  which  was  formerly  classed  with  the  Bryozoa?  is  no 
doubt  somewhat  nearly  related  to  Cephalodiscus,  but  is  further 
removed  than  the  latter  from  the  Enteropneusta. 

1  An  apparently  homologous  structure  is,  however,  figured  by  Bateson  on  a  young 
Balanoglossiis  Koivalevskii. 

*  Iii  the  first  volume  of  this  book,  indeed,  Rhabdopleura  appears  among  the  Bryozoa. 


. 


RHABDOPLEURA 


601 


This  animal  forms  colonies  by  gemmation.  Each  individual 
consists  of  a  body  and  a  contractile  stalk,  both  of  which  are 
enclosed  in  a  horny  tube.  This  tube  is  supine  at  first  but  rises 
erect  later.  It  is  secreted  in  successive  rings  by  the  buccal  shield. 
The  tentacles  can  be  protruded  through  the  aperture  of  the  tube, 


FIG.  472.— Rhabdopleura  Nonnani  Allm., 
individual,  from  the  right  side  (after  Lankester). 
1,  Buccal  shield ;  2,  feathered  tentacles ;  3,  region 
of  the  "collar  pore  " ;  4,  anus ;  5,  trunk ;  6,  stalk 
or  pedicle. 


the  body  being  withdrawn  again  into  the  tube  by  means  of  the  stalk. 
All  these  tubes  are  lateral  branches  of  a  creeping  basal  tube  which 
spreads  out  and  branches  on  the  substratum,  and  appears  to  be  divided 
into  chambers  by  septa. 

The  stalks  of  the  individuals  enter  this  radical  tube,  and  are 
continued  in  it  as  thin  strands  covered  with  cuticle,  which  in  travers- 
ing it  perforate  the  septa. 

The  axis  of  each  individual  stalk  is  formed  by  a  strand  of  tissue 


602 


COMPARATIVE  ANATOMY 


CHAP.  IX 


somewhat  of  the  consistency  of  cartilage.      Similar  skeletal   pieces 

support  the  tentacles  and  their  branches. 

Some  insight  into  the  chief  anatomical  features  of  the  individuals  will 

be  gained  from  the  figures  (Figs.  472,  473).     The  principal  differences 

between  Rhabdopleura  and  Cephalo- 
discus  are :  (1)  the  gill -slits  are 
wanting  in  the  former;  (2)  there 
are  only  two  feathered  tentacles ; 
(3)  the  proboscis  pores,  i.e.  the 
pores  of  the  coelom  of  the  buccal 
shield,  are  wanting. 

The  genital  organs  are  still 
imperfectly  known.  In  some  speci- 
mens a  testicle  tube,  which  runs 
longitudinally  and  asymmetrically 
along  one  side  of  the  body,  and 
bulges  out  the  body  wall,  has  been 
demonstrated :  this  tube  opens  out 
near  the  anus.  Rhabdopleura,  like 
Cephalodiscus,  is  a  deep-sea  form. 

Further  research  is  needed  be- 
fore we  can  establish  the  exact 
relationship  of  Cephalodiscus  and 
Rhabdopleura  to  the  Bryozoa. 


Literature. 


FIG.  473. —Rhabdopleura  Normani, 
median  longitudinal  section,  diagrammatic 
(after  Fowler).  1,  Tentacle  of  one  side,  in- 
dicated by  dotted  lines  ;  2,  anterior  paired 
coelom  (collar  coelom) ;  3,  anus  ;  4,  posterior 
paired  or  trunk  coelom ;  5,  hind-gut ;  6,  mid- 
gut  ;  7,  buccal  cavity ;  8,  mouth  ;  9,  anterior 
diverticulum  of  the  buccal  cavity  (probosciclal 
diverticulum) ;  10,  ccelom  of  the  buccal  shield 
(proboscidal  coelom) ;  11,  buccal  shield. 


G.  J.  Allmann.     On  Rhabdoplcura,  a  new 
form  of  Polyzoa,  from  deep-sea  dredg- 
ing   in     Shetland.        Quart.     Journ. 
Microsc.  Sc.     Vol.  IX.     1869. 
G.   Herbert  Fowler.      The  morphology   of 
•   Rhabdopleura  Normani  Allm.     Fest- 
schrift zum  Siebenzigsten  Geburtstag  R. 
Leuckart's.     1892. 
Sidney  F.  Harmer.     Appendix  to  M'Intosh  :  Report  on  Cephalodiscus  dodecalophus 

M'Intosh.     Rep.  Voy.  of  the  "Challenger"  Zool.     Vol.  XX.     1887. 
E.  R.  Lankester.     A  contribution  to  the  anatomy  of  Rhabdopleura.     Quart.  Journ. 

Microsc.  Sc.     Vol.  XXIV.     1884. 

W.  C.  M'Intosh.  Report  on  Cephalodiscus  dodecalophus  M'Intosh,  a  new  type  of  the 
Polyzoa,  procured  on  the  voyage  of  H.M.S.  "  Challenger."  Rep.  Voyage 
"  Challenger  "  Zool.  Vol.  XX.  1887. 

G.  0.  Sars.  On  Rhabdopleura  mirabilis.  Quart.  Journ.  Microsc.  Sc.  Vol.  XIV. 
1874. 


INDEX 


Numbers  in  Italics  give  System  atio  Position. 
refer  to  Figures 


^Numbers  in  Black  Type 


Amph.  =  Amphineura 
Ast.  =  Asteroidea 
Blast.  =Blastoidea 
Crin.  —  Crinoidea 
Cyst.  =  Cystidea 
Echin.  =  Echinodermata 
Echinoid.  =  Echinoidea 


Ent.  =  Enteropneusta 
Gastr.  =  Gastropoda 
Hoi.  =  Holothurioidea 
Lam.  =  Lamellibranchia 
Moll.  =  Mollusca 
Oph.  =  Ophiuroidea 


ABATUS  cavcrnosus,  apical  system.  324 
Abranchia,  12 
Acanthaster,  299 

,,  echinaster,  421 

Ellisii,  421 
Acanthochiton,  165 
Aca.nUwdoris,  13 
Acanthology,  387 

Aatiithotrochus  rniro.bilis,  "wheel."  337 
Acephala,  14,  177 
Accra,  10 


Acetabula  (Moll.),  117  ;  (Echinoid.),  390 

Acicididce,  6 

Acnwca  (  =  TccturoJ),  5 

Acmaeidce,  5 

Acri'cido.ris,  290 

Ac/'"t:rinidce,  308 

A?r»<:r!.iin*.  309 

Actceon,  110 

Actceonidce,  10 

Actiit»c/'inidce,  307 

Actiii'icrinus,  307 

,,  proboscidali.s,   apical  system, 

329 

,,  cr/-,i(uiliani(s,  423 

Actiiuxvcumis,  338 

Actinometm,    313  ;    food  grooves,  diagr., 
366 

„  «t  rota,  365 

Actiii'>j)oda,  285  \  oral  region,  section,  428 
Adambulacral  ossicles,  352  ;  radii,  316 
Adetes,  293 
Adradii,  316 


13 

,  alimentary  canal,  192 
nifibranchialis,  12 
,  293 
^Esthetes,  166 
jEtheria,  62 
Agaricocrinus,  307 
Agassizia,  293 
Agassizoerinus,  304 
Agelacrinus,  313 

„  cincinnatensis,  313 

Aglossa,  14,  177 
A media,  76 
Ambitus,  338 
Amblypygus,  345 
Ambulacra,  339 

Ambulacral  bmsh  (Echinoid.),  433;  gills 
(Echinoid.),  433;  ossicles,  352;  radii, 
316 

A  mmon  it  idea ,  ;?  J 
Amnion,  523 

Amoebocytes  (Moll.)  200  ;  (Echin.),  415 
Amphiaster,  297 
A  inph  idrom  us,  1 60 
Amphineura,  2 
Amphipeplea,  8 

,,  leuccmensis,  8 

A//(phisphyra,  46 
Amphisternal  test,  349 
Amphiura,  300 

„          magellanica,  495 
,,          squamata,  apical  system,  327  ; 
stone  canal,  422 


604 


COMPARATIVE  ANATOMY 


Amphiuridce,  300 
Amphoracrinus,  307 
Ampullae,  madreporic,  420 

,,        tentacle,  430 
Ampullaria,  100 

,,  (Lanistes),  Bolteinana,  shell, 

161 

(Oeratodes),  chiquitensis,  161 
crocistoma,  161 
Geveana,  161 
purpurea,  161 
(Geratodes),  rotula,  161 
Swainsoni,  161 
Ampullaridce,  6 
Ananchytidce,  293 
Anapta,  466 
Anaspidce,  10 
Anatina,  21 
Anatinidce,  21 
Anchylosis  (Grin. ),  378 
Ancula,  13 
Ancyloceras  stage,  68 
Ancylus,  8 
Ankyroderma,  287 
Annulus,  126 
Anochanus,  503 

Anodonta,    17 ;    circulation,    207 ;     gills, 
95  ;  larva,  264 ;  section,  221 
,,  cygncea,  34 

Aiwmia,  15  ;  shell,  63 
Anomiidce,  14 
Antedon,   313 ;    embryo  and   larva,   510, 

534-543 
Antedon  incisa,  312  ;  larva,  apical  system, 

318 

,,       phalangium,  young  stage,  375 
,,        rosaceus,  378 
,,        tuberculosa,  297 
Antedonidce,  373 
Anthenea,  296 
Antheneidce,  296 
Antispadix,  116 
Aorta,  198,  202 
Apetala,  293 
Apiocrinidce,  310 
Apiocrinus,  310 
Aplacophora,  3 
Aplustridce,  110 

Aplysia,  10,  78  ;  nervous  system,  140 
Aplysiella,  181 
Aplysiidce,  10 

Apophyses  (Chiton),  39  ;  (Echin.),  350 
Aptychi  (Ceph. ),  71 
Arachnoides,  293 
Arbacia,  290,  291 
Arbaciidce,  290 
Area,  15 

,,     barbata,  eye,  175 
„     Noce,  206 
Archceocidaridce,  289 
Archceocidaris  ( =  Echinocrinus),  289 
Ar chaster,  296 

296 


Arcliiacia,  295 

Archidoris,  13 

Architcenioglossa,  5 

A  rci  dee,  15 

Arcus  (Echinoid. ),  400 

Argentea  (Ceph.),  197 

Argonauta,  24,  24  ;  gonads,  female,  230  ; 

nervous  system,  148 
„  argo,  male,  243 

Anon,  8 

„       ater,  9 
Ariophanta,  8 
Aristocystis,  313 
Aristotle,  lantern  of,  400-403 
Articulamentum,  39 
Articulata,  309 
Ascocystis,  313 
Ascoglossa,  11 
Asiphoniata,  44 
Aspergillum  (Brechites),  21,  20 

„  dichotomum,  66 

Aspidobranchia,  4 

Aspidochirotce,  285  ;  organisation,  477 
Aspidodiadema,  290 
Aspidodiadematidce,  290 
Aspidosoma,  295 
Astartidce,  50 

Asteracanthion  glacicdis,  pedicellariaa,  395 
„  rubens,    madreporic    plate, 

421 

„  „          pedicellariae,  395 

Asterias,  299 

„         acutispina,  506 
,,         atlantica,  506 
calamaria,  506 
capensis,  421 
„         microdiscus,  506 
„        polyplax,  421 
„         rubens,  507 
„         spirabilis,  503 
,,         sticantha,  arm,  396 
,,         (Stollasterias),     volsellata,     arm, 

396 

„         tenuispina,  421 
„         mdgaris,  507 
Asteriidce,  299 
Asterina,  297 

,,          gibbosa,  circular  canal,  etc.,  425  ; 

ontogeny,  525-531 
Wega,  506 
Asterinidce,  297 
Asterodiscus,  297 

Asteroidea,  295  -  299  ;    alimentary   canal, 
etc.,  484;  arm,  section,  411;  optic  cushion, 
section,  468  ;  oral  skeleton,  352  ;   pedi- 
cellariae,  395  ;  stone  canal,  section,  421  ; 
water  vascular  system.  463 
Aster opsis,  297 
Asthenosoma,  290 

„  urens,  spine,  390  ;  test,  472  ; 

viscera,  443 
Astrochele,  301 
Astroclon,  301 


INDEX 


605 


Astrocnida,  301 
Astrocrln  idcr\  315 
Astrocrinu-s,  315 

„  Benniei,  315 

Hiphus,  301 
Astrogonium,  296 

den   ai'.rantiacus,  branchial    skele- 
ton, 351 

Axtropectinida,  296 
Attrophytiffce.  301 
Attrophyton,  301 

„  Lincki,  301 

Astroporpa,  301 
Astropyfja. 
Astroschema,  301 
Astrotoma,  301 
Atelecrinus,  313 
Atelestocrinus,  304 
Atlanta,  90,  109 

„         Peronii,  7 
Atlantidce,  6 
Atys,  46 

Auricula,  350,  402 

Auricularia,    506,    507,    508,    511,    512, 
513,  514 

„          of  Synapta,  512,  514 
A  v.  ricul idee,  8 
Ai-icv.la.  17 
Aciadidcf,  17 

Axial  organ  (  =  ovoid  gland),  437,  445,  446 
Azygobranchla,  5 

BACULITES  stage,  68 
Bc^anocrinus,  313 
Bcdanoglossus,  562 

canadensis,  565 
Kwoalevskii,  562  ;  branchial 
skeleton,  580 

i   „  Kiipfferi,  571 

„  Mcrschkorskii,  571 

Be.  rra  n dewin  idee,  309 
Barrandeocrinus,  309 
Barrel-shaped  larvae  (Echin.),  514 
BarycriiiHs,  304 
Basals,  318 

Basibrachial  cartilage  (Ceph.),  126 
Basipterygial  cartilage  (Ceph.),  127 
Basoiniiifitiiphora,  8 
Bathybiaster,  296 
Bathycrinida,  304 
Bathycrinus    Aldrichianus,    axial  canals, 

378 

Bathydoris,  13 
Batocrinus,  307 

,,          pyriforiiris,  307 
Baur's  vesicles,  467 
Bdemnites,  24  ',  shell  section,  69 
Belemnitidce,  24 
Belcm  nocrin  us,  304 
Bdemiwteuthis,  24 
Bellerophontidce,  5 
Belosepia,  24 

,,         shell  section,  69 


Benthaster,  298 

Benthodytes,  285 

Berghia,  13 

Bipinnaria  larva,  507  ;  dorsal  aspect,  528 

Bivalra,  14 

Bivium,  325,  347,  407 

Blastoidea,  314 

Blauneria,  109 

Bojauus,  organ  of,  215,  221 

Bornellidce,  13 

Bothriocidaris,  289 

Bothriocidaroida,  289 

Botyocrimis,  304 

Bourgueticrinidce,  310 

Bourguetinicrinus,  311 

Brachiolaria  larva,  508 

Branchiopneiista,  78 

Brechites  (Aspergillum),  21,  20 

Brisinga,  299 

Brisingidce,  299 

Brissopsis,  293 

Brissus,  293 

Buccal  shields  (Oph.),  336  ;  plates  (Echin- 

oid.),  344 
Buccinidce,  6 
Buccinnm,  160 
Bull-minus,  8 
Bidimulidce,  8,  9 
Bulimus,  8 

,.,       oblongus,  75 
,,       percersus,  160 
Bulla,  10 

,,     hydatis,  nervous  system,  140 
Buttidas,  10 

Bulloidea  (Cephalaspidce),  110 
Bursse  (Oph.),  494 
Byssus,  112-115,  114 

CALAMOCRINUS,  310 
Calamus,  69 
Calcareous  cells,  190 
Calceocrinus,  304 
Calliaster,  296 
Callicrinus,  309 
Callocystis,  332 
Callum,  64 
Calymne,  293 
Calyptroea,  108 
Ccdyptrceidce,  6 
Calyx,  319 
Camerata,  306 
Canaliculata,  310 
Cancdlariidce,  6 
Caprinidce,  18 
C'iptdidce,  6 
Cardiacea,  18 
Cardiaster,  293 
Cardiidce,  18 
Cardita,  17 
Carditldce,  17 
Cardium,  18 

„       ed ule,  nervous  system,  144 ;  tuber- 
culatum,  18 


606 


COMPARATIVE  ANATOMY 


Carina  (Echinoid.),  401 
Carinaria,  90,  109 
Carolia,  63 
Carpocrimis,  307 
Caryocrinus,  313 

,,          ornatus,  apical  capsule,  332 
Cassidaria  tyrrhena,  163 
Cassidiidce,  6 
Cassiduloidea,  293 
Cassidulus,  293 

„         pacificus,  perisome,  348 


Catillocrinus,  304 
Catopygus,  293 
Caudina,  287 
Cavolinia,  11 

„         tridentata,  91 
Cavoliniidce,  11 ;  diagram,  80 
Centre-dorsal,  375 
Centrostephanus  longispinus,  pedicellarisp, 

397 

Cephalaspidce  (Bulloidea),  10 
Cephalic  cartilage  (Gastr.),  126 ;  cone,  104  ; 

disc,  103 
Cephalodiscus  dodecalophus,  596  ;  sections, 

597,  598 

Cephalophora,  3,  177 

Cephalopoda,   21  -  25  ;    eye,   development, 
171 ;  embryo,  116  ;  gonad,  female,  230  ; 
male,  231  ;   ink-bag,  197 ;   heart,  199  ; 
retinal  cells,  173 
Cerata,  98 

Ceratodes  (see  Ampullaria),  161 
Cerithiidce,  6 
Cidaris,  290  ;  spine  of,  389 

canaliculata,  502 

hystrix,  peristome,  344 

membranipora,  502 

nutrix,  502 

papillata,  oral  area,  345 

tribuloides,  surface  of  test,  389 
Cidaroida,  290  ;  apophyses,  350 
Cionella,  75    . 
Cirrhoteuthidce,  24 
Cirrhoteuthis,  54  ;  shell  of,  127 
Cirrobranchia,  12 
Chcetaster,  298 
Chcetoderma,  3,  88 
Chcetodermatidce,  3 
Chcetodermatina,  3 
Chcetodermidce,  3 
Chama,  18 
Chamacea,  51 
Chambered  sinus,  446 
Chamidce,  18 
C/ienopidce,  6 
Chiasma    nervorum    brachialium   (Grin.), 

377,  460 

Chiastoneury,  135,  136,  137 
Chirodota,  288 

„         rotifera,  402 
Chiroteuthis,  24 
Chiti.  ",3,2;  spine,  40  ;  sections,  40,  212  ; 


ctenidium,  85,  87  ;  heart,  199  ; 
eye,  section,  167  ;    ovary,  227  ; 
nephridial  and  genital  systems, 
217 
Chiton  cajetanus,  165 

Icevis,  88,  165 

Pallasii,  82 

Polii,  165  ;  development,  249 

rubicundus,  129 

siculus,  165  ;  nervous  system,  130 
Chitonellus,  3  ;  section,  41 
Chitonidce,  2  \  gills,  87 
Chlamys,  17 

Choanomphalus  Maacki,  160 
Choristes,  108 
Chromatophores,  53 
Chromodoris,  13 
Cionella,  75 
Cladohepatica,  13 
Glaus-ilia,  8 
Clavagella,  21 
Clavagellidce,  21 
Clavulse  (Echinoid.),  391 
Cleiocrinus,  309 
Cleodora,  30 
Clio,  11 

„     striata,  anatomy,  190 
Clione,  90 
Clionidce,  11 
Clionopsidcc,  11 
Clionopsis,  90 
Clypeaster,  291,  292 

„  rosaceus,  apical  system,  322 

Clypeastridce,  291  ^ 

Clypeastroida,  291  ;  system  of  plates,  346 
Clypeus,  293 
Cnemidaster,  298 

,,  Wyvilli,  297 

Cadaster,  315  ;  ambulacrum  section,  383 

,.       bilobatus,  314 
Codasteridce,  315 
Codechinus.  290 
Codopleurus,  290 
Cottyrites,  293 

„        elliptica,  apical  system,  325 
Collyritidce,  293 
Colochirus,  287 

,,          cucumis,  calcareous  body,  337 
Colombellinidce,  6 
Columella,  123 
Columellar  muscle,  120-123 
Columna  (Crin. ),  373 
Columuals,  373 

Columnar  layer  of  shell  (Moll.),  57 
Comatulidce,  313 
Conchyolin,  26,  57 
Conidce,  6 
Conoclypeus,  291 
Coralliophila,  183 
Coralliophillidcc,  183 
Corbicula,  17 
Corbis,  93 
Corbula,  19 


INDEX 


607 


Corethraster,  298 

Corium  (Moll.),  39  ;  (Echin.),  414 

Corona,  339 

Coronaster,  299 

Corpus  epitheliale,  171 

Corylocrinus,  332 

Coryptella,  13 

Costals  (Grin.),  371 

Oranchia,  24 

Crassatella,  49 

Crassatellidce,  17 

Crenella,  115 

Crepidididce,  138 

Cribrella,  299 

,,        sexradiata,  506 
Crinoidea,  302-316  ;  arm,  372  ;  arm,  sec- 
tion, 413  ;  ovarial  pinnule,  section,  500 
Crioceras  stage,  68 
Crista  acustica,  169 
Oromyocrinus,  304 
Orossaster,  298 
Crotalocrinidoe,  308 
Crotalocrinus,  308 

,,  pulcher,  373 

,,  rugosus,  arm  disc,  372 

Cryptoblastus,  315 
Oryptochiton,  41 
Cryptodon  Moseleyi,  50 
Cryptoschisina,  315 
Gryptozonia,  297 
Crystalline  stylet,  191 
Ctenidium,  84,  85 
Ctenodiscus,  296 

*     ,,  procurator,  296 

Ctenopidce,  102 
Cucumaria,  287 

crocea,  502 

crudfera,  cruciform  body,  337 

„  doliolum,  section,  517 

„          frondosa,  488 

„  Lacazii,  464 

,,  losvigata,  502 

„  longipeda,  "stool,"  337 

„  mimita,  502 

„  planci,  287 

Culcita,  297 
Cidtellus,  19 
Gupressocrinus,  304 
Cuspidaria,  21 
Cuspidaridce,  21 
Cuttlefish  ( =  Cephalopoda],  21 
Cuvierian  organs  (Hoi.),  488 
Cyatliocrinidce,  304 
Cyathocrinus,  304  ;  apical  system,  329 

,,  lonyimanus,  304,  364 

Cyclas,  81 

„        cornea,  development,  262,  263 
Cyclophoridce,  5 
Cydophorus,  100 
Cydostoma,  100 

,,  elegans,  nervous  system,  139  ; 

radula,  182 
Cydostomidce,  6 


Cylichna,  46 

Cynibulia,  11  ;  larva,  257 
Cymbulidce,  11 
Cymbuliopsis,  11 
290 


Cypraea,  5 
Cypraeidce,  5 
Cyrena,  17 
Cyrenidce,  17 
Cyrtoceras  group,  68 


,,  vesica,  gonads,  499 

idea,  313 
CystoUastus,  313 

,,  Leuchtenbergi,    312  ;    apical 

side,  332 

Cystocidaris  ( =  Echinocystites),  289 
Cystocidaroida,  289 
Cystocrinoidea,  313 
Cystoid  stage,  544 
Cytherea  (Meretrix),  18 
„         chione,  shell,  63 

DAUDEBARDIA  (Helicophantd),  9 
„  brevipes,  9 

„  /""/#,     intestine,     section, 

39  ;     uephridia,    220  ; 
pallial  organs,  76,  77 
„  saulcyi,  76 

Decadocrinidce,  304 
Decadocrinus,  304 
Decapoda,  24 
Deima,  285 
DeimoMdce,  285 
Delphinulidce,  170 

Deltoid  plates  (Blast.),  331  ;  (Grin.),  364 
Dendrochirotce,  287 
Dendrocrinidce,  304 
Dendrocrinus,  304 
Dendronotidce,  13 
Dentalium,  13,  33  ;  shell,  59,  156 

,,          entcde,  113,    159  ;    alimentary 
canal,  193  ;  larva,  258  ;  on- 
togeny of,  258 
Dermatobranchus,  48 
Deziobranchced,  11 
Dextral  twist  (Gastr.),  56,  60,  160 
Diadema,  290 

„         setosum,  392  ;    compound   eye, 

469 

Diadematidoe,  290 
Diadematoida,  290 

,,  apophyses,  350 

Dialyneurous  nervous  system  (Gastr.),  138 
Diaphragm  (Ceph.),  127  ;  cartilage,  127 
Diaulula,  13 
j   Dibranchia,  24  ',    eye  development,   171  ; 

musculature,  127  ;  shells,  sections,  69 
Diceras,  18 
Dichocrinus,  SOS 
Dicyclic  base  (Criu.),  328 
Dicydica,  304 


608 


COMPARATIVE  ANATOMY 


Diffuse  liver  (Gastr.),  192 
Digonopora,  8 
Dimerocrinus,  367 
Dimyaria,  14,  124  ;  shell,  63 
Diotocardia,  4>  30 
Dipleurula  larva,  546 
Diplopodia,  290 
Discodoris,  13 
Discoidea,  291 
Distichals  (Grin.),  371 
Docoglossa,  5 
Dolabella,  10 
Doliidce,  6 
Dolium,  102 
Donacidce,  18 
Donax,  18 
Dondersia,  3 

•is,  251 
.  184 
Dondersiidce,  3 
Dorididce  cryptobranchiata,  13 

,,        phanerobranchiata,  13 
Doridiidce,  10 
Doridium,  46 
Doridopsidce,  13 
Doriopsis,  214 
Doris,  respiratory  and  circulatory  organs, 

98 

Dorocidaris  papillata,  388 
Dorsal  axis  (Blast.),  331  ;  cartilage  (Ceph.), 

127 

Dorycrinus,  307 
Dosidicus,  69 
Dotonidce,  IS  v 

Dreissensia  pvlwnorpha,  gills,  94 
Dreissensiidce,  17 
Disaster,  293 
Dytaster,  296 

ECHINASTER,  299 

,,  sepositus,  483 

EchinoMeridce,  299 
Echinidce,  290 
JEchinobrissus,  293 
Echinocardium,  293 


Echinocidaris,  290 

„  nigra,  ambulacrum,  393 

„  pustulosa,  291 

Echinocotms,  293 
Echinocorys,  293 
Echinocrepis,  295 
Echinocrinus  (Archceocidaris),  289 
Echinocyamus,  291 

,,  pusillus,     gastrula,     520  ; 

Pluteus  larva  and  young, 
520-524 ;  tentacle,  sec- 
tion, 464 

Echinocystis  (Cystocidaris),  289 
Echinodermata,    alimentary    canal,    475 ; 
representatives    of   principal    divisions, 
316 

293 


Echinodiscus  biforis,  424 

Echinoencrinus,  313 

„  armatus,  333 

Echinoidea,  288-295;  endocyclic,  322; 
exocyclic,  322  ;  larva,  509  ;  organisa- 
tion of  regular,  419  ;  pedicellarise, 
397  ;  Pluteus,  520-524  ;  radial  region, 
section,  410 

Echinolampas,  293 

Echinometra,  290 

Echinometridce,  290 

Echinoneidce,  293 

Echinoneus,  293 

Echinopsis,  290 

Echinospatagus,  293 

Echinosphcera,  386 

ISchinothrix,  290 

Echinothuria,  290 

Echinothuridce,  290 

Echinus,  290  ;  apical  system,  318  ;  masti- 
catory apparatus,  401 
,,        acutus,  398 


Edrioaster,  387 
Elceocrinus,  315 
Elasipoda,  285 
Eledone,  24 

,,         moschata,  secondary  body  cavity, 

diagram,  214 
Eleutherocrinus,  315 

,,  Cassedayi,    apical     side, 

331  ;  oral  side,  383 
Elpidia,  285 

,,       glacialis,  467 
Elpidiidce,  285 
Elysiadce,  12 

EiYbarginula,  4  >  shell,  59,  156 
Embryonic  cone,  257 
Enallocrinus,  308 
Encope,  293 

,,         Valenciennesi,   system   of  plates, 

346 
Encrinus,  304  ;  axial  canals,  378 

,,         liliiformis,  304 
Endoceratidce,  67 

Eudogastric  coil  (Gastr.),  67,  68,  159 
Enoploteuthis,  127  ;  musculature,  127 
Ensis,  19 

Enter opneusta,  561-596  ;  branchial  region, 
567,    580  ;   larvae,   586-590,   594 ;    pro- 
boscis, section,  583 
Entocolax  Ludwigii,  246,  247 
EntoconcJia,  183 

,t  mirabilis,  247,  248 

Entovalva,  229 
Eolampas,  293 
Eolis  D-rummondi,  183 
Epineural  canal,  449 
Epipodial  lobes  (Ceph.),  38 
Epipodium,  106 
Episternum,  349 
Epistroma,  349 
Eretmocrinus,  307 


INDEX 


609 


JSrisoc-rinus,  372 
ErycinO)  17 

Erycinidce,  17 
Emixteroidea,  296 
Eucalyptocrinidce,  309 
Euoalyptoervn  us,  309 

ix,  308 
313 
Eudiocrinus,  313 

Eiocliinoidea,  290 

,,  diadematoida,  469 

Eulamellibranchia,  17  ',  gills,  92 
Ei'lima,  183 
Enliuiidce,  6 
tin  HI  irrgerita,  108 
Eupachycrin  //.y,  372 
E'tptocamus,  13 
Enryalai,  301 
Ei'ru'i.le,  301 

Euthyneurous  condition  (Gastr.),  133,  158 
Exogastric  coil  (Gastr.),  159 
/>«/////•«,  62 

313 


FACELLINA,  13 
Falces  (Echinoid.  ),  400 
Faorina, 

Fascioles  (semites),  349 

F'-riti'nfi,  326 

Ferment  cells,  190 

Fibularia,  291 

Fibulariidce,  291 

Filibmnchia,  14  ',  gills,  92 

Fins  (Ceph.),  54  ;  (Oph.),  392 

Fiona,  13 

Firulu  coronata,  7 

Firoloides,  90 

F!x*mvlla,  4  ',   ctenidium,   85  ;   shell,  -59, 

156 

Fistitrellidce,  4 
Fistulana,  64 
Fistu.lata,  303 
Flabcllina,  13 
Fleming's  cells,  162 
Floscelle,  347 
Foramen  basale  (Echinoid.),  400  ;   exter- 

num  (Echinoid.),  400 
Forbexincriii  //.y,  309  ;  dorsal  cup,  369 
Fork  pieces  (Blast.),  331  ;  (Echinoid.),  400, 

401 
Funnel    (Ceph.  =  siphon),    265  ;    ciliated 

(Echin.),  437 
Ft'xidce,  6 

GALATEA,  17 

Galeomma,  17 

Galempygus,  322 

(»'"/<V/tff,  13 

Ganeria,  297 

Gasteropteridce,  10 

Gasteropteron  Meckdii,  10  ;  genital  organs, 

233 
Gastrochcena,  65 

VOL.  II 


Gastrocoma,  304 

Gastropoda,  3-13  ;  hypothetical  primitive, 
150,   151  ;   racial  form,   159  ;  sections, 
47,  110  ;  shell,  59,  156 
Genicopatagus,  293 
Genital    plates   (Echinoid.),   321  ;    stolon 

(Grin.),  446 
Gilbertsocrinus  (  =  011acrinus),  396 

,,  tuberculosis,     system      of 

plates,  367 

Gills,  adaptive,  97  ;  anal,  97 
Gissocrinus,  304 
Gladius,  69 
Glandiceps,  562 

,,  Hacksii,  570 

,,  talaboti,  570 

Glandina,  104 
Glands— 

,,        albuminous  (Gastr.),  232 

anal  (Gastr.),  177 
,,        blood-making  (Ceph.),  97 
,,        byssus  (Lam.),  112 
,,        calcareous  (Gastr.),  42 
,,        digestive,  190-195 

digitate  (Gastr.),  237 
,,        goblet  (Echin.),  415 
granular  (Echin.),  415 
hermaphrodite  (Gastr.),  235 
hypobranchial,  101 
labial  (Gastr.),  Ill,  178 
Leiblein's  (Gastr. ),  188 
lime  (Gastr.),  99 
lymph  (-ovoid)  (Echin.),  445 
mucous  (Gastr.),  4? 
nephridial  (Gastr.  \  219 
nidamental,  233,  241 
oviduct  (Ceph. ),  241 
ovoid    (  =  lymph)    (Echin.),    437, 

444 

pedal,  106,  111 
pericardial   (Keber's   organ),   214, 

215 

pigment  (Gastr.),  42 
poison  (Gastr.),  188 
purple  (Gastr.),  74,  101 
prostatic  (Gastr.),  233 
salivary,  184-187 
shell  (Gastr. ),  237  ;  larval,  253 
slime  (Gastr.),  237 
stalk  (Echin.),  399 
sugar  (Amph.),  187 
Glandular  pedicellariaj  (Echin.),  398 
t'-lit"'.-t(s,  13 
Gleba,  11 

Glochidium,  larva,  264  ;  parasiticum,  263 
Glomerulus  (Ent.),  582 
Glossophora,  177 
Glycynieridce,  19 
Glycymeris,  19 
Glyphocyphus,  290 
Glyptaster,  306 
Glyptasteridce,  306 
Glyptocrinus,  307 

2  R 


610 


COMPARATIVE  ANATOMY 


Glyptosphcerites,  313 
Goniaster,  296 
Goniodoris,  13 
Goniopygus,  290 
Gorgonocephalus,  301 
Granatoblastidce,  315 
Granatocrimis,  315 

,,  Norwoodi,  314 

Graphiocrinus,  304 
Gryplwea,  62 
Gymnasteria,  297 

,,  carinifera,  pedicellarife,    395 

Gymnasteriidce,  297 
Gymnosomata  (Pteropodd),  11 
Gyroceras,  group,  68 

H^EMOCYANINE,  200 

Haemoglobin,  200 

Haemolymph,  200 

Haliadea,  6 

Haliotidce,  4 

Haliotis,  gills,  90,  121  ;  shell,  59,  156 

Halopsyche,  90 

Hamites,  stage,  68 

Haplocrinm,  303 

,,  mespiliformis,  303,  334 

Harpidce,  6 

Hectocotylisation,  118,  242 
Helcion,  108 
Heliaster,  299 
Helmsteridce,  299 
Helicarion,  8 
Helicidce,  8 
Helicinidce,  5 

Helicophanta  (Daudebardia),  45 
Helider,  160 

Helix,  8 ;  anatomy,  236  ;  alimentary  canal, 
186  ;  circulatory  system,  204 ; 
sections,  99,  100,  104,  181  ;  shell, 
123 

,,      aspersa,  75 

,,      Ghiesbreghti,  183 
-  ,,     pomatia,  9  ;   nervous  system,  142  ; 
shell,  60 

,,       Waltoni,  258 
Hemiaster,  293 

,,          cavernosus,  480 
Hemicidaris,  299 
Hemicidaridce,  290 
Hemicosmites,  332 
ffemieuryale,  300 
Hemipatagus,  293 
Hemipholis,  327 

,,  cordifera,  disc  section,  435 

ffemipneustes,  293 
Hermceidce,  12 
Hero,  13 

Herpetocrinus,  304 
Heteroblastw,  315 
Heterocrinus,  304 
Heterolampas,  293 
Heterwnyaria,  15,  124 
Ifetercyoda,  6 


Hexacrinidce,  308 

Nexacrinus,  308 

Hinge  (Lam.),  61 

Hipponycidce,  6 

Hipponyx,  108 

Hippopus,  18 

Hippurites  (Rudistes),  62 

Hippuritidce,  18 

Histioteuthis,  54 

ffolaster,  293 

,,         suborbicularis,     apical      system, 
324 

Holectypoida,  290 

Holectypus,  290 

,,  depressus,  apical  system,  322 

Holohepatica,  12 

Holopidce,  304 

Holopus  Rangi,  305 

Holostomata,  44 

ffolothuria,  285,  316 

,,  impatiens,  464 

,,  Murrayi,  "stool,"  337 

,,  tubulosa,  organisation,  451 

Holothurioidea,  285-288 ;  calcareous  bodies, 
337  ;  ciliated  rings,  515  ;  diagram,  sec- 
tion, 407  ;  oesophagus  of  dendrochirote, 
404  ;  oral  region  of  actinopod,  428  ; 
organisation  of  aspidochirotan,  477  ; 
radial  region,  section,  409  ;  stone  canal, 
etc.  418 

Homalonyx,  45 

Homocrinus,  304 

Hood  (Ceph.),  37 

Hook  sacs  (Gastr.),  180 

Hyalina,  76 

Hybocrinus,  304 

Hydreiocrinus,  372 

Hydrobiidcc,  6 

Hydroccel,  511 

Hydrocoenidce,  5 

Hydrospire  (Blast.),  382 

Hymenaster,  298 

,,  ccelatus,  298 

.,  nobilis,  298 

,,  pellucidus,  apical  side,  503 

ffyocrinidce,  304 

Hyocrinus  Bethellianus,  305,  335 

Hyphcdaster,  296 

Hypobraftchicea,  98 

Hypocrinus,  304 

Hypoplax,  64 

lANTHINA,  108 

lanthinidce,  184 
Ichthyocrinidce,  309 
Ichthyocrinoidce,  308 
Ichthyocrinus,  309 
Idalia,  13 
Idiosepion,  24 
llyaster,  296 
Ilyodcemon,  418 
Inadunata,  303 

fistulata,  303,  363 


INDEX 


611 


//i<»/!>nata  larviformia,  303,  363 

lufrabasals,  318 

Ink-bag  (Ceph.),  177,  196  ;    morphology, 

197 

Inoceramus,  17 
I uti'iji'ljiiillinta,  shell,  63 
Interambulacra,  339 
Interambulacral  radii,  316 
Interdistichals,  363 
Internodes,  374 
Interpalmars,  363 
Inter  radii,  316 
Intersecundibrachs,  363 
Intertertibrachs,  363 
locrinn.s,  304 
Irpa,  418 
Irregidares,  315 
Isaster,  293 
Isoiiiyaria,  124 

JANUS,  13  ;  nervous  system,  141 
Jouanmtia,  19 

,,         Gumingii,  19 
Juglandocrinus,  332 

KEBER'S  organ  (red-brown  organ),  215 

Kellya,  17 

Kentrodoris,  13 

Kleinia  luzonica,  apical  system,  342 

Kolga,  285 

Kollicker's  canals  (Ceph.),  168 

LABIAL  palps  (Gastr.),  104;  (Lam.),  105 

La.bidiaster,  299 

Labidodemas,  491 

Labrum  (Echinoid.),  348 

Lacuna,  108 

Lcetmogcnie,  285 

Laganid.ce,  291 

Laganum,  291 

„          depressum,  apical  system,  323 
Latnellaridaf,  6 
Lamellibraiiclnata,    lJ^-21  \    byssus,    114; 

diagrams,  50  ;  heart,  199  ;  shell,  59,  62, 

156 

Lancet  plates  (Blast.),  380 
Lanistes  (see  Ampidlaria),  161 
Luri-ifnnnia,  303 
Lascea,  17 

Lateral  organs  (Gastr.),  165 
Lecythocrinus,  304 
Leda,  14 
Lepctidce,  5 
Lepidocentriis,  289 
Lepidomenia,  13 

hystrix,  132 
Leptoconckus,  183 
Lepton,  17 

Leptopty 'chaster  kerguelenensis,  503 
Leskiidce,  295 
Leuconia,  109 
Ligament  (Lam.),  61 
Lima,  105 


Limacidce,  8 

Limacina  helicina,  anatomy,  189 

,,        Lesuerui,  11 

,,        retroversa,  160 
Liinacinidce,  11 ;  diagram,  80 
Limapontiidce,  12 
Limax,  8  ;  vascular  system,  204 
Limidce,  53 
Limncea,  8 

„        abyssicola,  100 

„        stagnalis,  genital  organs,  235 
Limnceidce,  8 
Linckia,  298 

,,        midtifora,  244 
Linckiidce,  298 
Linthia,  293 
Lipocephala,  101,  177 
Lithodomus,  15 
Litiopidce,  108 
Littorinidce,  6 
Lituites,  68 
Lobiger,  12 
Loligo,   24  ;  gonad,  male,   231  ;    nervous 

system,  148 
,,        vulgaris,  23 
Loligopsis,  24  ',  shell,  section,  69 
Lomanotidce,  13 
Loven,  law  of,  341-344 
Lovenia,  293 

Lucina  Pennsylvanica,  shell,  63 
Lucinacea,  50 
Lucinidce,  17 
Luidia,  296 
Lunula,  341 
Lutraria,  51 
Lutrariidce,  19 
Lyonsia  Norwegica,  51 
Lyonsiidcie,  21 

MAGTRA,  18 

Mactridcc,  18 
Macula,  168 

,,        acustica,  168 
Madreporite,  321,  416-423 
Magttus,  183 
Malletia,  49 
Malleus,  17 
Mantle,  26  ;  cavity,  26 
Margarita  Groenlandica,  4 
Margaritana  (Unio)  Margartiifents,  17 
Marginaster,  297 
Marginellidce,  6 
Mariacrinus,  307 
Marionia,  13 
Marseniadce,  225 
Marsupia,  347 
Mamipiocrinus,  308 

„  ccelatus,    tegmen    calycis, 

369 
Marsupites,  304 

„  ornatus,  apical  system,  329 

Martesia,  65 
Megallaesthetes,  166 


612 


COMPARATIVE  ANATOMY 


Megistocrinus,  307 
Melampas,  109 
Melanidce,  6 
Meleagrina,  17 

,,  margaritifera,  17 

Melibe,  13 
Mdlita,  293 

,,        testudinata,  293 
Melocrinidce,  307 
Melocrinus,  307 

„          typus,  307 
Melonites,  289 

,,         multipora,  apical  system,  340 
Melonitidce,  289 
Membrana  limitans,  173 
Meona  ventricosa,  apical  system,  323 
Meretrix  (Gytherea),  18 
Meridosternal  test,  349 
Mesites,  313;  ambulacrum,  section,  386 
Mesollastus,  315 
Mesodesinatidce,  18 
Mesoplax,  64 
Mesorchium,  230 
Metablastus,  315 
'  Metacrinus,  313 

„  angulatus,  tegmen  calycis,  365 

„  Murrayi,  311 

Metaplax,  64 
Metapodium,  106 
Metrodira,  298 
Micrsesthetes,  166 
Micraster,  293 

,,         coranguinum,  apical  system,  324 
Miller  icrinus,  310 
Milneria,  49 
Mimaster,  296 
Mithrodia,  299 
Mitra,  101 
Mitridce,  6 
Modiola,  15 
Modiolaria,  15 
Moira,  503 
Mollusc,  hypothetical  primitive,  26 


„         chilensis,  488 
Molpadiidce,  287 
Monocyclic  base  (Grin.),  328 
Monocydica,  303 
Monogonopora,  8 
Monomyaria,  14,  124  ;  shell,  64 
Monopleuridce,  18 

Monotocardia,  5  ;  diagram,  31 ;  gills,  90 
Montacuta,  93 
Muelleria  (Moll.),  124 
Muelleriacea,  124 
Miietteridce,  124 
Muileria  (Echin.),  285 
Murex,  101 

„       trunculus,  alimentary  canal,  189 
Muricidte,  6 
Mutela,  17 
jHfutelinc',  51 
My  a,  I. 


Myacea,  18 
Myadce,  64 
Myiidce,  19 
Myochama,  51 
Myopsidce,  24 
Myriotrochus,  288 

„  Rinkii,  246 

Mytilidce,  15 
Mytilus  edulis,  15 


NACELLA,  108 

Nacreous  layer  of  shell,  57 

Narica,  108 

Naricidce,  107 

Natantia,  10 

Natica  Josephina,  107  ;   swelling  of  foot, 

119 

Naticidce,  6 
Nautiloidecti  22 

Nautilus,  22 ;  eye,  169  ;  diagram,  37  ; 
gonad.s,  230  ;  nervous  system, 
145,  146  ;  pallial  complex, 
82  ;  tentacles,  117 

„         Pompilius,  22 
Nautilus  group,  68 
Nectria,  296 

Needham's  (spermatophoral)  pouch,  238 
Neocrinoidea,  303 
Neomenia,  3 
Neomeniidce,  3 
Neomeniina,  3 
Nephropneusta,  78 
Neptunea  contraria,  160 
Neritacea,  4 
Neritidce,  5 
Neritince,  5 
Neritopsidce,  5 
Notaeum,  10 
Notarchus,  10 

„         punctatus,  nervous  system,  141 
Notaspidce,  10 
Notobranchcea,  90 
Notobranclweidai,  11 
Nuchal  plate,  126 
NudeoUastidce,  315 
Nucula,  14  ;  ctenidia,  85 

„        nucleus,  14 
Nuculidce,  14 
Nudibranchia,  12 

OCTOPODA,  24 
Octopodidce,  24 
Octopus,  24 ;  anatomy,  147  ;  nervous 

system,  148 
„      •     vulgaris,  25;  gonad,  female,  231 ; 

male,  239 

Ocular  plates  (Echinoid.),  321  ;  (Ast.)  354 
Odontoblasts,  183 
Odontophore,  336 
Oigopsidce,  24 
Oligopygus,  293 
Oliva,  30 


INDEX 


613 


Olividce,  6 

Ollacrimts  (see  Gilbertsocrinus) 

Ouimastrephes,   24  ',    gonad,    female,   230  ; 

shell,  sectiou,  69 
Ommatophore,  102 

Ommatostrephes,  24  ;  nervous  system,  148 
Oncididla,  46 
Onci'Uidce,  8 
0/ici'Uum,  44,  104 

celticum,    genital   organs,   235  ; 

larva,  256 

Oneirophanta  mutabilis,  rod,  237 
Onychoteuthis,  24 
Operculum,  31 
Ophiamntha,  300 

,,  marsupialis,  495  ;    vivipara, 

495 
Ophiactis,  300 

MiUleri,  506 
„        poa,  300 
„         Savigny,  506 
„         virens,  498  ;  disc,  section,  426 
Ophiarachna  incrassata,  vertebral  ossicles, 

356 

Ophidiaster  Germani,  421 
Ophi'.'ceramis,  327 
Ophiocnida,  300 

„  sexradia,,  506 

Ophiocoma,  300 

pumila,  506 
Valencia,  506  ; 
Ophiocreas,  301 
Ophioderma  (  =  0phiura),  495 
Ophiodiaster,  298 

diplax*  regeneration  of  arms, 

505 
Ophioglypfui,  300  ;  bursa,  495,  496  ;  disc, 

section,  497 

„  albida,  stomach,  494 

„  hexactis,  495 

„  lacertosa,  ovary,  section,  497 

Ophioglyphidce,  300 
Ophiohehis  umbella,  arm  joint,  357 
Ophiomastix,  327 
Ophiomitra  exigua,  327 
•  Ophiomusium,  300 

„  ralidum,  apical  system,  327 

Ophiomyxa,  300 

„  I'ii-ipara,  495 

Op/iiumyxidce,  300 
Ophiontreis,  422 
Ophwplocus,  422 

Ophiopteron  elegans,  brachial  joints,  391 
Ophiopya  longispinus,  oral  skeleton,  359 
Op h  iopyrgus,  327 
Ophiothela  dividiia,  506 
„         isidicola,  506 
Ophiotholia,  392 
Ophiothrix,  300 

fragilis,    ambulacral    tentacle, 
section,   466 ;   nervous   sys- 
tem, 456 
Ophwzona,  300 

VOL.  II 


Ophiura,  300 
Ophiurce,  300 
Ophiuroidea,  299-301  ;  arm,  section,  412  ; 

arm,  joint,    357 ;    disc,    section,    486 ; 

Pluteus,    533 ;    nervous    system,    456  ; 

ring  sinus,  496 

Opisthobranchia,  10,  33  ;  heart,  199 
Opisthopneumonia,  76 
Oral  angle  plates,  358 
„     lobes  (Lam.),  105 
Orocystis  Helmhackeri,  313 
Orophocrinus,  315 

„  stelliformw,  314 

Orphnurgits,  418 
Orthoceras  group,  68 
Orthopsis,  299 
Oscainius,  10 
Osphradium,  84,  162-164 
Ostracoteuthis,  shell,  section,  69 
Ostrea,  17 

„       edulis,  anatomy,  16 
Ostreidce,  17 
Owenia,  24 
Oxygyrus,  108 
Oxynoe,  12 
Oxynoidea,  12 

PAL&ASTER,  295 
Palceasteroidea,  295 
Palceechinoidea,  288 
Pcdceechinus,  289 

„  elegans,  289 

Palceobrissus,  293 
Palceocoma,  295 
Palceocrinoidea,  303 


„  Murrayi,  294 

Palceostoma,  295 
Palceotropus,  429 
Pallial  cavity,  26  ;  line,  64,  124  ;  sinus, 

64,  124 

Palliata  ( =  Tectibranchid),  46 
Pallium,  26 
Palmars  (Grin.),  371 
Palmipes,  297 
Paludina,  ctenidium,  85 

„         vivipara,     circulatory     system, 

203  ;  development,  252,  254, 

255 

Paludinidce,  5 
Pancreas  (Ceph.),  196 
Pandvridce,  21 
Pannychia,  285 
Papula,  439 
Paractinopoda,  288 
Paramenia  impexa,  216 
palifera,  184 
Parameniidce,  3 
Parapodia,  106 
Pa rar chaster,  296 
Parasalenui,  290 

Parasira  (Tremoctopus)  catenulata,  230 
Parelpidia,  418 

2  R  2 


614 


COMPARATIVE  ANATOMY 


Parmophorus  (Scutum),  5 

Patella,    72 ;     nephridia,     218 ;     nervous 

system,  138  ;  section,  188 
,,          vulgata,  4 
Patellidce,  5 
Patinella,  108 
Paxillse,  391,  503 
Pecten,  17  ;  eye,  174 
„       Jacobeus,  16 
Pectinibranchia,  5 
Pectinidce,  16 
Pectinura,  300 
Pectunculus,  15  ;  eye,  175 
Pedicellarise,  393-399 
Pedicellaster,  299 
Pedwellasteridce,  299 
Pedina,  290 
Pedipes,  109 

Pelagothuria  natatrix,  285,  286 
Pelagothuriidce,  285 
Pelanechinus,  290 
Pelecypoda,  14 
Pelmatozoa,  302-315 
Peltastes,  290 
Peltella  palliolum,  9 
Peltidce,  10 
Pen  (Ceph.),  69 
Peneagone,  285 
Pentaceros,  297 

„  turritus,  oral  skeleton,  473 

Pentacerotidce,  296 
Pentacrinidce,  313 
Pentacrinus,  313 

„  decorus,  377  ;  calyx,  section, 

382 
Pentactcea,  larva,  548 

Pentactula,  516 

Pentagonaster,  296 

PentagonasteridcK,  296 

Pentephyllum,  315 

Pentremites,  315,  314 ;  ambulacrum, 
section,  382  ;  apical  system,  330  ; 
organisation,  380 

Pentremitidce,  315 

Pentremitidea,  315 

Peradis,  11 

Periechocrinus,  307 

Periostracum,  58 

Periscoechinoidea,  289 

Peristome,  339 

Perna,  17 

„        Ephippiwn,  shell,  64 

Peronella  orbicularis,  424 

Peronia  (Moll),  45  ;  (Ech.),  290 

Perradii,  316 

Petalodium,  347 

Petricola,  52 

Petricolidce,  18 

Phcenoschisma,  315 

Phanerozonia,  296 

Pharus,  51,  115 

Philine,  46 

Philinulce,  10 


Philonexidce,  24 
Philonexis,  24 

„          (Octopus)  carence,  hectocotylisa- 

tion,  243 
Pholadacea,  19 
Pholadidce,  19 
Pholadidea,  19 
Pholadomyidce,  51 
Pholas,  19 

„       dactylus,  valve,  67 
Phormosoma,  290 
Phorus  exutus,  5 
Phragmocone,  68 
Phyllidiidce,  13 
Phyllirhoe,  genital  organs,  238 

,,  bucephalum,  12 

Phyllirhoidce,  13 
Phyllobranchidce,  12 
Phyllodes,  347 
Phyllophorus,  287 

„  urna,  502 


,,      fontinalis,  8 
Physetocrinus,  307 
Pinna,  17 

Pinnules  (Crin.),  371  ;  (Blast),  381 
Piracy 'stis,  336 
Pisidium,  17 
Pisocrinus,  304 
Placobranchidce,  12 
Placophora,  2 
Placuna,  15 
Planaxidce,  8 
PlanorUs,  8,  58 
Plastidogenic  organ,  445 
Plastron  (Ecbinoid.),  348 
Plates,  included,  340  ;  isolated,  340  ;  half, 

340  ;  primary,  340 
Platycrinidce,  307 
Platycrinus  triacontadactylis,  308 
„  tuberosus,  tegmen,  335 

Platydoris,  13 
Plesiocidaroida,  289 

Plesiospatangidce,  293 

Pleurae,  182 

Pleurobranchcea,  10 

„  Meckelii,    genital    organs, 

237 

Pleurdbranchia,  47 

Pleurobranchidce,  10 

Pleurobranchus,  18 

„  aurantiacus,  10 

Pleuroleuridce,  13 

Pleurophyllidia  lineata,  13 

Pleurophyllidiidce,  13 

Pleurotomaria,  5  ;  shell,  59,  156 

Pleurotomaridce,  5 

Pleurotomidce,  6 

Pliodon  Spekei,  125 

Pluteus,  larva,  508,  509,  522 

Plutonaster,  296 

Pneumoderma,  11,  79 

Pneumodermatidce,  11 


INDEX 


615 


Podocidaris,  290 
Podocyst,  258 
Polian  vesicles,  416,  423 
Polycera,  13 
Polyplacophora,  2 
Polytremaria,  5  ;  shell,  59,  156 
P&mpholyx  solida,  160 
Porcelainous  layer  of  shell,  57 
Porcellanaster,  296 
Porcellanasterid.ee,  296 
Pore  rhombs  (Cyst.),  384 
Porocrinus,  313 
Poromya,  51 
Poromyidce,  21 
Postpalmars  (Grin.),  371 
Poteriocrimis,  304 
Pourtalesia,  295 

„  Je/reysi,  295  ;  apical  system, 

325 

Paurtalesiidce,  295 
Prseputium,  235 
Prismatic  layer  of  shell,  57    . 
Proboscidifera  holostamata,  6 

„  siphonostomata,  6 

Promachocrinus,  313 
Proneomenia,  3 

„  Sluiteri,  3  ;  nervous  system, 

132  ;  sections,  42 

„  vagans,  184 

Proneonieniidce,  3 
Proostracum,  68 
Propodium,  106 

Prosobranchia,  4  '•>  gills*   90  5  heart,   199  ; 
proboscis,    179  ;    sections,   110  ;  snout, 
section,  181  ;  tentacles,  102 
Prosoplax,  64 
Prostata,  233 

Protobranchia,  14  ',  gills,  92 
Protocrinus,  313 

„  oviformis,  312 

Prunocystis,  332 
Pr;j,nnadetes,  293 
PrynDiodesmia,  293 
Paammobia,  18 
Pxiiiiriiiobiidce,  18 
Pseudarchaster,  296 
Pseudoconch,  47 
Pseudolamellibranchia,  15 
Pseiidomelanidce,  6 
Pseudomonocyclic  Crinoids,  329 
Psolus,  287 

„       ephippifer,  287 
Psychropotes,  285 

„  lonyicavda,  285 

Psychropotidce,  285 
Pteraster,  298 
Pterasteridce,  298 
Pterasterince,  503 
Pterobranchia,  12 
Pterocera,  107 
Pteropoda,  10  ;  larva,  257 
„          gymnosomata,  11 
„          thecosomata,  11 


Ptero&rachceidce,  6 
Pterotrachea,  109 

„  auditory  organ,  168 

„  (Firola)  coronata,  7 

Ptychodera,  562 

„  aurantiaca,  585 

„  bahamensis,  585 

„  clavigera,  570 

,,  erythrcea,  585 

„  minuta,  562 ;  branchial  region, 

section,  562  ;  head  region, 

section,  566,   568  ;   ccelom, 

diagrams,  576 

Pulinonata,  8,  32  ;  eye,  170  ;  radula,  183  ; 

renal  ducts,  diagram,  75 
Pupa,  8 
Pupidce,  8 
Purpura,  101 
Purpuridce,  6 
Pygaster,  291 
Pygurus,  293 
Pyramidellidce,  6 
Pyriform  vessel  (Gastr.),  220 
Pyrula,  44 

tuba,  73 
Pythonaster,  298 

EACHIAL  teeth,  182 

Rachiglossa,  6 

Radial  shields,  362 

Radials,  318 

Radiolitidce,  18 

Radula,  177,  180-183 

Red-brown  organ  (Keber's  organ),  215 

Regulares,  315 

Reptantia,  10 

Requienia,  18 

Metaster,  298 

Rete  mirabile  (Hoi.),  452 

Retiocrinidce,  306 

Retiocrinus,  306 

Rhabdopleura,  600-602 

„  Normani,    601  ;    section, 

602 

Rhachiglossa,  6 
Rhinophore,  48,  103 
Rhipidocrinus,  306 
Rhipidoglossa,  4 
Rhizochilus,  183 
Rhizocrinus,  311 

„  lofotensis,  335  ;  axial  canals, 

378  ;  stone  canal,  423 

„  Raivsoni,  335 

Rhodocrinidce,  306 
Rhodocrinus,  306 
Rhodope  Veranii,  281 
Rhopalodina,  287 

„  derivation  of,  408 

Rimula,  43 
Ringiculidce,  46 
Rissoidce,  108 
Rizzolia,  13 
Rossia,  24 


616 


COMPARATIVE  ANATOMY 


Rostellaria,  102 

„  rectirostris,  6 

Rostrifera,  6 

Kostruin  (Ceph.),  68  ;  (Gastr.),  102 
Rotula,  293 

Rotulae  (Echinoid.),  400 
Rudistes  (ffippurites),  62 
Huff  (Gastr.),  107 

SACCULI  (Grin.),  diagram,  490 

Salenia,  290  ;  apical  system,  319 

Saleniidce,  290 

Saxicava,  19 

Scceurgus,  243 

Scalariidce,  6 

Scaphander,  46 

Scaphandridce,  10 

Scaphites  stage,  68 

Scaphopoda,  13 

Schizaster,  293 

„          canaliferus,  pedicellariae,  397 
„  lacunosus,  294 

SchizoUastus,  315 

Schizocardium,  562 

„  brasiliense,    gills,    section, 

569 

Schizogony,  505 

Schwammerdam's  vesicle,  233 

Scissurella,  5 

Scotoplanes,  418 

Scrobicularia  piperata,  52 

Scrobiculariidce,  51 

Scurria,  5 

Scuta  buccalia,  360 

Scutella  adoratia,  360 

Scutella,  293 

,,        sexforis,  292 

Scutellidce,  291 

Scutum  (Parmorphus),  4 

Scyllceidce,  13 

Scytaster,  326 

Sea  urchins  ( =  Echinoid. ),  316 

Semiproboscidifera,  6 

Semites  (  =  fascicles),  349 

Semper's  organ,  166 

Sepia,  224  ;   alimentary  canal,  189  ;  body 
cavity,    213  ;   cephalic   cartilage, 
126  ;  ctenidiura,  85  ;  diagram,  36  ; 
gill,  96  ;  gonad,  male,  231  ;  onto- 
geny, 266,  267  ;  nervous  system, 
148  ;  renal  sacs,  213  ;  shell,  sec- 
tion, 69  ;  spermatophore,  242 
„      aculeata,  shell,  70 
„      officinalis,  circulatory  system,  209  ; 
eye,  172  ;  genital  organs,  female, 
240  ;  genital  organs,   male,  239  ; 
renal  sacs,  224 
„      Savigniana,  83 

Sepiadarium,  24 

Sepiola,  24  ',  nervous  system,  148 

Sepioloidea,  54 

Sepiotei'this,  24 

Septibr*  nchia,  21  ;  gills,  92 


Sickles  =  radii  (Echinoid.),  400 

Silenia  Sarsii,  21 

Siliquia,  115 

Sinistral  twist  (Gastr.),  56,  60,  160 

Sinupalliata,  shell,  63 

Siphon  (Ceph.),  37  ;  =  accessory  intestine 
(Echiuoid.),  481  ;  (Gastr.),  43;  (Lam.), 
exhalent,  49  ;  (Lam.),  inhalent,  49 


SipJwnodentalium,  13 

Siphonoplax,  64 

Sipfwnostomata,  44 

Siphuncle  (Ceph.),  37 

Solaridce,  6 

Solasteridce,  298 

Solen,  19 

Solenidce,  19 

Solenoc^lrtus,  19 

Solenogastres,  3 

Solenogastridce,  section,  212 

Solenomya,  91 

Solenomyidce,  14 

Solenopoda,  228 

Spadix,  116 

Spatagocystis,  295 

Spatangidce,  293 

Spatangoida,     293 ;      ambulacral     brush, 

section,  433 

Spatangoidea,  larva,  509 
Spatangomorpha,  293 
Spatangus,  293 

„          pvirpureus,  apical  system,  324  ; 

oral  region,  section,  441 
Speugel's  organ,  84 
Spermatophore,  241,  242 
Sphcerechinus,  290 

„  granularis,  pedicellarue,  397, 

398 

Sphseridia  (Echinoid.)  392  ;  section,  392 
Sphcerium,  17 
Sphceronis,  336 
Spiculum  amoris,  237 
Spiracles  (Blast.),  382 
Spirula,  24  ',  shell,  section,  69 
„       prototypos,  23 


Spiwilirostra,  24  ;  shell,  section,  ( 

Spondylidce,  53 

Spondylus,  62 

Spongiobranchcca,  11 

Spongylocentrotus,  290 

Star-fish  (  =  Asteroidea),  316 

Steganobranchia,  12 

Steganocrinus,  307 

Stelidiocrinus,  307 

Stellaster,  296 

Stelleridea  (=  Asteroidea),  295 

Stenoglossa,  6 

Stereosomata,  290 

Sternum,  348 

Stewart's  organs  (Echinoid.),  442 

Stichaster,  298 

„         albulus,  506 


INDEX 


617 


298 


Stiehopus,  285 

„         j<  iconic  us,  rod   and    supporting 

plate,  337 

Mil > ft'/-  Liu'-kii';  sections,  245,  246 
Stolasterias  (  =  Asterias  volseUata),  396 
8t<nimtii<he,  5 
Stone  canal,  416-423 
Streptoneurous    condition    (Gastr.),    133, 

158 

Streptosoniata,  290 
Strombidce,  6 
Strombus,  102 

Strong ylocentrotus  lividus,  398 
Strotocrimis,  307 

„  regalis,  apical  border,  368 

Ntm.thiolaridce,  6 
Stylommatophora,  8 
Subemargimda,  89 
Submytilacea,  17 
Succineidce,  8 
Xymbathocrinus,  304 

Synajjta,    288 ;     young,     516 ;     auditory 
vesicle,  section,  467 

„        digitata,  288  ;   larva  and  young, 
511 

,,         inhcerens,  calcareous  body,  337 

„        vittata,  470 
Synapticulae,  567 

Synaptiili.'*  288',  ciliated  urn,  438 
Synostosis  (Grin.),  376 
Syzygy  (Grin.),  376 

T^ENIOGLOSSA,  6 
Talarocrinus,  308 
Tapes,  18 

Tapetum  lucidum,  175 
309 
multibranchiatus,  310 

=  Palliata),    10;     section, 

110 

Ti'i-f/i/'it  (=A&naxt),  5 
Teeth    (Ast.),    473  ;    (Echin.),   400,    401  ; 

pharyngeal  (Gastr.),  180 
Tegmen  calycis,  302,  369 
Tegmentum,  39 
Tdeiocrinus,  307 
Tellina,  18 
Tillinacea,  18 
TcUinidcK,  18 
TV/// //"/'/'•>/ /•/</<',  290 
T'-nni<>i>li'nru.s,  290 
Terebra,  102 
Terebridce,  6 
T>>r«Hna,  65 
Teredinidce,  21 
Teredo.  Jl 

„        n<i.  call*.    20  ;     development,     259, 

260 

Tcri/ipes,  13 

Terminal  plates  (Ast.),  354 
Testacella,  S,  pallial  organs,  77 
„          fuxliotidea,  9,  44,  45 


Testacdlidce,  8 
Tethymelibidce,  13 
Tethys,  13 
Tetrabranchia,  22 
Thavmatocrinus,  309 

„  renovatiis,  310 

Tkecosamata  (Pteropoda),  11 
T/ieelia,  287 
Thracia,  21 
Thyca,  183 
,,       ectoconcha,  244-246  ;  sections,  244, 

246 
Thyone,  287 

„        chilensis,  417 
Thysanoteuthis,  24 
Tiarechimis,  289 

„  princeps,  289  ;  319 

Tiedemann's  bodies,  416-424 
Titiscania,  4 
Tornaria  larva,  507 

,,  Agassizi,  589 

Grenacheri,  587 

Krohni,  586,  590 

MiiUeri,  587 
Tornatinidce,  46 
Torus  angularis  (Oph.),  361 
Tosia,  326 
Toxiglossa,  6 
Toxopneiistes,  290  ;  masticatory  apparatus, 

402 
„  droebachiensis,  apical  system, 

320  ;  system  of  plates,  341 
Tremoctopus,  24 

„  violaceus,  196 

TricJiaster,  301 

„          elegans,  394 
Tricuspid  body  (Lam.),  194 
Tridacna,  18 
Tridacnidce,  18. 
Triforis,  160 
Trt'i/n/iia,  15 
TrigoniidcK,  15 
Triopa,  13 
Triploblastica,  545 
Tripneiistes,  290 
Tritonia,  13 
Tritoniadce,  13 
Ti-i/oiiiidas,  6 
Trivium,  325;  347,  407 
Trochidce,  5 
Trochophora,  253,  260 
Trochostwna,  287 
Trochiis,  gills,  90 
Troostoblastidce,  315 
Troostocrinus,  315 


Tubercle,  388 
TurMnellidce,  6 
Turbinidce,  5 
Turbo,  166 
Turgescence,  118 
Turrilite  stage,  68 
Tarritella,  30 


618 


COMPARATIVE  ANATOMY 


Turritellidce,  6 
Tylaster,  297 
Tylodina,  10 

UlNTACRINUS,  309 

Umbrella,  10 
Umbrellidce,  10 
Uncini,  182 
Unio,  17 

„     Margaratiferus   (Margaritana),    17, 
35 

„    pinctorum,  17 
Unionidce,  17 
Unionince,  50 
Uniqphora,  299 
Urechimis,  293 
Urns,  ciliated,  437 
Utriculus,  180 

VAGINULID&,  8 
Vaginulus,  45 
Valvaster,  299 
Valvatidce,  6 
Veliger,  larva,  1,  257 
Velum,  250 
Veneracea,  18 
Venericardia,  17 
Veneridce,  18 
Venus,  18 


Verinetidce,  6 

Vermetus,  108,  248 

Vertebral  ossicles  (Oph.),  356 

Verticordiidce,  51 

Vitrina,  8 

Volutidce,  6 

Vulsella,  17 

WACHSMUTH  and  Springer's  rule,  diagram, 

374 
Wandering  cells  (amoebocytes)  (Echiu.), 

415 
Water  lung  (Hoi.),  487  ;  pore  (Scaph.), 

221  ;  pores  (Crin.),  377 

XENOCRINUS,  306 
Xenophoridce,  6 
Xylopliaga,  21 

TOLDIA,  14 
Ypsilothuria,  409 

ZEUGOBRANCHIA,  4 
Zonites,  8 
Zoroaster,  298 

„         fulgens,  apical  system,  326 
Zoroasteridce,  298 
Zygoneury,  137 


THE    END 


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